I need to implement "big" arrays (~1800 elements) of a ternary datatype as runtime-efficiently as possible in C for cryptographic research. I thought of the following:
Using an array of any-sized integers, using 2 Bits to represent one element each
So I'd have
typedef uint32_t block;
const int blocksize = sizeof(block)<<3;
block dataArray[3]; // 3*32 bit => 48 Elements
uint8_t getElementAt(block *data, int position)
{
position = position * 2;
return (data[position/blocksize] >> (position % blocksize)) & 3;
}
returning me 0..2 which i can map to my three values.
Using an array of uint8_t addressing the elementy directly.
uint8_t data[48];
Sure, that needs at least four times more RAM but addressing and setting might be more efficient - is it?
Are there any other good possibilities I'm missing or special caveats in any of the two solutions?
The answer depends on how big the arrays will get, and how you want to optimize. I sketch some scenarios:
Runtime, small arrays.
Simply use unsigned long arr[N]. Reading only on machine word boundaries is the fastest, but uses a lot of memory. When the memory usage gets too big you actually do not want to do this, because cache performance outweighs the aligned reads.
Runtime, big arrays.
Use unsigned char arr[N]. This will give you fast reads/writes at a decent speed.
Good memory usage, mediocre speed.
Use unsigned long arr[N] and store each trit in two bits, unpacking using shifts and masks.
Better memory usage, slow.
Use unsigned long arr[N], and store floor(CHAR_BIT * sizeof(long) * log(2) / log(3)) numbers, by storing digits in base-3. You can pack 20 trits in 32 bits using this method.
Best memory usage, horrendous.
Store all the numbers as digits in one base-3 number, using a bignum implementation.
I am currently thinking about converting the data storage used in my CUDA kernels from Array of Structs (AoS) to Struct of Arrays (SoA).
I have a struct Element:
struct Element
{
float3 origin;
float3 direction;
uint8_t count1;
uint8_t count2;
unsigned int index;
float distance;
uint16_t instanceId;
uint64_t hash;
};
These structs are written in kernel1 each as a whole into an array residing in global memory and then subsets of the entries are used in multiple subsequent kernels.
I could now convert this to the following structure:
struct ElementSoA
{
float3 origin[N];
float3 direction[N];
uint8_t count1[N];
uint8_t count2[N];
unsigned int index[N];
float distance[N];
uint16_t instanceId[N];
uint64_t hash[N];
};
Questions:
1) Is the write performance affected if I have 8 separate, "small" writes in kernel1 instead of 1 "big" write?
2) Would it make sense to "pack" parts of the entries within ElementSoA, e.g. combining count1 and count2 into a
struct uint8_2
{
uint8_t count1;
uint8_t count2;
};
3) If "packing" is useful, is there a way to calculate the optimal structure of ElementSoA?
Suppose I have a list of the per-kernel read accesses like this:
kernel2: origin, direction, hash
kernel3: count1, count2, distance, hash
...
The reason I ask for a calculation of the optimal solution is that I have multiple structs and they contain even more entries than Element, so there is a huge number of combinations which I need to implement and test.
Assuming that thread i accesses element i and only element i. From the context this seems to be the case.
Is the write performance affected if I have 8 separate, "small" writes
in kernel1 instead of 1 "big" write?
Yes. It should be faster. The "big" write will be split into multiple small writes by the compiler, each of which will be strided. The memory subsystem works much better when access pattern is unstrided.
It's worth pointing out here that the float3 types you're using will also work like this, and will be split into three 32-bit transactions with a stride. There's no reason you can't convert these from AoS to SoA as well though.
Would it make sense to "pack" parts of the entries within ElementSoA?
Yes. Larger aligned power-of-two types (on current hardware, up to 128 bits) allow the hardware to load and store more efficiently. The difference isn't huge, but if it's easy to do, it's often worth it.
If "packing" is useful, is there a way to calculate the optimal
structure of ElementSoA?
