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From: "david at westcontrol dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/110249] __builtin_unreachable helps optimisation at -O1 but
 not at -O2
Date: Wed, 14 Jun 2023 11:38:45 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110249

--- Comment #2 from David Brown <david at westcontrol dot com> ---
My apologies - it does not optimise the code to a single aligned load at -O1
(at least, not with the combinations I have now tried).  The code was
originally C++, with a reference, which /does/ exhibit this behaviour of ha=
ving
better code at -O1 than -O2.  I had translated the reference to a pointer to
get more generic code that is valid as C and C++, but I don't see the same
effect with the pointer.

For a reference, the issue is as I first described:

#include <stdint.h>
#include <string.h>

uint64_t read64r(const uint64_t &x) {
    if ((uint64_t) &x % 8 ) {
        __builtin_unreachable();
    }
     uint64_t value;
     memcpy( &value, &x, sizeof(uint64_t) );
     return value;=20=20=20=20=20
}

I tested with 64-bit RISC-V and 32-bit ARM using trunk (gcc 13.1, I think) =
on
godbolt.org.  The only relevant flag was the optimisation level.

<https://godbolt.org/z/TaqdqeGch>=