Is dividing the work into 5 functions as opposed to one big function more memory efficient in C since at a given time there are fewer variables in memory, as the stack-frame gets deallocated more often? Does it depend on the compiler, and optimization? if so in what compilers is it faster?
Answer given there are a lot of local variables and the stack frames comes from a centralized main and not created on the top of each other.
I know other advantages of breaking out the function into smaller functions. Please answer this question, only in respect to memory usage.
It might reduce "high water mark" of stack usage for your program, and if so that might reduce the overall memory requirement of the program.
Yes, it depends on optimization. If the optimizer inlines the function calls, you might well find that all the variables of all the functions inlined are wrapped into one big stack frame. Any compiler worth using is capable of inlining[*], so the fact that it can happen doesn't depend on compiler. Exactly when it happens, will differ.
If your local variables are small, though, then it's fairly rare for your program to use more stack than has been automatically allocated to you at startup. Unless you go past what you're given initially, how much you use makes no difference to overall memory requirements.
If you're putting great big structures on the stack (multiple kilobytes), or if you're on a machine where a kilobyte is a lot of memory, then it might make a difference to overall memory usage. So, if by "a lot of local variables" you mean few dozen ints and pointers then no, nothing you do makes any significant difference. If by "a lot of local variables" you mean a few dozen 10k buffers, or if your function recurses very deep so that you have hundreds of levels of your few dozen ints, then it's a least possible it could make a difference, depending on the OS and configuration.
The model that stack and heap grow towards each other through general RAM, and the free memory in the middle can be used equally by either one of them, is obsolete. With the exception of a very few, very restricted systems, memory models are not designed that way any more. In modern OSes, we have so-called "virtual memory", and stack space is allocated to your program one page at a time. Most of them automatically allocate more pages of stack as it is used, up to a configured limit that's usually very large. A few don't automatically extend stack (Symbian last I used it, which was some years ago, didn't, although arguably Symbian is not a "modern" OS). If you're using an embedded OS, check what the manual says about stack.
Either way, the only thing that affects total memory use is how many pages of stack you need at any one time. If your system automatically extends stack, you won't even notice how much you're using. If it doesn't, you'll need to ensure that the program is given sufficient stack for its high-water mark, and that's when you might notice excessive stack use.
In short, this is one of those things that in theory makes a difference, but in practice that difference is almost always insignificant. It only matters if your program uses massive amounts of stack relative to the resources of the environment it runs in.
[*] People programming in C for PICs or something, using a C compiler that is basically a non-optimizing assembler, are allowed to be offended that I've called their compiler "not worth using". The stack on such devices is so different from "typical" systems that the answer is different anyway.
I think in most cases the area of memory allocated for the stack (for the entire program) remains constant. The amount in use will change based on the depth of call stack and that amount would be less when fewer variables are used (but note that function calls push the return address and stack pointer also).
Also it depends on how the functions are called. If two functions are called in series, for example, and the stack of the first is popped before the call to the second, then you'll be using less of the stack..but if the first function calls the second then you're back to where you were with one big function (plus the function call overhead).
There's no memory allocation on stack - just moving the stack pointer towards next value. While stack size itself is predefined. So there's no difference in memory usage (apart of situations when you get stack overflow).
Yes, in the same vein that using a finer coat of paint on a jet plane increases its aerodynamic properties. Ok, that's a bad analogy, but the point is that if there is ever a question of making things clear and telegraphic or trying to use more functions, go with telegraphic. In most cases these are not mutually exclusive anyway as the beginners tend to give subroutines or functions too much to do.
In terms of memory I think that if you are truly splitting up up work (f, then g, then h) then you will see some minute available memory increases but if these are interdependent then you will not.
As #Joel Burget says, memory management is not really a consideration in code structuring.
Just my take.
Splitting a huge function into smaller ones does have its benefits, among them is potentially more optimized memory usage.
Say, you have this function.
void huge_func(int input) {
char a[1024];
char b[1024];
// do something with input and a
// do something with input and b
}
And you split it to two.
void func_a(int input) {
char a[1024];
// do something with input and a
}
void func_b(int input) {
char b[1024];
// do something with input and b
}
Calling huge_func will take at least 2048 bytes of memory, and calling func_a then func_b achieves the same outcome with about half less memory. However, if inside func_a you call func_b, the amount of memory used is about the same as huge_func. Essentially, as what #sje397 wrote.
