Drawing a character in VGA memory with GNU C inline assembly

Drawing a character in VGA memory with GNU C inline assembly - c

I´m learning to do some low level VGA programming in DOS with C and inline assembly. Right now I´m trying to create a function that prints out a character on screen.
This is my code:
//This is the characters BITMAPS
uint8_t characters[464] = {
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x20,0x20,0x20,0x20,0x00,0x20,0x00,0x50,
0x50,0x00,0x00,0x00,0x00,0x00,0x50,0xf8,0x50,0x50,0xf8,0x50,0x00,0x20,0xf8,0xa0,
0xf8,0x28,0xf8,0x00,0xc8,0xd0,0x20,0x20,0x58,0x98,0x00,0x40,0xa0,0x40,0xa8,0x90,
0x68,0x00,0x20,0x40,0x00,0x00,0x00,0x00,0x00,0x20,0x40,0x40,0x40,0x40,0x20,0x00,
0x20,0x10,0x10,0x10,0x10,0x20,0x00,0x50,0x20,0xf8,0x20,0x50,0x00,0x00,0x20,0x20,
0xf8,0x20,0x20,0x00,0x00,0x00,0x00,0x00,0x60,0x20,0x40,0x00,0x00,0x00,0xf8,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x60,0x60,0x00,0x00,0x08,0x10,0x20,0x40,0x80,
0x00,0x70,0x88,0x98,0xa8,0xc8,0x70,0x00,0x20,0x60,0x20,0x20,0x20,0x70,0x00,0x70,
0x88,0x08,0x70,0x80,0xf8,0x00,0xf8,0x10,0x30,0x08,0x88,0x70,0x00,0x20,0x40,0x90,
0x90,0xf8,0x10,0x00,0xf8,0x80,0xf0,0x08,0x88,0x70,0x00,0x70,0x80,0xf0,0x88,0x88,
0x70,0x00,0xf8,0x08,0x10,0x20,0x20,0x20,0x00,0x70,0x88,0x70,0x88,0x88,0x70,0x00,
0x70,0x88,0x88,0x78,0x08,0x70,0x00,0x30,0x30,0x00,0x00,0x30,0x30,0x00,0x30,0x30,
0x00,0x30,0x10,0x20,0x00,0x00,0x10,0x20,0x40,0x20,0x10,0x00,0x00,0xf8,0x00,0xf8,
0x00,0x00,0x00,0x00,0x20,0x10,0x08,0x10,0x20,0x00,0x70,0x88,0x10,0x20,0x00,0x20,
0x00,0x70,0x90,0xa8,0xb8,0x80,0x70,0x00,0x70,0x88,0x88,0xf8,0x88,0x88,0x00,0xf0,
0x88,0xf0,0x88,0x88,0xf0,0x00,0x70,0x88,0x80,0x80,0x88,0x70,0x00,0xe0,0x90,0x88,
0x88,0x90,0xe0,0x00,0xf8,0x80,0xf0,0x80,0x80,0xf8,0x00,0xf8,0x80,0xf0,0x80,0x80,
0x80,0x00,0x70,0x88,0x80,0x98,0x88,0x70,0x00,0x88,0x88,0xf8,0x88,0x88,0x88,0x00,
0x70,0x20,0x20,0x20,0x20,0x70,0x00,0x10,0x10,0x10,0x10,0x90,0x60,0x00,0x90,0xa0,
0xc0,0xa0,0x90,0x88,0x00,0x80,0x80,0x80,0x80,0x80,0xf8,0x00,0x88,0xd8,0xa8,0x88,
0x88,0x88,0x00,0x88,0xc8,0xa8,0x98,0x88,0x88,0x00,0x70,0x88,0x88,0x88,0x88,0x70,
0x00,0xf0,0x88,0x88,0xf0,0x80,0x80,0x00,0x70,0x88,0x88,0xa8,0x98,0x70,0x00,0xf0,
0x88,0x88,0xf0,0x90,0x88,0x00,0x70,0x80,0x70,0x08,0x88,0x70,0x00,0xf8,0x20,0x20,
0x20,0x20,0x20,0x00,0x88,0x88,0x88,0x88,0x88,0x70,0x00,0x88,0x88,0x88,0x88,0x50,
0x20,0x00,0x88,0x88,0x88,0xa8,0xa8,0x50,0x00,0x88,0x50,0x20,0x20,0x50,0x88,0x00,
0x88,0x50,0x20,0x20,0x20,0x20,0x00,0xf8,0x10,0x20,0x40,0x80,0xf8,0x00,0x60,0x40,
0x40,0x40,0x40,0x60,0x00,0x00,0x80,0x40,0x20,0x10,0x08,0x00,0x30,0x10,0x10,0x10,
0x10,0x30,0x00,0x20,0x50,0x88,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0xf8,
0x00,0xf8,0xf8,0xf8,0xf8,0xf8,0xf8};
/**************************************************************************
* put_char *
* Print char *
**************************************************************************/
void put_char(int x ,int y,int ascii_char ,byte color){
__asm__(
"push %si\n\t"
"push %di\n\t"
"push %cx\n\t"
"mov color,%dl\n\t" //test color
"mov ascii_char,%al\n\t" //test char
"sub $32,%al\n\t"
"mov $7,%ah\n\t"
"mul %ah\n\t"
"lea $characters,%si\n\t"
"add %ax,%si\n\t"
"mov $7,%cl\n\t"
"0:\n\t"
"segCS %lodsb\n\t"
"mov $6,%ch\n\t"
"1:\n\t"
"shl $1,%al\n\t"
"jnc 2f\n\t"
"mov %dl,%ES:(%di)\n\t"
"2:\n\t"
"inc %di\n\t"
"dec %ch\n\t"
"jnz 1b\n\t"
"add $320-6,%di\n\t"
"dec %cl\n\t"
"jnz 0b\n\t"
"pop %cx\n\t"
"pop %di\n\t"
"pop %si\n\t"
"retn"
);
}
I´m guiding myself from this series of tutorials written in PASCAL: http://www.joco.homeserver.hu/vgalessons/lesson8.html .
I changed the assembly syntax according to the gcc compiler, but I´m still getting this errors:
Operand mismatch type for 'lea'
No such instruction 'segcs lodsb'
No such instruction 'retn'
EDIT:
I have been working on improving my code and at least now I see something on the screen. Here´s my updated code:
/**************************************************************************
* put_char *
* Print char *
**************************************************************************/
void put_char(int x,int y){
int char_offset;
int l,i,j,h,offset;
j,h,l,i=0;
offset = (y<<8) + (y<<6) + x;
__asm__(
"movl _VGA, %%ebx;" // VGA memory pointer
"addl %%ebx,%%edi;" //%di points to screen
"mov _ascii_char,%%al;"
"sub $32,%%al;"
"mov $7,%%ah;"
"mul %%ah;"
"lea _characters,%%si;"
"add %%ax,%%si;" //SI point to bitmap
"mov $7,%%cl;"
"0:;"
"lodsb %%cs:(%%si);" //load next byte of bitmap
"mov $6,%%ch;"
"1:;"
"shl $1,%%al;"
"jnc 2f;"
"movb %%dl,(%%edi);" //plot the pixel
"2:\n\t"
"incl %%edi;"
"dec %%ch;"
"jnz 1b;"
"addl $320-6,%%edi;"
"dec %%cl;"
"jnz 0b;"
: "=D" (offset)
: "d" (current_color)
);
}
If you see the image above I was trying to write the letter "S". The results are the green pixels that you see on the upper left side of the screen. No matter what x and y I give the functon it always plots the pixels on that same spot.
Can anyone help me correct my code?

