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From: "sagebar at web dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug ipa/105682] [12/13 Regression] Both `-Wsuggest-attribute=pure`
 and `-Wsuggest-attribute=const` on same function since
 r12-5177-g494bdadf28d0fb35
Date: Mon, 05 Sep 2022 17:15:09 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105682

--- Comment #8 from sagebar at web dot de ---
(In reply to Jan Hubicka from comment #6)
> I think the conlcusion here is correct.  callee has pure attribute and th=
at
> means that it has no side effects except for reading global memory. So
> whatever volatile assembly does has to satisfy this.
>=20
> Now since assembly is not declared as reading memory, GCC concludes that =
the
> function has no side effects and does not read global memory and this can=
 be
> uprgraded to const.

This assumes that reading memory is the only thing inline assembly can do to
access (part of) the global state. But consider the following example where=
 the
ARM FPU control word is read:

```
#include <fpu_control.h> // HINT: this is arm assembly
//#define _FPU_GETCW(cw) __asm__ __volatile__("vmrs %0, fpscr" : "=3Dr" (cw=
))
//#define _FPU_SETCW(cw) __asm__ __volatile__("vmsr fpscr, %0" : : "r" (cw))

__attribute__((pure))
fpu_control_t getcw() {
        fpu_control_t result;
        _FPU_GETCW(result);
        return result;
}

int main() {
        fpu_control_t before, after;
        before =3D getcw();
        _FPU_SETCW(0x1234);
        after =3D getcw()
        printf("oldcw: %d\n", before);
        printf("newcw: %d\n", after);
}
```

If you're saying that a `__asm__ __volatile__` that doesn't access memory
should be considered as `const`, then gcc should be allowed to remove the f=
irst
`getcw()` and simply assign the same value to `before` and `after` (since a
`const` function's return value only depends on its arguments, meaning it c=
alls
can be removed and re-ordered however gcc pleases).

I think you can see how that would be a problem in the above.

However: I would understand gcc doing this if `_FPU_GETCW` was implemented
using `__asm__` instead of `__asm__ __volatile__`.

(If I'm misunderstanding how `pure` is meant to work, please correct me. Bu=
t as
far as I understand it, the above is a correct usage examle)=