I might be wrong to say this but I do not think there is any compiler optimization that could help you reduce the usage of stack memory. I believe the layout of stack memory must ensure that sufficient memory is reserved for all declared variables, whether used or not.
Related
Like we do with macros:
#undef SOMEMACRO
Can we also undeclare or delete the variables in C, so that we can save a lot of memory?
I know about malloc() and free(), but I want to delete the variables completely so that if I use printf("%d", a); I should get error
test.c:4:14: error: ‘a’ undeclared (first use in this function)
No, but you can create small minimum scopes to achieve this since all scope local variables are destroyed when the scope is exit. Something like this:
void foo() {
// some codes
// ...
{ // create an extra minimum scope where a is needed
int a;
}
// a doesn't exist here
}
It's not a direct answer to the question, but it might bring some order and understanding on why this question has no proper answer and why "deleting" variables is impossible in C.
Point #1 What are variables?
Variables are a way for a programmer to assign a name to a memory space. This is important, because this means that a variable doesn't have to occupy any actual space! As long as the compiler has a way to keep track of the memory in question, a defined variable could be translated in many ways to occupy no space at all.
Consider: const int i = 10; A compiler could easily choose to substitute all instances of i into an immediate value. i would occupy 0 data memory in this case (depending on architecture it could increase code size). Alternatively, the compiler could store the value in a register and again, no stack nor heap space will be used. There's no point in "undefining" a label that exists mostly in the code and not necessarily in runtime.
Point #2 Where are variables stored?
After point #1 you already understand that this is not an easy question to answer as the compiler could do anything it wants without breaking your logic, but generally speaking, variables are stored on the stack. How the stack works is quite important for your question.
When a function is being called the machine takes the current location of the CPU's instruction pointer and the current stack pointer and pushes them into the stack, replacing the stack pointer to the next location on stack. It then jumps into the code of the function being called.
That function knows how many variables it has and how much space they need, so it moves the frame pointer to capture a frame that could occupy all the function's variables and then just uses stack. To simplify things, the function captures enough space for all it's variables right from the start and each variable has a well defined offset from the beginning of the function's stack frame*. The variables are also stored one after the other.
While you could manipulate the frame pointer after this action, it'll be too costly and mostly pointless - The running code only uses the last stack frame and could occupy all remaining stack if needed (stack is allocated at thread start) so "releasing" variables gives little benefit. Releasing a variable from the middle of the stack frame would require a defrag operation which would be very CPU costly and pointless to recover few bytes of memory.
Point #3: Let the compiler do its job
The last issue here is the simple fact that a compiler could do a much better job at optimizing your program than you probably could. Given the need, the compiler could detect variable scopes and overlap memory which can't be accessed simultaneously to reduce the programs memory consumption (-O3 compile flag).
There's no need for you to "release" variables since the compiler could do that without your knowledge anyway.
This is to complement all said before me about the variables being too small to matter and the fact that there's no mechanism to achieve what you asked.
* Languages that support dynamic-sized arrays could alter the stack frame to allocate space for that array only after the size of the array was calculated.
There is no way to do that in C nor in the vast majority of programming languages, certainly in all programming languages that I know.
And you would not save "a lot of memory". The amount of memory you would save if you did such a thing would be minuscule. Tiny. Not worth talking about.
The mechanism that would facilitate the purging of variables in such a way would probably occupy more memory than the variables you would purge.
The invocation of the code that would reclaim the code of individual variables would also occupy more space than the variables themselves.
So if there was a magic method purge() that purges variables, not only the implementation of purge() would be larger than any amount of memory you would ever hope to reclaim by purging variables in your program, but also, in int a; purge(a); the call to purge() would occupy more space than a itself.
