restrict

This article is about the C programming language keyword. For other uses, see Restriction (disambiguation).

In the C programming language, as of the C99 standard, restrict is a keyword that can be used in pointer declarations. The restrict keyword is a declaration of intent given by the programmer to the compiler.
It says that for the lifetime of the pointer, only it or a value directly derived from it (such as ​pointer + 1​) will be used to access the object to which it points. This limits the effects of pointer aliasing, aiding caching optimizations. If the declaration
of intent is not followed and the object is accessed by an independent pointer, this will result in undefined behavior.

Optimization

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

void updatePtrs(size_t *ptrA, size_t *ptrB, size_t *val)
{
    *ptrA += *val;
    *ptrB += *val;
}

In the above code, the pointers ptrA, ptrB, and 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 pointer
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 be equal 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, and 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 identical locations.
Now the compiler can generate better code as follows:

load R1 ← *val
load R2 ← *ptrA
add  R2 += R1
set  R2 → *ptrA
; Note that val is not reloaded,
; because the compiler knows it is unchanged
load R2 ← *ptrB
add  R2 += R1
set  R2 → *ptrB

Note that the above assembly code is shorter because val is loaded once.