There is no way to calculate this. One problem is that kernels have different characteristics. It maybe be that one is strongly bandwidth bound, so using efficient loads will help. Another may be compute bound, so more efficient loads won't help much.
On the question 'why do we need to use bit-fields?', searching on Google I found that bit fields are used for flags.
Now I am curious,
Is it the only way bit-fields are used practically?
Do we need to use bit fields to save space?
A way of defining bit field from the book:
struct {
unsigned int is_keyword : 1;
unsigned int is_extern : 1;
unsigned int is_static : 1;
} flags;
Why do we use int?
How much space is occupied?
I am confused why we are using int, but not short or something smaller than an int.
As I understand only 1 bit is occupied in memory, but not the whole unsigned int value. Is it correct?
A quite good resource is Bit Fields in C.
The basic reason is to reduce the used size. For example, if you write:
struct {
unsigned int is_keyword;
unsigned int is_extern;
unsigned int is_static;
} flags;
You will use at least 3 * sizeof(unsigned int) or 12 bytes to represent three small flags, that should only need three bits.
So if you write:
struct {
unsigned int is_keyword : 1;
unsigned int is_extern : 1;
unsigned int is_static : 1;
} flags;
This uses up the same space as one unsigned int, so 4 bytes. You can throw 32 one-bit fields into the struct before it needs more space.
This is sort of equivalent to the classical home brew bit field:
#define IS_KEYWORD 0x01
#define IS_EXTERN 0x02
#define IS_STATIC 0x04
unsigned int flags;
But the bit field syntax is cleaner. Compare:
if (flags.is_keyword)
against:
if (flags & IS_KEYWORD)
And it is obviously less error-prone.
Now I am curious, [are flags] the only way bitfields are used practically?
No, flags are not the only way bitfields are used. They can also be used to store values larger than one bit, although flags are more common. For instance:
typedef enum {
NORTH = 0,
EAST = 1,
SOUTH = 2,
WEST = 3
} directionValues;
struct {
unsigned int alice_dir : 2;
unsigned int bob_dir : 2;
} directions;
Do we need to use bitfields to save space?
Bitfields do save space. They also allow an easier way to set values that aren't byte-aligned. Rather than bit-shifting and using bitwise operations, we can use the same syntax as setting fields in a struct. This improves readability. With a bitfield, you could write
directions.alice_dir = WEST;
directions.bob_dir = SOUTH;
However, to store multiple independent values in the space of one int (or other type) without bitfields, you would need to write something like:
#define ALICE_OFFSET 0
#define BOB_OFFSET 2
directions &= ~(3<<ALICE_OFFSET); // clear Alice's bits
directions |= WEST<<ALICE_OFFSET; // set Alice's bits to WEST
directions &= ~(3<<BOB_OFFSET); // clear Bob's bits
directions |= SOUTH<<BOB_OFFSET; // set Bob's bits to SOUTH
The improved readability of bitfields is arguably more important than saving a few bytes here and there.
Why do we use int? How much space is occupied?
The space of an entire int is occupied. We use int because in many cases, it doesn't really matter. If, for a single value, you use 4 bytes instead of 1 or 2, your user probably won't notice. For some platforms, size does matter more, and you can use other data types which take up less space (char, short, uint8_t, etc.).
As I understand only 1 bit is occupied in memory, but not the whole unsigned int value. Is it correct?
No, that is not correct. The entire unsigned int will exist, even if you're only using 8 of its bits.
Another place where bitfields are common are hardware registers. If you have a 32 bit register where each bit has a certain meaning, you can elegantly describe it with a bitfield.
Such a bitfield is inherently platform-specific. Portability does not matter in this case.
We use bit fields mostly (though not exclusively) for flag structures - bytes or words (or possibly larger things) in which we try to pack tiny (often 2-state) pieces of (often related) information.
In these scenarios, bit fields are used because they correctly model the problem we're solving: what we're dealing with is not really an 8-bit (or 16-bit or 24-bit or 32-bit) number, but rather a collection of 8 (or 16 or 24 or 32) related, but distinct pieces of information.
