Re: Tell GCC functions do not modify struct

Martin Sebor via Gcc-help <gcc-help@xxxxxxxxxxx> · Mon, 14 Mar 2022 15:43:57 -0600

On 3/11/22 00:03, Yangfl via Gcc-help wrote:
Hello Everyone,

I'm playing with GCC optimization and come up with a quite common scenario:

struct A { int a; };

void print_a(const struct A *);

int test() {
     const struct A a = {3};
     print_a(&a);
     return a.a == 3;
}

GCC always products a load operation of a.a, that's reasonable. But my
question is, how to tell GCC that the function is guaranteed to not
modify the struct?

I've tried `access` attribute, but that does not help. Of course I can
use __builtin_unreachable around the function call to make a promise,
but that is not a struct-independent solution.

GCC doesn't yet use the access attribute for optimization.

The solution in standard C is to declare the pointer argument restrict:

  void print_a (const struct A * restrict);

Regrettably, GCC doesn't yet take advantage of this notation for
optimization either.  A request for this enhancement is being
tracked in pr81009:
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81009

Martin

Thanks