That's because the variables that you are talking about are very small. The printf("%d", a); example that you provided shows that you are thinking of somehow reclaiming the memory occupied by individual int variables. Even if there was a way to do that, you would be saving something of the order of 4 bytes. The total amount of memory occupied by such variables is extremely small, because it is a direct function of how many variables you, as a programmer, declare by hand-typing their declarations. It would take years of typing on a keyboard doing nothing but mindlessly declaring variables before you would declare a number of int variables occupying an amount of memory worth speaking of.
Well, you can use blocks ({ }) and defining a variable as late as possible to limit the scope where it exists.
But unless the variable's address is taken, doing so has no influence on the generated code at all, as the compiler's determination of the scope where it has to keep the variable's value is not significantly impacted.
If the variable's address is taken, failure of escape-analysis, mostly due to inlining-barriers like separate compilation or allowing semantic interpositioning, can make the compiler assume it has to keep it alive till later in the block than strictly neccessary. That's rarely significant (don't worry about a handful of ints, and most often a few lines of code longer keeping it alive are insignificant), but best to keep it in mind for the rare case where it might matter.
If you are that concerned about the tiny amount of memory that is on the stack, then you're probably going to be interested in understanding the specifics of your compiler as well. You'll need to find out what it does when it compiles. The actual shape of the stack-frame is not specified by the C language. It is left to the compiler to figure out. To take an example from the currently accepted answer:
void foo() {
// some codes
// ...
{ // create an extra minimum scope where a is needed
int a;
}
// a doesn't exist here
}
This may or may not affect the memory usage of the function. If you were to do this in a mainstream compiler like gcc or Visual Studio, you would find that they optimize for speed rather than stack size, so they pre-allocate all of the stack space they need at the start of the function. They will do analysis to figure out the minimum pre-allocation needed, using your scoping and variable-usage analysis, but those algorithms literally wont' be affected by extra scoping. They're already smarter than that.
Other compilers, especially those for embedded platforms, may allocate the stack frame differently. On these platforms, such scoping may be the trick you needed. How do you tell the difference? The only options are:
Read the documentation
Try it, and see what works
Also, make sure you understand the exact nature of your problem. I worked on a particular embedded project which eschewed the stack for everything except return values and a few ints. When I pressed the senior developers about this silliness, they explained that on this particular application, stack space was at more of a premium than space for globally allocated variables. They had a process they had to go through to prove that the system would operate as intended, and this process was much easier for them if they allocated everything up front and avoided recursion. I guarantee you would never arrive at such a convoluted solution unless you first knew the exact nature of what you were solving.
As another solution you could look at, you could always build your own stack frames. Make a union of structs, where each struct contains the variables for one stack frame. Then keep track of them yourself. You could also look at functions like alloca, which can allow for growing the stack frame during the function call, if your compiler supports it.
Would a union of structs work? Try it. The answer is compiler dependent. If all variables are stored in memory on your particular device, then this approach will likely minimize stack usage. However, it could also substantially confuse register coloring algorithms, and result in an increase in stack usage! Try and see how it goes for you!
This is my problem in essence. In the life of a function, I generate some integers, then use the array of integers in an algorithm that is also part of the same function. The array of integers will only be used within the function, so naturally it makes sense to store the array on the stack.
The problem is I don't know the size of the array until I'm finished generating all the integers.
I know how to allocate a fixed size and variable sized array on the stack. However, I do not know how to grow an array on the stack, and that seems like the best way to solve my problem. I'm fairly certain this is possible to do in assembly, you just increment stack pointer and store an int for each int generated, so the array of ints would be at the end of the stack frame. Is this possible to do in C though?
I would disagree with your assertion that "so naturally it makes sense to store the array on the stack". Stack memory is really designed for when you know the size at compile time. I would argue that dynamic memory is the way to go here
C doesn't define what the "stack" is. It only has static, automatic and dynamic allocations. Static and automatic allocations are handled by the compiler, and only dynamic allocation puts the controls in your hands. Thus, if you want to manually deallocate an object and allocate a bigger one, you must use dynamic allocation.
Don't use dynamic arrays on the stack (compare Why is the use of alloca() not considered good practice?), better allocate memory from the heap using malloc and resize it using realloc.
Never Use alloca()
IMHO this point hasn't been made well enough in the standard references.
