Question

Is there any advantage to specifying the MSVC/GCC non-standard __restrict qualifier on a function pointer parameter if it is the only pointer parameter? For example,

int longCalculation(int a, int* __restrict b)

My guess is it should allow better optimization since it implies b does not point to a, but all examples I’ve seen __restrict two pointers to indicate no aliasing between them.

Answer 1

As mentioned in the comments b can’t point to a anyways, so there is no aliasing potential there anyways. So if the function is pure in the sense that it works only on its parameters there shouldn’t be any real benefits.

However if the function uses global variables internally then __restrict might offer benefits once again, since it makes clear that b doesn’t point to any of those global variables.

An interesting case might be the situation where you allocate and deallocate memory inside the function. The compiler could theoretically be sure that b doesn’t point to that memory, however whether or not it realizes that I’m not sure and might depend how the allocation is called.

Personally however I prefer to keep __restrict out of the signature and do something like this

int longCalculation(int a, int* b){ 
   assert(...);//ensure that b doesn't point to anything used
   int* __restrict bx = b;
   ...
}

IMO this has the following advantages:

• The function signature doesn’t expose the non standard __restrict used
• The ability to ensure that the variables actually conform to __restrict using assert, since passing aliasing pointers to a function expecting them to be nonaliasing can lead to hard to track down bugs.