See below for an analysis of some things that are specifically wrong with your put_char function, and a version that might work. (I'm not sure about the %cs segment override, but other than that it should do what you intend).
Learning DOS and 16-bit asm isn't the best way to learn asm
First of all, DOS and 16-bit x86 are thoroughly obsolete, and are not easier to learn than normal 64-bit x86. Even 32-bit x86 is obsolete, but still in wide use in the Windows world.
32-bit and 64-bit code don't have to care about a lot of 16-bit limitations / complications like segments or limited register choice in addressing modes. Some modern systems do use segment overrides for thread-local storage, but learning how to use segments in 16-bit code is barely connected to that.
One of the major benefits to knowing asm is for debugging / profiling / optimizing real programs. If you want to understand how to write C or other high-level code that can (and actually does) compile to efficient asm, you'll probably be looking at compiler output. This will be 64-bit (or 32-bit). (e.g. see Matt Godbolt's CppCon2017 talk: “What Has My Compiler Done for Me Lately? Unbolting the Compiler's Lid” which has an excellent intro to reading x86 asm for total beginners, and to looking at compiler output).
Asm knowledge is useful when looking at performance-counter results annotating a disassembly of your binary (perf stat ./a.out && perf report -Mintel: see Chandler Carruth's CppCon2015 talk: "Tuning C++: Benchmarks, and CPUs, and Compilers! Oh My!"). Aggressive compiler optimizations mean that looking at cycle / cache-miss / stall counts per source line are much less informative than per instruction.
Also, for your program to actually do anything, it has to either talk to hardware directly, or make system calls. Learning DOS system calls for file access and user input is a complete waste of time (except for answering the steady stream of SO questions about how to read and print multi-digit numbers in 16-bit code). They're quite different from the APIs in the current major OSes. Developing new DOS applications is not useful, so you'd have to learn another API (as well as ABI) when you get to the stage of doing something with your asm knowledge.
Learning asm on an 8086 simulator is even more limiting: 186, 286, and 386 added many convenient instructions like imul ecx, 15, making ax less "special". Limiting yourself to only instructions that work on 8086 means you'll figure out "bad" ways to do things. Other big ones are movzx / movsx, shift by an immediate count (other than 1), and push immediate. Besides performance, it's also easier to write code when these are available, because you don't have to write a loop to shift by more than 1 bit.
Suggestions for better ways to teach yourself asm
I mostly learned asm from reading compiler output, then making small changes. I didn't try to write stuff in asm when I didn't really understand things, but if you're going to learn quickly (rather than just evolve an understanding while debugging / profiling C), you probably need to test your understanding by writing your own code. You do need to understand the basics, that there are 8 or 16 integer registers + the flags and instruction pointer, and that every instruction makes a well-defined modification to the current architectural state of the machine. (See the Intel insn ref manual for complete descriptions of every instruction (links in the x86 wiki, along with much more good stuff).
You might want to start with simple things like writing a single function in asm, as part of a bigger program. Understanding the kind of asm needed to make system calls is useful, but in real programs it's normally only useful to hand-write asm for inner loops that don't involve any system calls. It's time-consuming to write asm to read input and print results, so I'd suggest doing that part in C. Make sure you read the compiler output and understand what's going on, and the difference between an integer and a string, and what strtol and printf do, even if you don't write them yourself.
Once you think you understand enough of the basics, find a function in some program you're familiar with and/or interested in, and see if you can beat the compiler and save instructions (or use faster instructions). Or implement it yourself without using the compiler output as a starting point, whichever you find more interesting. This answer might be interesting, although the focus there was finding C source that got the compiler to produce the optimal ASM.
How to try to solve your own problems (before asking an SO question)
There are many SO questions from people asking "how do I do X in asm", and the answer is usually "the same as you would in C". Don't get so caught up in asm being unfamiliar that you forget how to program. Figure out what needs to happen to the data the function operates on, then figure out how to do that in asm. If you get stuck and have to ask a question, you should have most of a working implementation, with just one part that you don't know what instructions to use for one step.
You should do this with 32 or 64bit x86. I'd suggest 64bit, since the ABI is nicer, but 32bit functions will force you to make more use of the stack. So that might help you understand how a call instruction puts the return address on the stack, and where the args the caller pushed actually are after that. (This appears to be what you tried to avoid dealing with by using inline asm).
Programming hardware directly is neat, but not a generally useful skill
Learning how to do graphics by directly modifying video RAM is not useful, other than to satisfy curiosity about how computers used to work. You can't use that knowledge for anything. Modern graphics APIs exist to let multiple programs draw in their own regions of the screen, and to allow indirection (e.g. draw on a texture instead of the screen directly, so 3D window-flipping alt-tab can look fancy). There too many reasons to list here for not drawing directly on video RAM.
Drawing on a pixmap buffer and then using a graphics API to copy it to the screen is possible. Still, doing bitmap graphics at all is more or less obsolete, unless you're generating images for PNG or JPEG or something (e.g. optimize converting histogram bins to a scatter plot in the back-end code for a web service). Modern graphics APIs abstract away the resolution, so your app can draw things at a reasonable size regardless of how big each pixel is. (small but extremely high rez screen vs. big TV at low rez).
It is kind of cool to write to memory and see something change on-screen. Or even better, hook up LEDs (with small resistors) to the data bits on a parallel port, and run an outb instruction to turn them on/off. I did this on my Linux system ages ago. I made a little wrapper program that used iopl(2) and inline asm, and ran it as root. You can probably do similar on Windows. You don't need DOS or 16bit code to get your feet wet talking to the hardware.
in/out instructions, and normal loads/stores to memory-mapped IO, and DMA, are how real drivers talk to hardware, including things far more complicated than parallel ports. It's fun to know how your hardware "really" works, but only spend time on it if you're actually interested, or want to write drivers. The Linux source tree includes drivers for boatloads of hardware, and is often well commented, so if you like reading code as much as writing code, that's another way to get a feel for what read drivers do when they talk to hardware.
It's generally good to have some idea how things work under the hood. If you want to learn about how graphics used to work ages ago (with VGA text mode and color / attribute bytes), then sure, go nuts. Just be aware that modern OSes don't use VGA text mode, so you aren't even learning what happens under the hood on modern computers.
Many people enjoy https://retrocomputing.stackexchange.com/, reliving a simpler time when computers were less complex and couldn't support as many layers of abstraction. Just be aware that's what you're doing. I might be a good stepping stone to learning to write drivers for modern hardware, if you're sure that's why you want to understand asm / hardware.
Inline asm
You are taking a totally incorrect approach to using inline ASM. You seem to want to write whole functions in asm, so you should just do that. e.g. put your code in asmfuncs.S or something. Use .S if you want to keep using GNU / AT&T syntax; or use .asm if you want to use Intel / NASM / YASM syntax (which I would recommend, since the official manuals all use Intel syntax. See the x86 wiki for guides and manuals.)
GNU inline asm is the hardest way to learn ASM. You have to understand everything that your asm does, and what the compiler needs to know about it. It's really hard to get everything right. For example, in your edit, that block of inline asm modifies many registers that you don't list as clobbered, including %ebx which is a call-preserved register (so this is broken even if that function isn't inlined). At least you took out the ret, so things won't break as spectacularly when the compiler inlines this function into the loop that calls it. If that sounds really complicated, that's because it is, and part of why you shouldn't use inline asm to learn asm.
This answer to a similar question from misusing inline asm while trying to learn asm in the first place has more links about inline asm and how to use it well.
Getting this mess working, maybe
This part could be a separate answer, but I'll leave it together.
Besides your whole approach being fundamentally a bad idea, there is at least one specific problem with your put_char function: you use offset as an output-only operand. gcc quite happily compiles your whole function to a single ret instruction, because the asm statement isn't volatile, and its output isn't used. (Inline asm statements without outputs are assumed to be volatile.)
I put your function on godbolt, so I could look at what assembly the compiler generates surrounding it. That link is to the fixed maybe-working version, with correctly-declared clobbers, comments, cleanups, and optimizations. See below for the same code, if that external link ever breaks.
I used gcc 5.3 with the -m16 option, which is different from using a real 16bit compiler. It still does everything the 32bit way (using 32bit addresses, 32bit ints, and 32bit function args on the stack), but tells the assembler that the CPU will be in 16bit mode, so it will know when to emit operand-size and address-size prefixes.
Even if you compile your original version with -O0, the compiler computes offset = (y<<8) + (y<<6) + x;, but doesn't put it in %edi, because you didn't ask it to. Specifying it as another input operand would have worked. After the inline asm, it stores %edi into -12(%ebp), where offset lives.
Other stuff wrong with put_char:
You pass 2 things (ascii_char and current_color) into your function through globals, instead of function arguments. Yuck, that's disgusting. VGA and characters are constants, so loading them from globals doesn't look so bad. Writing in asm means you should ignore good coding practices only when it helps performance by a reasonable amount. Since the caller probably had to store those values into the globals, you're not saving anything compared to the caller storing them on the stack as function args. And for x86-64, you'd be losing perf because the caller could just pass them in registers.
Also:
j,h,l,i=0; // sets i=0, does nothing to j, h, or l.
// gcc warns: left-hand operand of comma expression has no effect
j;h;l;i=0; // equivalent to this
j=h=l=i=0; // This is probably what you meant
All the local variables are unused anyway, other than offset. Were you going to write it in C or something?
You use 16bit addresses for characters, but 32bit addressing modes for VGA memory. I assume this is intentional, but I have no idea if it's correct. Also, are you sure you should use a CS: override for the loads from characters? Does the .rodata section go into the code segment? Although you didn't declare uint8_t characters[464] as const, so it's probably just in the .data section anyway. I consider myself fortunate that I haven't actually written code for a segmented memory model, but that still looks suspicious.
If you're really using djgpp, then according to Michael Petch's comment, your code will run in 32bit mode. Using 16bit addresses is thus a bad idea.
Optimizations
You can avoid using %ebx entirely by doing this, instead of loading into ebx and then adding %ebx to %edi.
"add _VGA, %%edi\n\t" // load from _VGA, add to edi.
You don't need lea to get an address into a register. You can just use
"mov %%ax, %%si\n\t"
"add $_characters, %%si\n\t"
$_characters means the address as an immediate constant. We can save a lot of instructions by combining this with the previous calculation of the offset into the characters array of bitmaps. The immediate-operand form of imul lets us produce the result in %si in the first place:
"movzbw _ascii_char,%%si\n\t"
//"sub $32,%%ax\n\t" // AX = ascii_char - 32
"imul $7, %%si, %%si\n\t"
"add $(_characters - 32*7), %%si\n\t" // Do the -32 at the same time as adding the table address, after multiplying
// SI points to characters[(ascii_char-32)*7]
// i.e. the start of the bitmap for the current ascii character.
Since this form of imul only keeps the low 16b of the 16*16 -> 32b multiply, the 2 and 3 operand forms imul can be used for signed or unsigned multiplies, which is why only imul (not mul) has those extra forms. For larger operand-size multiplies, 2 and 3 operand imul is faster, because it doesn't have to store the high half in %[er]dx.
You could simplify the inner loop a bit, but it would complicate the outer loop slightly: you could branch on the zero flag, as set by shl $1, %al, instead of using a counter. That would make it also unpredictable, like the jump over store for non-foreground pixels, so the increased branch mispredictions might be worse than the extra do-nothing loops. It would also mean you'd need to recalculate %edi in the outer loop each time, because the inner loop wouldn't run a constant number of times. But it could look like:
... same first part of the loop as before
// re-initialize %edi to first_pixel-1, based on outer-loop counter
"lea -1(%%edi), %%ebx\n"
".Lbit_loop:\n\t" // map the 1bpp bitmap to 8bpp VGA memory
"incl %%ebx\n\t" // inc before shift, to preserve flags
"shl $1,%%al\n\t"
"jnc .Lskip_store\n\t" // transparency: only store on foreground pixels
"movb %%dl,(%%ebx)\n" //plot the pixel
".Lskip_store:\n\t"
"jnz .Lbit_loop\n\t" // flags still set from shl
"addl $320,%%edi\n\t" // WITHOUT the -6
"dec %%cl\n\t"
"jnz .Lbyte_loop\n\t"
Note that the bits in your character bitmaps are going to map to bytes in VGA memory like {7 6 5 4 3 2 1 0}, because you're testing the bit shifted out by a left shift. So it starts with the MSB. Bits in a register are always "big endian". A left shift multiplies by two, even on a little-endian machine like x86. Little-endian only affects ordering of bytes in memory, not bits in a byte, and not even bytes inside registers.
A version of your function that might do what you intended.
This is the same as the godbolt link.
void put_char(int x,int y){
int offset = (y<<8) + (y<<6) + x;
__asm__ volatile ( // volatile is implicit for asm statements with no outputs, but better safe than sorry.
"add _VGA, %%edi\n\t" // edi points to VGA + offset.
"movzbw _ascii_char,%%si\n\t" // Better: use an input operand
//"sub $32,%%ax\n\t" // AX = ascii_char - 32
"imul $7, %%si, %%si\n\t" // can't fold the load into this because it's not zero-padded
"add $(_characters - 32*7), %%si\n\t" // Do the -32 at the same time as adding the table address, after multiplying
// SI points to characters[(ascii_char-32)*7]
// i.e. the start of the bitmap for the current ascii character.
"mov $7,%%cl\n"
".Lbyte_loop:\n\t"
"lodsb %%cs:(%%si)\n\t" //load next byte of bitmap
"mov $6,%%ch\n"
".Lbit_loop:\n\t" // map the 1bpp bitmap to 8bpp VGA memory
"shl $1,%%al\n\t"
"jnc .Lskip_store\n\t" // transparency: only store on foreground pixels
"movb %%dl,(%%edi)\n" //plot the pixel
".Lskip_store:\n\t"
"incl %%edi\n\t"
"dec %%ch\n\t"
"jnz .Lbit_loop\n\t"
"addl $320-6,%%edi\n\t"
"dec %%cl\n\t"
"jnz .Lbyte_loop\n\t"
: "+&D" (offset) // EDI modified by the asm, compiler needs to know that, so use a read-write "+" input. Early-clobber "&" because we read the other input after modifying this.
: "d" (current_color) // used read-only
: "%eax", "%ecx", "%esi", "memory"
// omit the memory clobber if your C never touches VGA memory, and your asm never loads/stores anywhere else.
// but that's not the case here: the asm loads from memory written by C
// without listing it as a memory operand (even a pointer in a register isn't sufficient)
// so gcc might optimize away "dead" stores to it, or reorder the asm with loads/stores to it.
);
}
Re: the "memory" clobber, see How can I indicate that the memory *pointed* to by an inline ASM argument may be used?
I didn't use dummy output operands to leave register allocation up to the compiler's discretion, but that's a good idea to reduce the overhead of getting data in the right places for inline asm. (extra mov instructions). For example, here there was no need to force the compiler to put offset in %edi. It could have been any register we aren't already using.