The problems we solve using bit fields are problems where "packing" the information tightly has measurable benefits and/or "unpacking" the information doesn't have a penalty. For example, if you're exposing 1 byte through 8 pins and the bits from each pin go through their own bus that's already printed on the board so that it leads exactly where it's supposed to, then a bit field is ideal. The benefit in "packing" the data is that it can be sent in one go (which is useful if the frequency of the bus is limited and our operation relies on frequency of its execution), and the penalty of "unpacking" the data is non-existent (or existent but worth it).
On the other hand, we don't use bit fields for booleans in other cases like normal program flow control, because of the way computer architectures usually work. Most common CPUs don't like fetching one bit from memory - they like to fetch bytes or integers. They also don't like to process bits - their instructions often operate on larger things like integers, words, memory addresses, etc.
So, when you try to operate on bits, it's up to you or the compiler (depending on what language you're writing in) to write out additional operations that perform bit masking and strip the structure of everything but the information you actually want to operate on. If there are no benefits in "packing" the information (and in most cases, there aren't), then using bit fields for booleans would only introduce overhead and noise in your code.
To answer the original question »When to use bit-fields in C?« … according to the book "Write Portable Code" by Brian Hook (ISBN 1-59327-056-9, I read the German edition ISBN 3-937514-19-8) and to personal experience:
Never use the bitfield idiom of the C language, but do it by yourself.
A lot of implementation details are compiler-specific, especially in combination with unions and things are not guaranteed over different compilers and different endianness. If there's only a tiny chance your code has to be portable and will be compiled for different architectures and/or with different compilers, don't use it.
We had this case when porting code from a little-endian microcontroller with some proprietary compiler to another big-endian microcontroller with GCC, and it was not fun. :-/
This is how I have used flags (host byte order ;-) ) since then:
# define SOME_FLAG (1 << 0)
# define SOME_OTHER_FLAG (1 << 1)
# define AND_ANOTHER_FLAG (1 << 2)
/* test flag */
if ( someint & SOME_FLAG ) {
/* do this */
}
/* set flag */
someint |= SOME_FLAG;
/* clear flag */
someint &= ~SOME_FLAG;
No need for a union with the int type and some bitfield struct then. If you read lots of embedded code those test, set, and clear patterns will become common, and you spot them easily in your code.
Why do we need to use bit-fields?
When you want to store some data which can be stored in less than one byte, those kind of data can be coupled in a structure using bit fields.
In the embedded word, when one 32 bit world of any register has different meaning for different word then you can also use bit fields to make them more readable.
I found that bit fields are used for flags. Now I am curious, is it the only way bit-fields are used practically?
No, this not the only way. You can use it in other ways too.
Do we need to use bit fields to save space?
Yes.
As I understand only 1 bit is occupied in memory, but not the whole unsigned int value. Is it correct?
No. Memory only can be occupied in multiple of bytes.
Bit fields can be used for saving memory space (but using bit fields for this purpose is rare). It is used where there is a memory constraint, e.g., while programming in embedded systems.
But this should be used only if extremely required because we cannot have the address of a bit field, so address operator & cannot be used with them.
A good usage would be to implement a chunk to translate to—and from—Base64 or any unaligned data structure.
struct {
unsigned int e1:6;
unsigned int e2:6;
unsigned int e3:6;
unsigned int e4:6;
} base64enc; // I don't know if declaring a 4-byte array will have the same effect.
struct {
unsigned char d1;
unsigned char d2;
unsigned char d3;
} base64dec;
union base64chunk {
struct base64enc enc;
struct base64dec dec;
};
base64chunk b64c;
// You can assign three characters to b64c.enc, and get four 0-63 codes from b64dec instantly.
This example is a bit naive, since Base64 must also consider null-termination (i.e. a string which has not a length l so that l % 3 is 0). But works as a sample of accessing unaligned data structures.
