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Restrict

[ri-strikt]

In the C programming language, as of the C99 standard, restrict is a keyword which may be used when declaring pointers. The restrict keyword is a message from the programmer to the compiler saying 'this is the only pointer to that memory block'. The data pointed to by a pointer declared with the restrict qualifier may not be pointed to by any other pointer, or strange runtime behavior may result.

Optimization

If the compiler knows that there is only one pointer to a memory block, it can produce better code. The following example makes it clear:

void UpdatePtrs(size_t* ptrA, size_t* ptrB, size_t* val) {

   *ptrA += *val;

   *ptrB += *val;

}

In the above code, the pointers ptrA , ptrB, val might refer to the same memory location, so the compiler will generate a less optimal code : load R1 ← *val ; Load the value of val pointer load R2 ← *ptrA ; Load the value of ptrA ponter add R2 += R1 ; Perform Addition set R2 → *ptrA ; Update the value of ptrA pointer Similarly for ptrB, note that val is loaded twice, Because ptrA may point to val. load R1 ← *val load R2 ← *ptrB add R2 += R1 set R2 → *ptrB

However if the restrict keyword is used and the above function is declared as void UpdatePtrs(size_t* restrict ptrA, size_t* restrict ptrB, size_t* restrict val) then the compiler is allowed to assume that ptrA , ptrB, val point to different locations and updating one pointer will not affect the other pointers. The programmer, not the compiler, is responsible for ensuring that the pointers do not point to identic locations.

Now the compiler can generate better code as follows: load R1 ← *val load R2 ← *ptrA load R3 ← *ptrB add R2 += R1 add R3 += R1 set R2 → *ptrA set R3 → *ptrB Note that the above assembly code is better and the val is loaded once.

Another example is of memcpy. The two pointers used as arguments to memcpy(void*, void*, nbytes) are declared with restrict, which tells the compiler of the memcpy function that the two data areas do not overlap. This allows the compiler to produce a faster memcpy function. A programmer might send the same pointer as both arguments, in which case behavior of the memcpy function is undefined.