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From: "malat at debian dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/110622] New: x86: Miscompilation at O1 level (O0 is
 working)
Date: Tue, 11 Jul 2023 06:28:29 +0000
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X-Bugzilla-Version: 13.1.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110622

            Bug ID: 110622
           Summary: x86: Miscompilation at O1 level (O0 is working)
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: malat at debian dot org
  Target Milestone: ---

I can trigger an assertion in highway unit test suite on Debian/i386 when u=
sing
-O1 (does not happen at -O0).

Symptoms:

[...]
f32x2: Log1p(5.5968112995194757e-20) expected 5.5968112995194757e-20 actual=
 0
ulp 5.28754e+08 max ulp 3
[...]

For some reason the code return exactly zero (0) during a math computation =
of a
log1p function. The code runs fine at O2 on all other Debian arches (amd64,
mipsel, armel, ppc32 ...).

Since the return value is exactly 0, I suspect this is not a case of excess
precision (x87). This is not a hardware issue as I can reproduce it on mult=
iple
hosts (Debian buildds machine, and x86 porterbox).

If I extract the math logic and call it in a `main` function, then the comp=
iler
is able to optimize the code correctly (even at O2) and return appropriate
result (correct tolerance).

I did not observe any suspicious behavior under valgrind and/or -fsanitize.=