Another example: Using this feature to break a TCP packet header into its components (or other network protocol packet header you want to discuss), although it is a more advanced and less end-user example. In general: this is useful regarding PC internals, SO, drivers, an encoding systems.
Another example: analyzing a float number.
struct _FP32 {
unsigned int sign:1;
unsigned int exponent:8;
unsigned int mantissa:23;
}
union FP32_t {
_FP32 parts;
float number;
}
(Disclaimer: Don't know the file name / type name where this is applied, but in C this is declared in a header; Don't know how can this be done for 64-bit floating-point numbers since the mantissa must have 52 bits and—in a 32 bit target—ints have 32 bits).
Conclusion: As the concept and these examples show, this is a rarely used feature because it's mostly for internal purposes, and not for day-by-day software.
To answer the parts of the question no one else answered:
Ints, not Shorts
The reason to use ints rather than shorts, etc. is that in most cases no space will be saved by doing so.
Modern computers have a 32 or 64 bit architecture and that 32 or 64 bits will be needed even if you use a smaller storage type such as a short.
The smaller types are only useful for saving memory if you can pack them together (for example a short array may use less memory than an int array as the shorts can be packed together tighter in the array). For most cases, when using bitfields, this is not the case.
Other uses
Bitfields are most commonly used for flags, but there are other things they are used for. For example, one way to represent a chess board used in a lot of chess algorithms is to use a 64 bit integer to represent the board (8*8 pixels) and set flags in that integer to give the position of all the white pawns. Another integer shows all the black pawns, etc.
You can use them to expand the number of unsigned types that wrap. Ordinary you would have only powers of 8,16,32,64... , but you can have every power with bit-fields.
struct a
{
unsigned int b : 3 ;
} ;
struct a w = { 0 } ;
while( 1 )
{
printf("%u\n" , w.b++ ) ;
getchar() ;
}
To utilize the memory space, we can use bit fields.
As far as I know, in real-world programming, if we require, we can use Booleans instead of declaring it as integers and then making bit field.
If they are also values we use often, not only do we save space, we can also gain performance since we do not need to pollute the caches.
However, caching is also the danger in using bit fields since concurrent reads and writes to different bits will cause a data race and updates to completely separate bits might overwrite new values with old values...
Bitfields are much more compact and that is an advantage.
But don't forget packed structures are slower than normal structures. They are also more difficult to construct since the programmer must define the number of bits to use for each field. This is a disadvantage.
Why do we use int? How much space is occupied?
One answer to this question that I haven't seen mentioned in any of the other answers, is that the C standard guarantees support for int. Specifically:
A bit-field shall have a type that is a qualified or unqualified version of _Bool, signed int, unsigned int, or some other implementation defined type.
It is common for compilers to allow additional bit-field types, but not required. If you're really concerned about portability, int is the best choice.
Nowadays, microcontrollers (MCUs) have peripherals, such as I/O ports, ADCs, DACs, onboard the chip along with the processor.
Before MCUs became available with the needed peripherals, we would access some of our hardware by connecting to the buffered address and data buses of the microprocessor. A pointer would be set to the memory address of the device and if the device saw its address along with the R/W signal and maybe a chip select, it would be accessed.
Oftentimes we would want to access individual or small groups of bits on the device.
In our project, we used this to extract a page table entry and page directory entry from a given memory address:
union VADDRESS {
struct {
ULONG64 BlockOffset : 16;
ULONG64 PteIndex : 14;
ULONG64 PdeIndex : 14;
ULONG64 ReservedMBZ : (64 - (16 + 14 + 14));
};
ULONG64 AsULONG64;
};
Now suppose, we have an address:
union VADDRESS tempAddress;
tempAddress.AsULONG64 = 0x1234567887654321;
Now we can access PTE and PDE from this address:
cout << tempAddress.PteIndex;
what i am trying to accomplish is
user enters bit field widths, say 17 5 8 19 0 (can be more or less bit fields) 0 states end of bit field inputs
then user enters in values to be stored in a allocated array set to the bit field sizes.
say 1 2 3 4
how do i scan in several bit field values to be put in a struct like this?
struct{
unsigned int bit0:1;
unsigned int bit1:1;
unsigned int bit2:1;
unsigned int bit3:1;
}pack;
Then would this be correct when setting up the array to bit field size?
