Restrict

In the C programming language, restrict is a keyword, introduced by the C99 standard, that can be used in pointer declarations. By adding this type qualifier, a programmer hints to the compiler that for the lifetime of the pointer, no other pointer will be used to access the object to which it points. This allows the compiler to make optimizations (for example, vectorization) that would not otherwise have been possible. restrict limits the effects of pointer aliasing, aiding optimizations. If the declaration of intent is not followed and the object is accessed by an independent pointer, this will result in undefined behavior. If the compiler knows that there is only one pointer to a memory block, it can produce better optimized code. For instance: 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 may generate less optimal code: Hypothetical RISC Machine. ldr r12, [val] ; Load memory at val to r12. ldr r3, [ptrA] ; Load memory at ptrA to r3. add r3, r3, r12 ; Perform addition: r3 = r3 + r12. str r3, [ptrA] ; Store r3 to memory location ptrA, updating the value. ldr r3, [ptrB] ; 'load' may have to wait until preceding 'store' completes. ldr r12, [val] ; Have to load a second time to ensure consistency. add r3, r3, r12 str r3, [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 the memory location referenced by one pointer will not affect the memory locations referenced by the other pointers. The programmer, not the compiler, is responsible for ensuring that the pointers do not point to identical locations. The compiler can e.g. rearrange the code, first loading all memory locations, then performing the operations before committing the results back to memory.