Restrict - Optimization

Optimization

If the compiler knows that there is only one pointer to a memory block, it can produce better 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 will generate 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.