Related

Why does Knuth use this clunky decrement?

I'm looking at some of Prof. Don Knuth's code, written in CWEB that is converted to C. A specific example is dlx1.w, available from Knuth's website
At one stage, the .len value of a struct nd[cc] is decremented, and it is done in a clunky way:
o,t=nd[cc].len-1;
o,nd[cc].len=t;
(This is a Knuth-specific question, so maybe you already know that "o," is a preprocessor macro for incrementing "mems", which is a running total of effort expended, as measured by accesses to 64-bit words.) The value remaining in "t" is definitely not used for anything else. (The example here is on line 665 of dlx1.w, or line 193 of dlx1.c after ctangle.)
My question is: why does Knuth write it this way, rather than
nd[cc].len--;
which he does actually use elsewhere (line 551 of dlx1.w):
oo,nd[k].len--,nd[k].aux=i-1;
(And "oo" is a similar macro for incrementing "mems" twice -- but there is some subtlety here, because .len and .aux are stored in the same 64-bit word. To assign values to S.len and S.aux, only one increment to mems would normally be counted.)
My only theory is that a decrement consists of two memory accesses: first to look up, then to assign. (Is that correct?) And this way of writing it is a reminder of the two steps. This would be unusually verbose of Knuth, but maybe it is an instinctive aide-memoire rather than didacticism.
For what it's worth, I've searched in CWEB documentation without finding an answer. My question probably relates more to Knuth's standard practices, which I am picking up bit by bit. I'd be interested in any resources where these practices are laid out (and maybe critiqued) as a block -- but for now, let's focus on why Knuth writes it this way.

