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From: Yangfl <mmyangfl@gmail.com>
Date: Fri, 11 Mar 2022 15:03:47 +0800
Message-ID: <CAAXyoMPRqbJMo9mV9japafA9HLE68Ze3-KUfex7OMqJKQ9q_tw@mail.gmail.com>
Subject: Tell GCC functions do not modify struct
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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.

Thanks