I'm using "bit field inputs" in place of however i would scan them in for now.
pack = (int *)malloc(sizeof(struct)*bit field inputs);
i believe i asked my original question wrong,
what im trying to do here is take say value 1 and put it in to a bit field width say 17
and keep repeating this for up to 5 values.
if the user inputs the bit field widths how would i take value one and store it in a field width of 17?
If you need dynamic bit field widths, you need to do your own bit-fiddling. For compatibility, you should do it anyway.
If not, you must read it into a standard type like int and then asign to the bitfield, so the compiler does your bitpacking for you. But beware, the standard gives few guarantees regarding the compilers choices in this.
Also, never cast the return value of malloc: Do I cast the result of malloc?.
It is up to the compiler to determine the order and padding for bitfields. There are no guarantees.
In general, you should only use bitfields "internally" in your program. Anytime you serialize bitfields, you may run into incompatibilities.
You would generally be better off by serializing into a structure with known size and alignment, then explicitly copying the values into your bitfield to use internally.
After re-reading your question again, I think you would be better off using bit-masking operations on contiguous bytes. This allows you to control the memory layout of your internal representation.
It sounds like you want to:
read and store the bit offsets from the input string
sum up how many bits the user wants to use from their input string
malloc some bytes of contiguous memory that is large enough to
contain the user bits
provide accessor functions to the bit position and length of data,
then use masking to set the bits.
Or if you don't care about size, just make everything an array of unsigned ints and let the compiler do the alignment for you.
I was reading this link
http://dec.bournemouth.ac.uk/staff/awatson/micro/articles/9907feat2.htm
I could not understand this following statements from the link, Please help me understand about this.
The programmer just writes some macros that shift or mask the
appropriate bits to get what is desired. However, if the data involves
longer binary encoded records, the C API runs into a problem. I have,
over the years, seen many lengthy, complex binary records described
with the short or long integer bit-field definition facilities. C
limits these bit fields to subfields of integer-defined variables,
which implies two limitations: first of all, that bit fields may be no
wider, in bits, than the underlying variable; and secondly, that no
bit field should overlap the underlying variable boundaries. Complex
records are usually composed of several contiguous long integers
populated with bit-subfield definitions.
ANSI-compliant compilers are free to impose these size and alignment
restrictions and to specify, in an implementation-dependent but
predictable way, how bit fields are packed into the underlying machine
word structure. Structure memory alignment often isn’t portable, but
bit field memory is even less so.
What i have understood from these statements is that the macros can be used to mask the bits to left or right shift. But i had this doubt in my mind why do they use macros? - I thought by defining it in macros the portability can be established irrespective of 16-bit or 32-bit OS..Is it true?I could not understand the two disadvantages mentioned in the above statement.1.bit fields may be no wider 2.no bit field should overlap the underlying variable boundaries
and the line,
Complex records are usually composed of several contiguous long integers
populated with bit-subfield definitions.
1.bit fields may be no wider
Let's say you want a bitfield that is 200 bits long.
struct my_struct {
int my_field:200; /* Illegal! No integer type has 200 bits --> compile error!
} v;
2.no bit field should overlap the underlying variable boundaries
Let's say you want two 30 bit bitfields and that the compiler uses a 32 bit integer as the underlying variable.
struct my_struct {
unsigned int my_field1:30;
unsigned int my_field2:30; /* Without padding this field will overlap a 32-bit boundary */
} v;
Ususally, the compiler will add padding automatically, generating a struct with the following layout:
struct my_struct {
unsigned int my_field1:30;
:2 /* padding added by the compiler */
unsigned int my_field2:30; /* Without padding this field will overlap a 32-bit boundary */
:2 /* padding added by the compiler */
} v;