In the C programming language, as of the C99 standard

C99

C99 is a modern dialect of the C programming language. It extends the previous version with new linguistic and library features, and helps implementations make better use of available computer hardware and compiler technology.-History:...

, restrict is a keyword

Keyword (computer programming)

In computer programming, a keyword is a word or identifier that has a particular meaning to the programming language. The meaning of keywords — and, indeed, the meaning of the notion of keyword — differs widely from language to language....

 that can be used in pointer declarations. The restrict keyword is a declaration of intent given by the programmer to the compiler

Compiler

A compiler is a computer program that transforms source code written in a programming language into another computer language...

. It says that for the lifetime of the pointer, only it or a value directly derived from it (such as ) 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

Pointer alias

In computer programming, aliasing refers to the situation where the same memory location can be accessed using different names.For instance, if a function takes two pointers A and B which have the same value, then the name A[0] aliases the name B[0]. In this case we say the pointers A and B alias...

, 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.

External links

• Demystifying The Restrict Keyword: explanation and examples of use