The restrict type qualifier

A pointer is the address of a location in memory. More than one pointer can access the same chunk of memory and modify it during the course of a program. The restrict (or __restrict or __restrict__)¹ type qualifier is an indication to the compiler that, if the memory addressed by the restrict -qualified pointer is modified, no other pointer will access that same memory. The compiler may choose to optimize code involving restrict -qualified pointers in a way that might otherwise result in incorrect behavior. It is the responsibility of the programmer to ensure that restrict -qualified pointers are used as they were intended to be used. Otherwise, undefined behavior may result.

If a particular chunk of memory is not modified, it can be aliased through more than one restricted pointer. The following example shows restricted pointers as parameters of foo(), and how an unmodified object can be aliased through two restricted pointers.

void foo(int n, int * restrict  a, int * restrict b, int * restrict  c)
{
    int i;
    for (i = 0; i < n; i++)
        a[i] = b[i] + c[i];
}

Assignments between restricted pointers are limited, and no distinction is made between a function call and an equivalent nested block.

{
      int * restrict  x;
      int * restrict  y;
      x = y; // undefined
      {
         int * restrict  x1 = x; // okay
         int * restrict  y1 = y; // okay
         x = y1;  // undefined
      }
   }

In nested blocks containing restricted pointers, only assignments of restricted pointers from outer to inner blocks are allowed. The exception is when the block in which the restricted pointer is declared finishes execution. At that point in the program, the value of the restricted pointer can be carried out of the block in which it was declared.

Notes:

The restrict qualifier is represented by the following keywords (all have the same semantics):
- The restrict keyword is recognized under compilation with bgxlc or bgc99 or with the -qlanglvl=stdc99 or -qlanglvl=extc99 options or -qkeyword=restrict. The __restrict and __restrict__ keywords are recognized at all language levels.
- The restrict, __restrict and __restrict__ keywords are recognized by default.
Using the -qrestrict option is equivalent to adding the restrict keyword to the pointer parameters within the specified functions.

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