One rule of thumb is:
If you're not prepared to statically allocate the maximum possible size as a
fixed length C array then you shouldn't do it dynamically with alloca() either.
Why? The reason you're trying to avoid malloc() is performance.
alloca() will be slower and won't work in any circumstance static allocation will fail. It's generally less likely to succeed than malloc() too.
One thing is sure. Statically allocating the maximum will outdo both malloc() and alloca().
Static allocation is typically damn near a no-op. Most systems will advance the stack pointer for the function call anyway. There's no appreciable difference for how far.
So what you're telling me is you care about performance but want to hold back on a no-op solution? Think about why you feel like that.
The overwhelming likelihood is you're concerned about the size allocated.
But as explained it's free and it gets taken back. What's the worry?
If the worry is "I don't have a maximum or don't know if it will overflow the stack" then you shouldn't be using alloca() because you don't have a maximum and know it if it will overflow the stack.
If you do have a maximum and know it isn't going to blow the stack then statically allocate the maximum and go home. It's a free lunch - remember?
That makes alloca() either wrong or sub-optimal.
Every time you use alloca() you're either wasting your time or coding in one of the difficult-to-test-for arbitrary scaling ceilings that sleep quietly until things really matter then f**k up someone's day.
Don't.
PS: If you need a big 'workspace' but the malloc()/free() overhead is a bottle-neck for example called repeatedly in a big loop, then consider allocating the workspace outside the loop and carrying it from iteration to iteration. You may need to reallocate the workspace if you find a 'big' case but it's often possible to divide the number of allocations by 100 or even 1000.
Footnote:
There must be some theoretical algorithm where a() calls b() and if a() requires a massive environment b() doesn't and vice versa.
In that event there could be some kind of freaky play-off where the stack overflow is prevented by alloca(). I have never heard of or seen such an algorithm. Plausible specimens will be gratefully received!
The innards of the C compiler requires stack sizes to be fixed or calculable at compile time. It's been a while since I used C (now a C++ convert) and I don't know exactly why this is. http://gribblelab.org/CBootcamp/7_Memory_Stack_vs_Heap.html provides a useful comparison of the pros and cons of the two approaches.
I appreciate your assembly code analogy but C is largely managed, if that makes any sense, by the Operating System, which imposes/provides the task, process and stack notations.
In order to address your issue dynamic memory allocation looks ideal.
int *a = malloc(sizeof(int));
and dereference it to store the value .
Each time a new integer needs to be added to the existing list of integers
int *temp = realloc(a,sizeof(int) * (n+1)); /* n = number of new elements */
if(temp != NULL)
a = temp;
Once done using this memory free() it.
Is there an upper limit on the size? If you can impose one, so the size is at most a few tens of KiB, then yes alloca is appropriate (especially if this is a leaf function, not one calling other functions that might also allocate non-tiny arrays this way).
Or since this is C, not C++, use a variable-length array like int foo[n];.
But always sanity-check your size, otherwise it's a stack-clash vulnerability waiting to happen. (Where a huge allocation moves the stack pointer so far that it ends up in the middle of another memory region, where other things get overwritten by local variables and return addresses.) Some distros enable hardening options that make GCC generate code to touch every page in between when moving the stack pointer by more than a page.
It's usually not worth it to check the size and use alloc for small, malloc for large, since you also need another check at the end of your function to call free if the size was large. It might give a speedup, but this makes your code more complicated and more likely to get broken during maintenance if future editors don't notice that the memory is only sometimes malloced. So only consider a dual strategy if profiling shows this is actually important, and you care about performance more than simplicity / human-readability / maintainability for this particular project.
A size check for an upper limit (else log an error and exit) is more reasonable, but then you have to choose an upper limit beyond which your program will intentionally bail out, even though there's plenty of RAM you're choosing not to use. If there is a reasonable limit where you can be pretty sure something's gone wrong, like the input being intentionally malicious from an exploit, then great, if(size>limit) error(); int arr[size];.
If neither of those conditions can be satisfied, your use case is not appropriate for C automatic storage (stack memory) because it might need to be large. Just use dynamic allocation autom don't want malloc.