A preliminary remark: with Knuth-style literate programming (i.e. when reading WEB or CWEB programs) the “real” program, as conceived by Knuth, is neither the “source” .w file nor the generated (tangled) .c file, but the typeset (woven) output. The source .w file is best thought of as a means to produce it (and of course also the .c source that's fed to the compiler). (If you don't have cweave and TeX handy; I've typeset some of these programs here; this program DLX1 is here.)
So in this case, I'd describe the location in the code as module 25 of DLX1, or subroutine "cover":
Anyway, to return to the actual question: note that this (DLX1) is one of the programs written for The Art of Computer Programming. Because reporting the time taken by a program “seconds” or “minutes” becomes meaningless from year to year, he reports how long a program took in number of “mems” plus “oops”, that's dominated by the “mems”, i.e. the number of memory accesses to 64-bit words (usually). So the book contains statements like “this program finds the answer to this problem in 3.5 gigamems of running time”. Further, the statements are intended to be fundamentally about the program/algorithm itself, not the specific code generated by a specific version of a compiler for certain hardware. (Ideally when the details are very important he writes the program in MMIX or MMIXAL and analyses its operations on the MMIX hardware, but this is rare.) Counting the mems (to be reported as above) is the purpose of inserting o and oo instructions into the program. Note that it's more important to get this right for the “inner loop” instructions that are executed a lot of times, such as everything in the subroutine cover in this case.
This is elaborated in Section 1.3.1′ (part of Fascicle 1):
Timing. […] The running time of a program depends not only on the clock rate but also on the number of functional units that can be active simultaneously and the degree to which they are pipelined; it depends on the techniques used to prefetch instructions before they are executed; it depends on the size of the random-access memory that is used to give the illusion of 264 virtual bytes; and it depends on the sizes and allocation strategies of caches and other buffers, etc., etc.
For practical purposes, the running time of an MMIX program can often be estimated satisfactorily by assigning a fixed cost to each operation, based on the approximate running time that would be obtained on a high-performance machine with lots of main memory; so that’s what we will do. Each operation will be assumed to take an integer number of υ, where υ (pronounced “oops”) is a unit that represents the clock cycle time in a pipelined implementation. Although the value of υ decreases as technology improves, we always keep up with the latest advances because we measure time in units of υ, not in nanoseconds. The running time in our estimates will also be assumed to depend on the number of memory references or mems that a program uses; this is the number of load and store instructions. For example, we will assume that each LDO (load octa) instruction costs µ + υ, where µ is the average cost of a memory reference. The total running time of a program might be reported as, say, 35µ+ 1000υ, meaning “35 mems plus 1000 oops.” The ratio µ/υ has been increasing steadily for many years; nobody knows for sure whether this trend will continue, but experience has shown that µ and υ deserve to be considered independently.
And he does of course understand the difference from reality:
Even though we will often use the assumptions of Table 1 for seat-of-the-pants estimates of running time, we must remember that the actual running time might be quite sensitive to the ordering of instructions. For example, integer division might cost only one cycle if we can find 60 other things to do between the time we issue the command and the time we need the result. Several LDB (load byte) instructions might need to reference memory only once, if they refer to the same octabyte. Yet the result of a load command is usually not ready for use in the immediately following instruction. Experience has shown that some algorithms work well with cache memory, and others do not; therefore µ is not really constant. Even the location of instructions in memory can have a significant effect on performance, because some instructions can be fetched together with others. […] Only the meta-simulator can be trusted to give reliable information about a program’s actual behavior in practice; but such results can be difficult to interpret, because infinitely many configurations are possible. That’s why we often resort to the much simpler estimates of Table 1.
Finally, we can use Godbolt's Compiler Explorer to look at the code generated by a typical compiler for this code. (Ideally we'd look at MMIX instructions but as we can't do that, let's settle for the default there, which seems to be x68-64 gcc 8.2.) I removed all the os and oos.
For the version of the code with:
/*o*/ t = nd[cc].len - 1;
/*o*/ nd[cc].len = t;
the generated code for the first line is:
movsx rax, r13d
sal rax, 4
add rax, OFFSET FLAT:nd+8
mov eax, DWORD PTR [rax]
lea r14d, [rax-1]
and for the second line is:
movsx rax, r13d
sal rax, 4
add rax, OFFSET FLAT:nd+8
mov DWORD PTR [rax], r14d
For the version of the code with:
/*o ?*/ nd[cc].len --;
the generated code is:
movsx rax, r13d
sal rax, 4
add rax, OFFSET FLAT:nd+8
mov eax, DWORD PTR [rax]
lea edx, [rax-1]
movsx rax, r13d
sal rax, 4
add rax, OFFSET FLAT:nd+8
mov DWORD PTR [rax], edx
which as you can see (even without knowing much about x86-64 assembly) is simply the concatenation of the code generated in the former case (except for using register edx instead of r14d), so it's not as if writing the decrement in one line has saved you any mems. In particular, it would not be correct to count it as a single one, especially in something like cover that is called a huge number of times in this algorithm (dancing links for exact cover).
So the version as written by Knuth is correct, for its goal of counting the number of mems. He could also write oo,nd[cc].len--; (counting two mems) as you observed, but perhaps it might look like a bug at first glance in that case. (BTW, in your example in the question of oo,nd[k].len--,nd[k].aux=i-1; the two mems come from the load and the store in --; not two stores.)

This whole practice seems to be based on a mistaken idea/model of how C works, that there's some correspondence between the work performed by the abstract machine and by the actual program as executed (i.e. the "C is portable assembler" fallacy). I don't think we can answer a lot more about why that exact code fragment appears, except that it happens to be an unusual idiom for counting loads and stores on the abstract machine as separate.

NASM Assembly Add Array elements [duplicate]

mov ecx, 16
looptop: .
.
.
loop looptop
How many times will this loop execute?
What happens if ecx = 0 to start with? Does loop jump or fall-through in that case?

loop is exactly like dec ecx / jnz, except it doesn't set flags.
It's like the bottom of a do {} while(--ecx != 0); in C. If execution enters the loop with ecx = 0, wrap-around means the loop will run 2^32 times. (Or 2^64 times in 64-bit mode, because it uses RCX.)
Unlike rep movsb/stosb/etc., it doesn't check for ECX=0 before decrementing, only after1.
The address-size determines whether it uses CX, ECX, or RCX. So in 64-bit code, addr32 loop is like dec ecx / jnz, while a regular loop is like dec rcx / jnz. Or in 16-bit code, it normally uses CX, but an address-size prefix (0x67) will make it use ecx. As Intel's manual says, it ignores REX.W, because that sets the operand-size, not the address-size.
rep string instructions use the address-size prefix the same way, overriding the address size but also RCX vs. ECX (or CX vs. ECX in modes other than 64-bit). The operand-size for string instructions is already used to determine movsw vs. movsd vs. movsq, and you want address/repeat size to be orthogonal to that. Having loop and jrcxz/jecxz follow that behaviour is just continuing the design intent from 8086 of loop being intended for use with string operations when a simple rep couldn't get the job done; see below.
Related: Why are loops always compiled into "do...while" style (tail jump)? for more about loop structure in asm, while() {} vs. do {} while() and how to lay them out.
Footnote 1: jcxz (or x86-64 jrcxz) was intended for use before the top of a do {} while style loop, to skip it if it should run 0 times. On modern CPUs test rcx, rcx / jz is more efficient.
Stephen Morse, architect of 8086, wrote about the intended uses of loop/jcxz with string instructions in that section of his book, The 8086 Primer, available for free on his web site: https://www.stevemorse.org/8086/index.html. See the "complex string instructions" subsection, starting at the bottom of page 71. (Or start reading from earlier in the chapter, the whole String Instructions section starts on page 66. But note #ecm's review of a few things the book seems to explain poorly or incorrectly.)
If you're wondering about the design intent of x86 instructions, you won't find a better source than this. That's separate from the best / most efficient way to use them, especially on modern x86, but very good intro for beginners into what you can do with asm instructions as building blocks.
Extra debugging tips
If you ever want to know the details on an instruction, check the manual: either Intel's official vol.2 PDF instruction set reference manual, or an html extract with each entry on a different page (http://felixcloutier.com/x86/). But note that the HTML leaves out the intro and appendices that have details on how to interpret stuff, like when it says "flags are set according to the result" for instructions like add.
And you can (and should) also just try stuff in a debugger: single-step and watch registers change. Use a smaller starting value for ecx so you get to the interesting ecx=1 part sooner. See also the x86 tag wiki for links to manuals, guides, and asm debugging tips at the bottom.
And BTW, if the instructions inside the loop that aren't shown modify ecx, it could loop any number of times. For the question to have a simple and unique answer, you need a guarantee that the instructions between the label and the loop instruction don't modify ecx. (They could save/restore it, but if you're going to do that it's usually better to just use a different register as the loop counter. push/pop inside a loop makes your code hard to read.)
Rant about over-use of LOOP even when you already need to increment something else in the loop. LOOP isn't the only way to loop, and usually it's the worst.
You should normally never use the loop instruction unless optimizing for code-size at the expense of speed, because it's slow. Compilers don't use it. (So CPU vendors don't bother to make it fast; catch 22.) Use dec / jnz, or an entirely different loop condition. (See also http://agner.org/optimize/ to learn more about what's efficient.)
Loops don't even have to use a counter; it's often just as good if not better to compare a pointer to an end address, or to check for some other condition. (Pointless use of loop is one of my pet peeves, especially when you already have something in another register that would work as a loop counter.) Using cx as a loop counter often just ties up one of your precious few registers when you could have used cmp/jcc on another register you were incrementing anyway.
IMO, loop should be considered one of those obscure x86 instructions that beginners shouldn't be distracted with. Like stosd (without a rep prefix), aam or xlatb. It does have real uses when optimizing for code size, though. (That's sometimes useful in real life for machine code (like for boot sectors), not just for stuff like code golf.)
IMO, just teach / learn how conditional branches work, and how to make loops out of them. Then you won't get stuck into thinking there's something special about a loop that uses loop. I've seen an SO question or comment that said something like "I thought you had to declare loops", and didn't realize that loop was just an instruction.
</rant>. Like I said, loop is one of my pet peeves. It's an obscure code-golfing instruction, unless you're optimizing for an actual 8086.

How exactly does the x86 LOOP instruction work?

mov ecx, 16
looptop: .
.
.
loop looptop
How many times will this loop execute?
What happens if ecx = 0 to start with? Does loop jump or fall-through in that case?

Efficiency of C vs Assembler

How much faster is the following assembler code:
shl ax, 1
Versus the following C code:
num = num * 2;
How can I even find out?

Your assembly variant might be faster, might be slower. What made you think that it is necessarily faster?
On the x86 platform, there are quite a few ways to multiply something by 2. I would expect a compiler to do add ax, ax, which is intuitively more efficient than your shl because it doesn't involve a potentially stored constant ('1' in your case).
Also, for quite a long time, on a x86 platform the preferred way of multiplying things by constants was not a shift, but rather a lea operation (when possible). In the above example that would be lea eax, [eax*2]. (Multiplication by 3 would be done through lea eax, [eax*2+eax])
The belief in shift operations being somehow "faster" is a nice old story for newbies, which has virtually no relevance today. And, as usual, most of the time your compiler (if it is up-to-date) has much better knowledge about the underlying hardware platform than people with naive love for shift operations.

Is this, by any chance, an academic question? I assume you understand it is in the general category of "getting a haircut to lose weight".

