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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] New: __builtin_unreachable helps optimisation at -O1
 but not at -O2
Date: Wed, 14 Jun 2023 11:01:34 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110249

            Bug ID: 110249
           Summary: __builtin_unreachable helps optimisation at -O1 but
                    not at -O2
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: david at westcontrol dot com
  Target Milestone: ---

Sometimes it can be useful to use __builtin_unreachable() to give the compi=
ler
hints that can improve optimisation.  For example, it can be used here to t=
ell
the compiler that the parameter is always 8-byte aligned:

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

uint64_t read64(const uint64_t * p) {
    if ((uint64_t) p % 8 ) {
        __builtin_unreachable();
    }
     uint64_t value;
     memcpy( &value, p, sizeof(uint64_t) );
     return value;=20=20=20=20=20
}

For some targets, such as 32-bit ARM and especially RISC-V, this can make a
difference to the generated code.  In testing, when given -O1 the compiler
takes advantage of the explicit undefined behaviour to see that the pointer=
 is
aligned, and generates a single 64-bit load.  With -O2, however, it seems t=
hat
information is lost - perhaps due to earlier optimisation passes - and now =
slow
unaligned load code is generated.

Ideally, such optimisation information from undefined behaviour (explicit v=
ia a
builtin, or implicit via other code) should be kept - -O2 should have at le=
ast
as much information as -O1.  An alternative would be the addition of a more
directed "__builtin_assume" function that could be used.

(I know that in this particular case, __builtin_assume_aligned is available=
 and
works exactly as intended at -O1 and -O2, but I think this is a more general
issue than just alignments.)=