Windows x86/x64 the default user-space stack size is 1MiB, I think. On x86-64 Linux it's 8MiB. (ulimit -s). Thread stacks are allocated with the same size. But remember, your function will be part of a chain of function calls (so if every function used a large fraction of the total size, you'd have a problem if they called each other). And any stack memory you dirty won't get handed back to the OS even after the function returns, unlike malloc/free where a large allocation can give back the memory instead of leaving it on the free list.
Kernel thread stack are much smaller, like 16 KiB total for x86-64 Linux, so you never want VLAs or alloca in kernel code, except maybe for a tiny max size, like up to 16 or maybe 32 bytes, not large compared to the size of a pointer that would be needed to store a kmalloc return value.
malloc is not guaranteed to return 0'ed memory. The conventional wisdom is not only that, but that the contents of the memory malloc returns are actually non-deterministic, e.g. openssl used them for extra randomness.
However, as far as I know, malloc is built on top of brk/sbrk, which do "return" 0'ed memory. I can see why the contents of what malloc returns may be non-0, e.g. from previously free'd memory, but why would they be non-deterministic in "normal" single-threaded software?
Is the conventional wisdom really true (assuming the same binary and libraries)
If so, Why?
Edit Several people answered explaining why the memory can be non-0, which I already explained in the question above. What I'm asking is why the program using the contents of what malloc returns may be non-deterministic, i.e. why it could have different behavior every time it's run (assuming the same binary and libraries). Non-deterministic behavior is not implied by non-0's. To put it differently: why it could have different contents every time the binary is run.
Malloc does not guarantee unpredictability... it just doesn't guarantee predictability.
E.g. Consider that
return 0;
Is a valid implementation of malloc.
The initial values of memory returned by malloc are unspecified, which means that the specifications of the C and C++ languages put no restrictions on what values can be handed back. This makes the language easier to implement on a variety of platforms. While it might be true that in Linux malloc is implemented with brk and sbrk and the memory should be zeroed (I'm not even sure that this is necessarily true, by the way), on other platforms, perhaps an embedded platform, there's no reason that this would have to be the case. For example, an embedded device might not want to zero the memory, since doing so costs CPU cycles and thus power and time. Also, in the interest of efficiency, for example, the memory allocator could recycle blocks that had previously been freed without zeroing them out first. This means that even if the memory from the OS is initially zeroed out, the memory from malloc needn't be.
The conventional wisdom that the values are nondeterministic is probably a good one because it forces you to realize that any memory you get back might have garbage data in it that could crash your program. That said, you should not assume that the values are truly random. You should, however, realize that the values handed back are not magically going to be what you want. You are responsible for setting them up correctly. Assuming the values are truly random is a Really Bad Idea, since there is nothing at all to suggest that they would be.
If you want memory that is guaranteed to be zeroed out, use calloc instead.
Hope this helps!
malloc is defined on many systems that can be programmed in C/C++, including many non-UNIX systems, and many systems that lack operating system altogether. Requiring malloc to zero out the memory goes against C's philosophy of saving CPU as much as possible.
The standard provides a zeroing cal calloc that can be used if you need to zero out the memory. But in cases when you are planning to initialize the memory yourself as soon as you get it, the CPU cycles spent making sure the block is zeroed out are a waste; C standard aims to avoid this waste as much as possible, often at the expense of predictability.
Memory returned by mallocis not zeroed (or rather, is not guaranteed to be zeroed) because it does not need to. There is no security risk in reusing uninitialized memory pulled from your own process' address space or page pool. You already know it's there, and you already know the contents. There is also no issue with the contents in a practical sense, because you're going to overwrite it anyway.
Incidentially, the memory returned by malloc is zeroed upon first allocation, because an operating system kernel cannot afford the risk of giving one process data that another process owned previously. Therefore, when the OS faults in a new page, it only ever provides one that has been zeroed. However, this is totally unrelated to malloc.
(Slightly off-topic: The Debian security thing you mentioned had a few more implications than using uninitialized memory for randomness. A packager who was not familiar with the inner workings of the code and did not know the precise implications patched out a couple of places that Valgrind had reported, presumably with good intent but to desastrous effect. Among these was the "random from uninitilized memory", but it was by far not the most severe one.)