If you are using GCC, ask to see the generated assembly with option -S. You may find it's the same as your assembler instruction.
To answer the original question, on Out-Of-Order processors instruction speed is measured by throughput and latency, and you would measure both using the rdtsc assembly instruction. But someone else did it for you for a lot of processors, so you don't need to bother. PDF

In most circumstances, it won't make a difference. Multiplication is fast on nearly all modern hardware. In particular, it is usually fast enough that unless you have meticulously hand-optimized code, the pipeline will hide the entirety of the latency and you will see no speed difference at all between the two cases.
You may be able to measure a performance difference on multiplies and shifts when you execute them in isolation, but there will typically not be any difference in the context of the rest of your compiled code. (As I noted, this may not hold true if the code is meticulously optimized).
Now, that said, shifts are still generally faster than multiplies, and almost any reasonable compiler will map a fixed power-of-two multiply into a shift, anyway (assuming that the semantics are actually equivalent on the target architecture).
Edit: one more thing you may want to try if you really care about this is x+x. I know of at least one architecture on which this can actually be faster than shifting, depending on the surrounding context.

If you have a decent compiler it will produce the same or similar code. The best way is to disassemble and checked the created code.

The answer depends, as you've been able to see here, upon many things. What the compiler will do with your C code depends on a lot of things. If we're talking x86-32 the following should be generally applicable.
At the basic level your C code indicates a memory variable which would require at least one instruction to multiply by two: "shl mem,1" and in such a simple case the C code will be slower.
If num is a local variable the compiler may decide to put it in a register (if it is used often enough and/or the function is small enough) and then you will have your "shl reg,1" instruction - maybe.
What instruction is fastest has everything to do with how they are implemented in the processor. Shl may not be the best choice since it affects the C and Z flags which slows it down. A few years ago the recommendation was "lea reg,[reg+reg]" (all reg are the same) because lea didn't affect any flags and there were variants such as (using the eax register on x86-32 platform as an example):
lea eax,[eax+eax] ; *2
lea eax,[eax+eax*2] ; *3
lea eax,[eax+eax*4] ; *5
lea eax,[eax+eax*8] ; *9
I don't know what is the norm today but your compiler probably will.
As for measuring search for information here on the rdtsc instruction which is the hands-down best alternative as it counts actual clock cycles.

Put them in a loop with a counter that goes so high that it runs for at least a second in the fastest case. Use your favorite timing mechanism to see how long each takes.
The assembly case should be done with inline assembly in the same C program as you use for the pure C test. Otherwise, you're not comparing apples to apples.
By the way, I think you should add a third test:
num <<= 1;
The question then is whether that does the same thing as the assembly version.

If, for your target platform, shifting left is the quickest way to multiply a number by two, then the chances are your compiler will do that when compiling the code. Look at the disassembly to check
So, for that one line, it's probably exactly the same speed. However, as you're unlikely to have a function containing just that one line, you might well find the compiler would defer the shift until the value is used, or otherwise mix it up with surrounding code, making it less clear cut. A good optimizing compiler will generally do a good job of beating poor to average hand written assembly.

If the compiler up to date now ( vc9 ) was really doing a good job it would outperform vc6 by a wide margin and this dont occur, this is why I even prefer to use VC6 for some code that run faster than code compiled in mingw with -O3 and VC9 with /Ox

When is assembly faster than C? [closed]

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One of the stated reasons for knowing assembler is that, on occasion, it can be employed to write code that will be more performant than writing that code in a higher-level language, C in particular. However, I've also heard it stated many times that although that's not entirely false, the cases where assembler can actually be used to generate more performant code are both extremely rare and require expert knowledge of and experience with assembly.
This question doesn't even get into the fact that assembler instructions will be machine-specific and non-portable, or any of the other aspects of assembler. There are plenty of good reasons for knowing assembly besides this one, of course, but this is meant to be a specific question soliciting examples and data, not an extended discourse on assembler versus higher-level languages.
Can anyone provide some specific examples of cases where assembly will be faster than well-written C code using a modern compiler, and can you support that claim with profiling evidence? I am pretty confident these cases exist, but I really want to know exactly how esoteric these cases are, since it seems to be a point of some contention.

Here is a real world example: Fixed point multiplies on old compilers.
These don't only come handy on devices without floating point, they shine when it comes to precision as they give you 32 bits of precision with a predictable error (float only has 23 bit and it's harder to predict precision loss). i.e. uniform absolute precision over the entire range, instead of close-to-uniform relative precision (float).
Modern compilers optimize this fixed-point example nicely, so for more modern examples that still need compiler-specific code, see
Getting the high part of 64 bit integer multiplication: A portable version using uint64_t for 32x32 => 64-bit multiplies fails to optimize on a 64-bit CPU, so you need intrinsics or __int128 for efficient code on 64-bit systems.
_umul128 on Windows 32 bits: MSVC doesn't always do a good job when multiplying 32-bit integers cast to 64, so intrinsics helped a lot.
C doesn't have a full-multiplication operator (2N-bit result from N-bit inputs). The usual way to express it in C is to cast the inputs to the wider type and hope the compiler recognizes that the upper bits of the inputs aren't interesting:
// on a 32-bit machine, int can hold 32-bit fixed-point integers.
int inline FixedPointMul (int a, int b)
{
long long a_long = a; // cast to 64 bit.
long long product = a_long * b; // perform multiplication
return (int) (product >> 16); // shift by the fixed point bias
}
The problem with this code is that we do something that can't be directly expressed in the C-language. We want to multiply two 32 bit numbers and get a 64 bit result of which we return the middle 32 bit. However, in C this multiply does not exist. All you can do is to promote the integers to 64 bit and do a 64*64 = 64 multiply.
x86 (and ARM, MIPS and others) can however do the multiply in a single instruction. Some compilers used to ignore this fact and generate code that calls a runtime library function to do the multiply. The shift by 16 is also often done by a library routine (also the x86 can do such shifts).
So we're left with one or two library calls just for a multiply. This has serious consequences. Not only is the shift slower, registers must be preserved across the function calls and it does not help inlining and code-unrolling either.
If you rewrite the same code in (inline) assembler you can gain a significant speed boost.
In addition to this: using ASM is not the best way to solve the problem. Most compilers allow you to use some assembler instructions in intrinsic form if you can't express them in C. The VS.NET2008 compiler for example exposes the 32*32=64 bit mul as __emul and the 64 bit shift as __ll_rshift.
Using intrinsics you can rewrite the function in a way that the C-compiler has a chance to understand what's going on. This allows the code to be inlined, register allocated, common subexpression elimination and constant propagation can be done as well. You'll get a huge performance improvement over the hand-written assembler code that way.
For reference: The end-result for the fixed-point mul for the VS.NET compiler is:
int inline FixedPointMul (int a, int b)
{
return (int) __ll_rshift(__emul(a,b),16);
}
The performance difference of fixed point divides is even bigger. I had improvements up to factor 10 for division heavy fixed point code by writing a couple of asm-lines.
Using Visual C++ 2013 gives the same assembly code for both ways.
gcc4.1 from 2007 also optimizes the pure C version nicely. (The Godbolt compiler explorer doesn't have any earlier versions of gcc installed, but presumably even older GCC versions could do this without intrinsics.)
See source + asm for x86 (32-bit) and ARM on the Godbolt compiler explorer. (Unfortunately it doesn't have any compilers old enough to produce bad code from the simple pure C version.)
Modern CPUs can do things C doesn't have operators for at all, like popcnt or bit-scan to find the first or last set bit. (POSIX has a ffs() function, but its semantics don't match x86 bsf / bsr. See https://en.wikipedia.org/wiki/Find_first_set).
Some compilers can sometimes recognize a loop that counts the number of set bits in an integer and compile it to a popcnt instruction (if enabled at compile time), but it's much more reliable to use __builtin_popcnt in GNU C, or on x86 if you're only targeting hardware with SSE4.2: _mm_popcnt_u32 from <immintrin.h>.
Or in C++, assign to a std::bitset<32> and use .count(). (This is a case where the language has found a way to portably expose an optimized implementation of popcount through the standard library, in a way that will always compile to something correct, and can take advantage of whatever the target supports.) See also https://en.wikipedia.org/wiki/Hamming_weight#Language_support.
Similarly, ntohl can compile to bswap (x86 32-bit byte swap for endian conversion) on some C implementations that have it.
Another major area for intrinsics or hand-written asm is manual vectorization with SIMD instructions. Compilers are not bad with simple loops like dst[i] += src[i] * 10.0;, but often do badly or don't auto-vectorize at all when things get more complicated. For example, you're unlikely to get anything like How to implement atoi using SIMD? generated automatically by the compiler from scalar code.

Many years ago I was teaching someone to program in C. The exercise was to rotate a graphic through 90 degrees. He came back with a solution that took several minutes to complete, mainly because he was using multiplies and divides etc.
I showed him how to recast the problem using bit shifts, and the time to process came down to about 30 seconds on the non-optimizing compiler he had.
I had just got an optimizing compiler and the same code rotated the graphic in < 5 seconds. I looked at the assembly code that the compiler was generating, and from what I saw decided there and then that my days of writing assembler were over.

