3.8.2 Restricted Pointers

In order for a compiler to effectively perform parallel execution of a loop, it needs to determine if certain lvalues designate distinct regions of storage. Aliases are lvalues whose regions of storage are not distinct. Determining if two pointers to objects are aliases is a difficult and time consuming process because it could require analysis of the entire program. Consider function vsq() below:

Example 3–22 A Loop With Two Pointers

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

The compiler can parallelize the execution of the different iterations of the loops if it knows that pointers a and b access different objects. If there is an overlap in objects accessed through pointers a and b then it would be unsafe for the compiler to execute the loops in parallel. At compile time, the compiler does not know if the objects accessed by a and b overlap by simply analyzing the function vsq(); the compiler may need to analyze the whole program to get this information.

Restricted pointers are used to specify pointers which designate distinct objects so that the compiler can perform pointer alias analysis. The following is an example of function vsq() in which function parameters are declared as restricted pointers:

void vsq(int n, double * restrict a, double * restrict b)

Pointers a and b are declared as restricted pointers, so the compiler knows that a and b point to distinct regions of storage. With this alias information, the compiler is able to parallelize the loop.

The keyword restrict is a type-qualifier, like volatile, and it shall only qualify pointer types. restrict is recognized as a keyword when you use -xc99=all (except with -Xs). There are situations in which you may not want to change the source code. You can specify that pointer-valued function-parameters be treated as restricted pointers by using the following command line option:

-xrestrict=[func1,…,funcn]

If a function list is specified, then pointer parameters in the specified functions are treated as restricted; otherwise, all pointer parameters in the entire C file are treated as restricted. For example, -xrestrict=vsq, qualifies the pointers a and b given in the first example of the function vsq() with the keyword restrict.

It is critical that you use restrict correctly. If pointers qualified as restricted pointers point to objects which are not distinct, the compiler can incorrectly parallelize loops resulting in undefined behavior. For example, assume that pointers a and b of function vsq() point to objects which overlap, such that b[i] and a[i+1] are the same object. If a and b are not declared as restricted pointers the loops will be executed serially. If a and b are incorrectly qualified as restricted pointers the compiler may parallelize the execution of the loops, which is not safe, because b[i+1] should only be computed after b[i] is computed.