I think that the assumption that it is non-deterministic is plain wrong, particularly as you ask for a non-threaded context. (In a threaded context due to scheduling alea you could have some non-determinism).
Just try it out. Create a sequential, deterministic application that
does a whole bunch of allocations
fills the memory with some pattern, eg fill it with the value of a counter
free every second of these allocations
newly allocate the same amount
run through these new allocations and register the value of the first byte in a file (as textual numbers one per line)
run this program twice and register the result in two different files. My idea is that these files will be identical.
Even in "normal" single-threaded programs, memory is freed and reallocated many times. Malloc will return to you memory that you had used before.
Even single-threaded code may do malloc then free then malloc and get back previously used, non-zero memory.
There is no guarantee that brk/sbrk return 0ed-out data; this is an implementation detail. It is generally a good idea for an OS to do that to reduce the possibility that sensitive information from one process finds its way into another process, but nothing in the specification says that it will be the case.
Also, the fact that malloc is implemented on top of brk/sbrk is also implementation-dependent, and can even vary based on the size of the allocation; for example, large allocations on Linux have traditionally used mmap on /dev/zero instead.
Basically, you can neither rely on malloc()ed regions containing garbage nor on it being all-0, and no program should assume one way or the other about it.
The simplest way I can think of putting the answer is like this:
If I am looking for wall space to paint a mural, I don't care whether it is white or covered with old graffiti, since I'm going to prime it and paint over it. I only care whether I have enough square footage to accommodate the picture, and I care that I'm not painting over an area that belongs to someone else.
That is how malloc thinks. Zeroing memory every time a process ends would be wasted computational effort. It would be like re-priming the wall every time you finish painting.
There is an whole ecosystem of programs living inside a computer memmory and you cannot control the order in which mallocs and frees are happening.
Imagine that the first time you run your application and malloc() something, it gives you an address with some garbage. Then your program shuts down, your OS marks that area as free. Another program takes it with another malloc(), writes a lot of stuff and then leaves. You run your program again, it might happen that malloc() gives you the same address, but now there's different garbage there, that the previous program might have written.
I don't actually know the implementation of malloc() in any system and I don't know if it implements any kind of security measure (like randomizing the returned address), but I don't think so.
It is very deterministic.
In a non-recursive function is it generally OK to place large data types on the stack?
One place where it is handy is in a C function that needs to clean things up.
e.g.
I have a C function that allows me to read/write arbitrary packets of data like this.
int do_something()
{
char buf[9000];
struct ot_packet_t p;
pkt_init(&p);
pkt_set_type(&p, "WDAT");
pkt_write_uint32(&p, some_var);
pkt_write_data(&p, some_data, some_len);
// other stuff...
// if need to early exit... buf & p cleaned up. An RAII approach.
send_packet(buf, pkt_get_length(&p));
}
I've tagged this question with C++ as well as it could apply to C++ too.
Although at least on C++ I would generally use auto_ptr to clean up larger objects allocated on the heap. In C I don't think this is as tidy right?
In a single-threaded application on a main-stream PC, then there isn't much need to be concerned: a few KiB here, a few Kib there; eventually, maybe, you reach a MiB or two on the stack. The stack grows automatically with minimal cost to the application.
If you're dealing with a multi-threaded application, things change. Each thread has its own stack, and (as I understand it) that stack size is fixed when the thread starts. If you start cluttering the thread's stack with large local variables, you may consume all the stack and run out of space. Therefore, it is better to use dynamic memory allocation for the larger objects to avoid running into such problems. Since you can can also do dynamic allocation for the single-threaded application, if there's a reasonable chance the program will evolve into a multi-threaded program, then maybe you should use dynamic allocation from the start. Note that dynamic allocation is slower than automatic allocation on the stack.
If you're dealing with a system that has limited memory, then you want to avoid creating large objects, period. You have to worry, at least, about how many exist at any one time, and make sure that they are released as soon as possible.
"It depends." On the machine, the device, the size of the block, and how big the stack already is beneath you.
In your example, a ~9k block in a nonrecursive function will pretty much always be fine. (Exceptions here would be small embedded devices, or if you had lots of other functions that were doing the same thing on the same stack.)