Pretty much anytime the compiler sees floating point code, a hand written version will be quicker if you're using an old bad compiler. (2019 update: This is not true in general for modern compilers. Especially when compiling for anything other than x87; compilers have an easier time with SSE2 or AVX for scalar math, or any non-x86 with a flat FP register set, unlike x87's register stack.)
The primary reason is that the compiler can't perform any robust optimisations. See this article from MSDN for a discussion on the subject. Here's an example where the assembly version is twice the speed as the C version (compiled with VS2K5):
#include "stdafx.h"
#include <windows.h>
float KahanSum(const float *data, int n)
{
float sum = 0.0f, C = 0.0f, Y, T;
for (int i = 0 ; i < n ; ++i) {
Y = *data++ - C;
T = sum + Y;
C = T - sum - Y;
sum = T;
}
return sum;
}
float AsmSum(const float *data, int n)
{
float result = 0.0f;
_asm
{
mov esi,data
mov ecx,n
fldz
fldz
l1:
fsubr [esi]
add esi,4
fld st(0)
fadd st(0),st(2)
fld st(0)
fsub st(0),st(3)
fsub st(0),st(2)
fstp st(2)
fstp st(2)
loop l1
fstp result
fstp result
}
return result;
}
int main (int, char **)
{
int count = 1000000;
float *source = new float [count];
for (int i = 0 ; i < count ; ++i) {
source [i] = static_cast <float> (rand ()) / static_cast <float> (RAND_MAX);
}
LARGE_INTEGER start, mid, end;
float sum1 = 0.0f, sum2 = 0.0f;
QueryPerformanceCounter (&start);
sum1 = KahanSum (source, count);
QueryPerformanceCounter (&mid);
sum2 = AsmSum (source, count);
QueryPerformanceCounter (&end);
cout << " C code: " << sum1 << " in " << (mid.QuadPart - start.QuadPart) << endl;
cout << "asm code: " << sum2 << " in " << (end.QuadPart - mid.QuadPart) << endl;
return 0;
}
And some numbers from my PC running a default release build*:
C code: 500137 in 103884668
asm code: 500137 in 52129147
Out of interest, I swapped the loop with a dec/jnz and it made no difference to the timings - sometimes quicker, sometimes slower. I guess the memory limited aspect dwarfs other optimisations. (Editor's note: more likely the FP latency bottleneck is enough to hide the extra cost of loop. Doing two Kahan summations in parallel for the odd/even elements, and adding those at the end, could maybe speed this up by a factor of 2.)
Whoops, I was running a slightly different version of the code and it outputted the numbers the wrong way round (i.e. C was faster!). Fixed and updated the results.

Without giving any specific example or profiler evidence, you can write better assembler than the compiler when you know more than the compiler.
In the general case, a modern C compiler knows much more about how to optimize the code in question: it knows how the processor pipeline works, it can try to reorder instructions quicker than a human can, and so on - it's basically the same as a computer being as good as or better than the best human player for boardgames, etc. simply because it can make searches within the problem space faster than most humans. Although you theoretically can perform as well as the computer in a specific case, you certainly can't do it at the same speed, making it infeasible for more than a few cases (i.e. the compiler will most certainly outperform you if you try to write more than a few routines in assembler).
On the other hand, there are cases where the compiler does not have as much information - I'd say primarily when working with different forms of external hardware, of which the compiler has no knowledge. The primary example probably being device drivers, where assembler combined with a human's intimate knowledge of the hardware in question can yield better results than a C compiler could do.
Others have mentioned special purpose instructions, which is what I'm talking in the paragraph above - instructions of which the compiler might have limited or no knowledge at all, making it possible for a human to write faster code.

In my job, there are three reasons for me to know and use assembly. In order of importance:
Debugging - I often get library code that has bugs or incomplete documentation. I figure out what it's doing by stepping in at the assembly level. I have to do this about once a week. I also use it as a tool to debug problems in which my eyes don't spot the idiomatic error in C/C++/C#. Looking at the assembly gets past that.
Optimizing - the compiler does fairly well in optimizing, but I play in a different ballpark than most. I write image processing code that usually starts with code that looks like this:
for (int y=0; y < imageHeight; y++) {
for (int x=0; x < imageWidth; x++) {
// do something
}
}
the "do something part" typically happens on the order of several million times (ie, between 3 and 30). By scraping cycles in that "do something" phase, the performance gains are hugely magnified. I don't usually start there - I usually start by writing the code to work first, then do my best to refactor the C to be naturally better (better algorithm, less load in the loop etc). I usually need to read assembly to see what's going on and rarely need to write it. I do this maybe every two or three months.
doing something the language won't let me. These include - getting the processor architecture and specific processor features, accessing flags not in the CPU (man, I really wish C gave you access to the carry flag), etc. I do this maybe once a year or two years.

Only when using some special purpose instruction sets the compiler doesn't support.
To maximize the computing power of a modern CPU with multiple pipelines and predictive branching you need to structure the assembly program in a way that makes it a) almost impossible for a human to write b) even more impossible to maintain.
Also, better algorithms, data structures and memory management will give you at least an order of magnitude more performance than the micro-optimizations you can do in assembly.

Although C is "close" to the low-level manipulation of 8-bit, 16-bit, 32-bit, 64-bit data, there are a few mathematical operations not supported by C which can often be performed elegantly in certain assembly instruction sets:
Fixed-point multiplication: The product of two 16-bit numbers is a 32-bit number. But the rules in C says that the product of two 16-bit numbers is a 16-bit number, and the product of two 32-bit numbers is a 32-bit number -- the bottom half in both cases. If you want the top half of a 16x16 multiply or a 32x32 multiply, you have to play games with the compiler. The general method is to cast to a larger-than-necessary bit width, multiply, shift down, and cast back:
int16_t x, y;
// int16_t is a typedef for "short"
// set x and y to something
int16_t prod = (int16_t)(((int32_t)x*y)>>16);`
In this case the compiler may be smart enough to know that you're really just trying to get the top half of a 16x16 multiply and do the right thing with the machine's native 16x16multiply. Or it may be stupid and require a library call to do the 32x32 multiply that's way overkill because you only need 16 bits of the product -- but the C standard doesn't give you any way to express yourself.
Certain bitshifting operations (rotation/carries):
// 256-bit array shifted right in its entirety:
uint8_t x[32];
for (int i = 32; --i > 0; )
{
x[i] = (x[i] >> 1) | (x[i-1] << 7);
}
x[0] >>= 1;
This is not too inelegant in C, but again, unless the compiler is smart enough to realize what you are doing, it's going to do a lot of "unnecessary" work. Many assembly instruction sets allow you to rotate or shift left/right with the result in the carry register, so you could accomplish the above in 34 instructions: load a pointer to the beginning of the array, clear the carry, and perform 32 8-bit right-shifts, using auto-increment on the pointer.
For another example, there are linear feedback shift registers (LFSR) that are elegantly performed in assembly: Take a chunk of N bits (8, 16, 32, 64, 128, etc), shift the whole thing right by 1 (see above algorithm), then if the resulting carry is 1 then you XOR in a bit pattern that represents the polynomial.
Having said that, I wouldn't resort to these techniques unless I had serious performance constraints. As others have said, assembly is much harder to document/debug/test/maintain than C code: the performance gain comes with some serious costs.
edit: 3. Overflow detection is possible in assembly (can't really do it in C), this makes some algorithms much easier.

Short answer? Sometimes.
Technically every abstraction has a cost and a programming language is an abstraction for how the CPU works. C however is very close. Years ago I remember laughing out loud when I logged onto my UNIX account and got the following fortune message (when such things were popular):
The C Programming Language -- A
language which combines the
flexibility of assembly language with
the power of assembly language.
It's funny because it's true: C is like portable assembly language.
It's worth noting that assembly language just runs however you write it. There is however a compiler in between C and the assembly language it generates and that is extremely important because how fast your C code is has an awful lot to do with how good your compiler is.
When gcc came on the scene one of the things that made it so popular was that it was often so much better than the C compilers that shipped with many commercial UNIX flavours. Not only was it ANSI C (none of this K&R C rubbish), was more robust and typically produced better (faster) code. Not always but often.
I tell you all this because there is no blanket rule about the speed of C and assembler because there is no objective standard for C.
Likewise, assembler varies a lot depending on what processor you're running, your system spec, what instruction set you're using and so on. Historically there have been two CPU architecture families: CISC and RISC. The biggest player in CISC was and still is the Intel x86 architecture (and instruction set). RISC dominated the UNIX world (MIPS6000, Alpha, Sparc and so on). CISC won the battle for the hearts and minds.
Anyway, the popular wisdom when I was a younger developer was that hand-written x86 could often be much faster than C because the way the architecture worked, it had a complexity that benefitted from a human doing it. RISC on the other hand seemed designed for compilers so noone (I knew) wrote say Sparc assembler. I'm sure such people existed but no doubt they've both gone insane and been institutionalized by now.
Instruction sets are an important point even in the same family of processors. Certain Intel processors have extensions like SSE through SSE4. AMD had their own SIMD instructions. The benefit of a programming language like C was someone could write their library so it was optimized for whichever processor you were running on. That was hard work in assembler.
There are still optimizations you can make in assembler that no compiler could make and a well written assembler algoirthm will be as fast or faster than it's C equivalent. The bigger question is: is it worth it?
Ultimately though assembler was a product of its time and was more popular at a time when CPU cycles were expensive. Nowadays a CPU that costs $5-10 to manufacture (Intel Atom) can do pretty much anything anyone could want. The only real reason to write assembler these days is for low level things like some parts of an operating system (even so the vast majority of the Linux kernel is written in C), device drivers, possibly embedded devices (although C tends to dominate there too) and so on. Or just for kicks (which is somewhat masochistic).

