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From: "arek_koz at o2 dot pl" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c/99494] New: isfinite gets optimized away with -Ofast
Date: Tue, 09 Mar 2021 19:01:53 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99494

            Bug ID: 99494
           Summary: isfinite gets optimized away with -Ofast
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: arek_koz at o2 dot pl
  Target Milestone: ---
              Host: x86_64-pc-linux-gnu
            Target: x86_64-pc-linux-gnu
             Build: x86_64-pc-linux-gnu

Created attachment 50342
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D50342&action=3Dedit
stripped _Py_HashDouble

CPython compiled with -Ofast hangs forever on python -c 'hash(float("inf"))=
'.

Attached is a program based on stripped _Py_HashDouble from CPython sources.

It should print 314159 when it reads an infinite number from the input.
It always prints 1 instead (the function gets reduced to "return 1").

When replacing -Ofast with -O3, the problem is gone. Likely to appear on ot=
her
build/host/target configurations.

Version: gcc (Gentoo 10.2.0-r5 p6) 10.2.0
Target architecture: x86_64 (generic)
GCC is compiled to execute on: x86_64 (znver2)
Invoked command: x86_64-pc-linux-gnu-gcc -Ofast -o bug bug.c

My best guess:
GCC -Ofast assumes that isfinite(v) is always true for any double v.=