As a purely personal rule of thumb, I generally don't use the stack for buffers of anything above a few K in normal application-level code. If you're doing packet transfer as in your example, and it was in a loop, you might see performance overhead from doing a dynamic allocation every time, so you'd either keep it on the stack, keep a static buffer somewhere, or some sort of reusable pool of buffers to use.
My function will be called thousands of times. If i want to make it faster, will changing the local function variables to static be of any use? My logic behind this is that, because static variables are persistent between function calls, they are allocated only the first time, and thus, every subsequent call will not allocate memory for them and will become faster, because the memory allocation step is not done.
Also, if the above is true, then would using global variables instead of parameters be faster to pass information to the function every time it is called? i think space for parameters is also allocated on every function call, to allow for recursion (that's why recursion uses up more memory), but since my function is not recursive, and if my reasoning is correct, then taking off parameters will in theory make it faster.
I know these things I want to do are horrible programming habits, but please, tell me if it is wise. I am going to try it anyway but please give me your opinion.
The overhead of local variables is zero. Each time you call a function, you are already setting up the stack for the parameters, return values, etc. Adding local variables means that you're adding a slightly bigger number to the stack pointer (a number which is computed at compile time).
Also, local variables are probably faster due to cache locality.
If you are only calling your function "thousands" of times (not millions or billions), then you should be looking at your algorithm for optimization opportunities after you have run a profiler.
Re: cache locality (read more here):
Frequently accessed global variables probably have temporal locality. They also may be copied to a register during function execution, but will be written back into memory (cache) after a function returns (otherwise they wouldn't be accessible to anything else; registers don't have addresses).
Local variables will generally have both temporal and spatial locality (they get that by virtue of being created on the stack). Additionally, they may be "allocated" directly to registers and never be written to memory.
The best way to find out is to actually run a profiler. This can be as simple as executing several timed tests using both methods and then averaging out the results and comparing, or you may consider a full-blown profiling tool which attaches itself to a process and graphs out memory use over time and execution speed.
Do not perform random micro code-tuning because you have a gut feeling it will be faster. Compilers all have slightly different implementations of things and what is true on one compiler on one environment may be false on another configuration.
To tackle that comment about fewer parameters: the process of "inlining" functions essentially removes the overhead related to calling a function. Chances are a small function will be automatically in-lined by the compiler, but you can suggest a function be inlined as well.
In a different language, C++, the new standard coming out supports perfect forwarding, and perfect move semantics with rvalue references which removes the need for temporaries in certain cases which can reduce the cost of calling a function.
I suspect you're prematurely optimizing, however, you should not be this concerned with performance until you've discovered your real bottlenecks.
Absolutly not! The only "performance" difference is when variables are initialised
int anint = 42;
vs
static int anint = 42;
In the first case the integer will be set to 42 every time the function is called in the second case ot will be set to 42 when the program is loaded.
However the difference is so trivial as to be barely noticable. Its a common misconception that storage has to be allocated for "automatic" variables on every call. This is not so C uses the already allocated space in the stack for these variables.
Static variables may actually slow you down as its some aggresive optimisations are not possible on static variables. Also as locals are in a contiguous area of the stack they are easier to cache efficiently.
There is no one answer to this. It will vary with the CPU, the compiler, the compiler flags, the number of local variables you have, what the CPU's been doing before you call the function, and quite possibly the phase of the moon.
Consider two extremes; if you have only one or a few local variables, it/they might easily be stored in registers rather than be allocated memory locations at all. If register "pressure" is sufficiently low that this may happen without executing any instructions at all.
At the opposite extreme there are a few machines (e.g., IBM mainframes) that don't have stacks at all. In this case, what we'd normally think of as stack frames are actually allocated as a linked list on the heap. As you'd probably guess, this can be quite slow.
When it comes to accessing the variables, the situation's somewhat similar -- access to a machine register is pretty well guaranteed to be faster than anything allocated in memory can possible hope for. OTOH, it's possible for access to variables on the stack to be pretty slow -- it normally requires something like an indexed indirect access, which (especially with older CPUs) tends to be fairly slow. OTOH, access to a global (which a static is, even though its name isn't globally visible) typically requires forming an absolute address, which some CPUs penalize to some degree as well.