I'm surprised no one said this. The strlen() function is much faster if written in assembly! In C, the best thing you can do is
int c;
for(c = 0; str[c] != '\0'; c++) {}
while in assembly you can speed it up considerably:
mov esi, offset string
mov edi, esi
xor ecx, ecx
lp:
mov ax, byte ptr [esi]
cmp al, cl
je end_1
cmp ah, cl
je end_2
mov bx, byte ptr [esi + 2]
cmp bl, cl
je end_3
cmp bh, cl
je end_4
add esi, 4
jmp lp
end_4:
inc esi
end_3:
inc esi
end_2:
inc esi
end_1:
inc esi
mov ecx, esi
sub ecx, edi
the length is in ecx. This compares 4 characters at time, so it's 4 times faster. And think using the high order word of eax and ebx, it will become 8 times faster that the previous C routine!

A use case which might not apply anymore but for your nerd pleasure: On the Amiga, the CPU and the graphics/audio chips would fight for accessing a certain area of RAM (the first 2MB of RAM to be specific). So when you had only 2MB RAM (or less), displaying complex graphics plus playing sound would kill the performance of the CPU.
In assembler, you could interleave your code in such a clever way that the CPU would only try to access the RAM when the graphics/audio chips were busy internally (i.e. when the bus was free). So by reordering your instructions, clever use of the CPU cache, the bus timing, you could achieve some effects which were simply not possible using any higher level language because you had to time every command, even insert NOPs here and there to keep the various chips out of each others radar.
Which is another reason why the NOP (No Operation - do nothing) instruction of the CPU can actually make your whole application run faster.
[EDIT] Of course, the technique depends on a specific hardware setup. Which was the main reason why many Amiga games couldn't cope with faster CPUs: The timing of the instructions was off.

Point one which is not the answer.
Even if you never program in it, I find it useful to know at least one assembler instruction set. This is part of the programmers never-ending quest to know more and therefore be better. Also useful when stepping into frameworks you don't have the source code to and having at least a rough idea what is going on. It also helps you to understand JavaByteCode and .Net IL as they are both similar to assembler.
To answer the question when you have a small amount of code or a large amount of time. Most useful for use in embedded chips, where low chip complexity and poor competition in compilers targeting these chips can tip the balance in favour of humans. Also for restricted devices you are often trading off code size/memory size/performance in a way that would be hard to instruct a compiler to do. e.g. I know this user action is not called often so I will have small code size and poor performance, but this other function that look similar is used every second so I will have a larger code size and faster performance. That is the sort of trade off a skilled assembly programmer can use.
I would also like to add there is a lot of middle ground where you can code in C compile and examine the Assembly produced, then either change you C code or tweak and maintain as assembly.
My friend works on micro controllers, currently chips for controlling small electric motors. He works in a combination of low level c and Assembly. He once told me of a good day at work where he reduced the main loop from 48 instructions to 43. He is also faced with choices like the code has grown to fill the 256k chip and the business is wanting a new feature, do you
Remove an existing feature
Reduce the size of some or all of the existing features maybe at the cost of performance.
Advocate moving to a larger chip with a higher cost, higher power consumption and larger form factor.
I would like to add as a commercial developer with quite a portfolio or languages, platforms, types of applications I have never once felt the need to dive into writing assembly. I have how ever always appreciated the knowledge I gained about it. And sometimes debugged into it.
I know I have far more answered the question "why should I learn assembler" but I feel it is a more important question then when is it faster.
so lets try once more
You should be thinking about assembly
working on low level operating system function
Working on a compiler.
Working on an extremely limited chip, embedded system etc
Remember to compare your assembly to compiler generated to see which is faster/smaller/better.
David.

Matrix operations using SIMD instructions is probably faster than compiler generated code.

A few examples from my experience:
Access to instructions that are not accessible from C. For instance, many architectures (like x86-64, IA-64, DEC Alpha, and 64-bit MIPS or PowerPC) support a 64 bit by 64 bit multiplication producing a 128 bit result. GCC recently added an extension providing access to such instructions, but before that assembly was required. And access to this instruction can make a huge difference on 64-bit CPUs when implementing something like RSA - sometimes as much as a factor of 4 improvement in performance.
Access to CPU-specific flags. The one that has bitten me a lot is the carry flag; when doing a multiple-precision addition, if you don't have access to the CPU carry bit one must instead compare the result to see if it overflowed, which takes 3-5 more instructions per limb; and worse, which are quite serial in terms of data accesses, which kills performance on modern superscalar processors. When processing thousands of such integers in a row, being able to use addc is a huge win (there are superscalar issues with contention on the carry bit as well, but modern CPUs deal pretty well with it).
SIMD. Even autovectorizing compilers can only do relatively simple cases, so if you want good SIMD performance it's unfortunately often necessary to write the code directly. Of course you can use intrinsics instead of assembly but once you're at the intrinsics level you're basically writing assembly anyway, just using the compiler as a register allocator and (nominally) instruction scheduler. (I tend to use intrinsics for SIMD simply because the compiler can generate the function prologues and whatnot for me so I can use the same code on Linux, OS X, and Windows without having to deal with ABI issues like function calling conventions, but other than that the SSE intrinsics really aren't very nice - the Altivec ones seem better though I don't have much experience with them). As examples of things a (current day) vectorizing compiler can't figure out, read about bitslicing AES or SIMD error correction - one could imagine a compiler that could analyze algorithms and generate such code, but it feels to me like such a smart compiler is at least 30 years away from existing (at best).
On the other hand, multicore machines and distributed systems have shifted many of the biggest performance wins in the other direction - get an extra 20% speedup writing your inner loops in assembly, or 300% by running them across multiple cores, or 10000% by running them across a cluster of machines. And of course high level optimizations (things like futures, memoization, etc) are often much easier to do in a higher level language like ML or Scala than C or asm, and often can provide a much bigger performance win. So, as always, there are tradeoffs to be made.

I can't give the specific examples because it was too many years ago, but there were plenty of cases where hand-written assembler could out-perform any compiler. Reasons why:
You could deviate from calling conventions, passing arguments in registers.
You could carefully consider how to use registers, and avoid storing variables in memory.
For things like jump tables, you could avoid having to bounds-check the index.
Basically, compilers do a pretty good job of optimizing, and that is nearly always "good enough", but in some situations (like graphics rendering) where you're paying dearly for every single cycle, you can take shortcuts because you know the code, where a compiler could not because it has to be on the safe side.
In fact, I have heard of some graphics rendering code where a routine, like a line-draw or polygon-fill routine, actually generated a small block of machine code on the stack and executed it there, so as to avoid continual decision-making about line style, width, pattern, etc.
That said, what I want a compiler to do is generate good assembly code for me but not be too clever, and they mostly do that. In fact, one of the things I hate about Fortran is its scrambling the code in an attempt to "optimize" it, usually to no significant purpose.
Usually, when apps have performance problems, it is due to wasteful design. These days, I would never recommend assembler for performance unless the overall app had already been tuned within an inch of its life, still was not fast enough, and was spending all its time in tight inner loops.
Added: I've seen plenty of apps written in assembly language, and the main speed advantage over a language like C, Pascal, Fortran, etc. was because the programmer was far more careful when coding in assembler. He or she is going to write roughly 100 lines of code a day, regardless of language, and in a compiler language that's going to equal 3 or 400 instructions.

More often than you think, C needs to do things that seem to be unneccessary from an Assembly coder's point of view just because the C standards say so.
Integer promotion, for example. If you want to shift a char variable in C, one would usually expect that the code would do in fact just that, a single bit shift.
The standards, however, enforce the compiler to do a sign extend to int before the shift and truncate the result to char afterwards which might complicate code depending on the target processor's architecture.

You don't actually know whether your well-written C code is really fast if you haven't looked at the disassembly of what compiler produces. Many times you look at it and see that "well-written" was subjective.
So it's not necessary to write in assembler to get fastest code ever, but it's certainly worth to know assembler for the very same reason.