Bottom line: even the advice to profile your code may be misplaced -- the difference may easily be so tiny that even a profiler won't detect it dependably, and the only way to be sure is to examine the assembly language that's produced (and spend a few years learning assembly language well enough to know say anything when you do look at it). The other side of this is that when you're dealing with a difference you can't even measure dependably, the chances that it'll have a material effect on the speed of real code is so remote that it's probably not worth the trouble.
It looks like the static vs non-static has been completely covered but on the topic of global variables. Often these will slow down a programs execution rather than speed it up.
The reason is that tightly scoped variables make it easy for the compiler to heavily optimise, if the compiler has to look all over your application for instances of where a global might be used then its optimising won't be as good.
This is compounded when you introduce pointers, say you have the following code:
int myFunction()
{
SomeStruct *A, *B;
FillOutSomeStruct(B);
memcpy(A, B, sizeof(A);
return A.result;
}
the compiler knows that the pointer A and B can never overlap and so it can optimise the copy. If A and B are global then they could possibly point to overlapping or identical memory, this means the compiler must 'play it safe' which is slower. The problem is generally called 'pointer aliasing' and can occur in lots of situations not just memory copies.
http://en.wikipedia.org/wiki/Pointer_alias
Using static variables may make a function a tiny bit faster. However, this will cause problems if you ever want to make your program multi-threaded. Since static variables are shared between function invocations, invoking the function simultaneously in different threads will result in undefined behaviour. Multi-threading is the type of thing you may want to do in the future to really speed up your code.
Most of the things you mentioned are referred to as micro-optimizations. Generally, worrying about these kind of things is a bad idea. It makes your code harder to read, and harder to maintain. It's also highly likely to introduce bugs. You'll likely get more bang for your buck doing optimizations at a higher level.
As M2tM suggests, running a profiler is also a good idea. Check out gprof for one which is quite easy to use.
You can always time your application to truly determine what is fastest. Here is what I understand: (all of this depends on the architecture of your processor, btw)
C functions create a stack frame, which is where passed parameters are put, and local variables are put, as well as the return pointer back to where the caller called the function. There is no memory management allocation here. It usually a simple pointer movement and thats it. Accessing data off the stack is also pretty quick. Penalties usually come into play when you're dealing with pointers.
As for global or static variables, they're the same...from the standpoint that they're going to be allocated in the same region of memory. Accessing these may use a different method of access than local variables, depends on the compiler.
The major difference between your scenarios is memory footprint, not so much speed.
Using static variables can actually make your code significantly slower. Static variables must exist in a 'data' region of memory. In order to use that variable, the function must execute a load instruction to read from main memory, or a store instruction to write to it. If that region is not in the cache, you lose many cycles. A local variable that lives on the stack will most surely have an address that is in the cache, and might even be in a cpu register, never appearing in memory at all.
I agree with the others comments about profiling to find out stuff like that, but generally speaking, function static variables should be slower. If you want them, what you are really after is a global. Function statics insert code/data to check if the thing has been initialized already that gets run every time your function is called.
Profiling may not see the difference, disassembling and knowing what to look for might.
I suspect you are only going to get a variation as much as a few clock cycles per loop (on average depending on the compiler, etc). Sometimes the change will be dramatic improvement or dramatically slower, and that wont necessarily be because the variables home has moved to/from the stack. Lets say you save four clock cycles per function call for 10000 calls on a 2ghz processor. Very rough calculation: 20 microseconds saved. Is 20 microseconds a lot or a little compared to your current execution time?
You will likely get more a performance improvement by making all of your char and short variables into ints, among other things. Micro-optimization is a good thing to know but takes lots of time experimenting, disassembling, timing the execution of your code, understanding that fewer instructions does not necessarily mean faster for example.
Take your specific program, disassemble both the function in question and the code that calls it. With and without the static. If you gain only one or two instructions and this is the only optimization you are going to do, it is probably not worth it. You may not be able to see the difference while profiling. Changes in where the cache lines hit could show up in profiling before changes in the code for example.