Tight loops, like when playing with images, since an image may cosist of millions of pixels. Sitting down and figuring out how to make best use of the limited number of processor registers can make a difference. Here's a real life sample:
http://danbystrom.se/2008/12/22/optimizing-away-ii/
Then often processors have some esoteric instructions which are too specialized for a compiler to bother with, but on occasion an assembler programmer can make good use of them. Take the XLAT instruction for example. Really great if you need to do table look-ups in a loop and the table is limited to 256 bytes!
Updated: Oh, just come to think of what's most crucial when we speak of loops in general: the compiler has often no clue on how many iterations that will be the common case! Only the programmer know that a loop will be iterated MANY times and that it therefore will be beneficial to prepare for the loop with some extra work, or if it will be iterated so few times that the set-up actually will take longer than the iterations expected.

I have read all the answers (more than 30) and didn't find a simple reason: assembler is faster than C if you have read and practiced the Intel® 64 and IA-32 Architectures Optimization Reference Manual, so the reason why assembly may be slower is that people who write such slower assembly didn't read the Optimization Manual.
In the good old days of Intel 80286, each instruction was executed at a fixed count of CPU cycles. Still, since Pentium Pro, released in 1995, Intel processors became superscalar, utilizing Complex Pipelining: Out-of-Order Execution & Register Renaming. Before that, on Pentium, produced in 1993, there were U and V pipelines. Therefore, Pentium introduced dual pipelines that could execute two simple instructions at one clock cycle if they didn't depend on one another. However, this was nothing compared with the Out-of-Order Execution & Register Renaming that appeared in Pentium Pro. This approach introduced in Pentium Pro is practically the same nowadays on most recent Intel processors.
Let me explain the Out-of-Order Execution in a few words. The fastest code is where instructions do not depend on previous results, e.g., you should always clear whole registers (by movzx) to remove dependency from previous values of the registers you are working with, so they may be renamed internally by the CPU to allow instruction execute in parallel or in a different order. Or, on some processors, false dependency may exist that may also slow things down, like false dependency on Pentium 4 for inc/dec, so you may wish to use add eax, 1 instead or inc eax to remove dependency on the previous state of the flags.
You can read more on Out-of-Order Execution & Register Renaming if time permits. There is plenty of information available on the Internet.
There are also many other essential issues like branch prediction, number of load and store units, number of gates that execute micro-ops, memory cache coherence protocols, etc., but the crucial thing to consider is the Out-of-Order Execution.
Most people are simply not aware of the Out-of-Order Execution. Therefore, they write their assembly programs like for 80286, expecting their instructions will take a fixed time to execute regardless of the context. At the same time, C compilers are aware of the Out-of-Order Execution and generate the code correctly. That's why the code of such uninformed people is slower, but if you become knowledgeable, your code will be faster.
There are also lots of optimization tips and tricks besides the Out-of-Order Execution. Just read the Optimization Manual above mentioned :-)
However, assembly language has its own drawbacks when it comes to optimization. According to Peter Cordes (see the comment below), some of the optimizations compilers do would be unmaintainable for large code-bases in hand-written assembly. For example, suppose you write in assembly. In that case, you need to completely change an inline function (an assembly macro) when it inlines into a function that calls it with some arguments being constants. At the same time, a C compiler makes its job a lot simpler—and inlining the same code in different ways into different call sites. There is a limit to what you can do with assembly macros. So to get the same benefit, you'd have to manually optimize the same logic in each place to match the constants and available registers you have.

I think the general case when assembler is faster is when a smart assembly programmer looks at the compiler's output and says "this is a critical path for performance and I can write this to be more efficient" and then that person tweaks that assembler or rewrites it from scratch.

It all depends on your workload.
For day-to-day operations, C and C++ are just fine, but there are certain workloads (any transforms involving video (compression, decompression, image effects, etc)) that pretty much require assembly to be performant.
They also usually involve using CPU specific chipset extensions (MME/MMX/SSE/whatever) that are tuned for those kinds of operation.

It might be worth looking at Optimizing Immutable and Purity by Walter Bright it's not a profiled test but shows you one good example of a difference between handwritten and compiler generated ASM. Walter Bright writes optimising compilers so it might be worth looking at his other blog posts.

LInux assembly howto, asks this question and gives the pros and cons of using assembly.

I have an operation of transposition of bits that needs to be done, on 192 or 256 bits every interrupt, that happens every 50 microseconds.
It happens by a fixed map(hardware constraints). Using C, it took around 10 microseconds to make. When I translated this to Assembler, taking into account the specific features of this map, specific register caching, and using bit oriented operations; it took less than 3.5 microsecond to perform.

The simple answer... One who knows assembly well (aka has the reference beside him, and is taking advantage of every little processor cache and pipeline feature etc) is guaranteed to be capable of producing much faster code than any compiler.
However the difference these days just doesn't matter in the typical application.

http://cr.yp.to/qhasm.html has many examples.

One of the posibilities to the CP/M-86 version of PolyPascal (sibling to Turbo Pascal) was to replace the "use-bios-to-output-characters-to-the-screen" facility with a machine language routine which in essense was given the x, and y, and the string to put there.
This allowed to update the screen much, much faster than before!
There was room in the binary to embed machine code (a few hundred bytes) and there was other stuff there too, so it was essential to squeeze as much as possible.
It turnes out that since the screen was 80x25 both coordinates could fit in a byte each, so both could fit in a two-byte word. This allowed to do the calculations needed in fewer bytes since a single add could manipulate both values simultaneously.
To my knowledge there is no C compilers which can merge multiple values in a register, do SIMD instructions on them and split them out again later (and I don't think the machine instructions will be shorter anyway).

One of the more famous snippets of assembly is from Michael Abrash's texture mapping loop (expained in detail here):
add edx,[DeltaVFrac] ; add in dVFrac
sbb ebp,ebp ; store carry
mov [edi],al ; write pixel n
mov al,[esi] ; fetch pixel n+1
add ecx,ebx ; add in dUFrac
adc esi,[4*ebp + UVStepVCarry]; add in steps
Nowadays most compilers express advanced CPU specific instructions as intrinsics, i.e., functions that get compiled down to the actual instruction. MS Visual C++ supports intrinsics for MMX, SSE, SSE2, SSE3, and SSE4, so you have to worry less about dropping down to assembly to take advantage of platform specific instructions. Visual C++ can also take advantage of the actual architecture you are targetting with the appropriate /ARCH setting.

Given the right programmer, Assembler programs can always be made faster than their C counterparts (at least marginally). It would be difficult to create a C program where you couldn't take out at least one instruction of the Assembler.

gcc has become a widely used compiler. Its optimizations in general are not that good. Far better than the average programmer writing assembler, but for real performance, not that good. There are compilers that are simply incredible in the code they produce. So as a general answer there are going to be many places where you can go into the output of the compiler and tweak the assembler for performance, and/or simply re-write the routine from scratch.

Longpoke, there is just one limitation: time. When you don't have the resources to optimize every single change to code and spend your time allocating registers, optimize few spills away and what not, the compiler will win every single time. You do your modification to the code, recompile and measure. Repeat if necessary.
Also, you can do a lot in the high-level side. Also, inspecting the resulting assembly may give the IMPRESSION that the code is crap, but in practice it will run faster than what you think would be quicker. Example:
int y = data[i];
// do some stuff here..
call_function(y, ...);
The compiler will read the data, push it to stack (spill) and later read from stack and pass as argument. Sounds shite? It might actually be very effective latency compensation and result in faster runtime.
// optimized version
call_function(data[i], ...); // not so optimized after all..
The idea with the optimized version was, that we have reduced register pressure and avoid spilling. But in truth, the "shitty" version was faster!
Looking at the assembly code, just looking at the instructions and concluding: more instructions, slower, would be a misjudgment.
The thing here to pay attention is: many assembly experts think they know a lot, but know very little. The rules change from architecture to next, too. There is no silver-bullet x86 code, for example, which is always the fastest. These days is better to go by rules-of-thumb:
memory is slow
cache is fast
try to use cached better
how often you going to miss? do you have latency compensation strategy?
you can execute 10-100 ALU/FPU/SSE instructions for one single cache miss
application architecture is important..
.. but it does't help when the problem isn't in the architecture
Also, trusting too much into compiler magically transforming poorly-thought-out C/C++ code into "theoretically optimum" code is wishful thinking. You have to know the compiler and tool chain you use if you care about "performance" at this low-level.
Compilers in C/C++ are generally not very good at re-ordering sub-expressions because the functions have side effects, for starters. Functional languages don't suffer from this caveat but don't fit the current ecosystem that well. There are compiler options to allow relaxed precision rules which allow order of operations to be changed by the compiler/linker/code generator.
This topic is a bit of a dead-end; for most it's not relevant, and the rest, they know what they are doing already anyway.
It all boils down to this: "to understand what you are doing", it's a bit different from knowing what you are doing.

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