[Bug ada/110564] New: Incorrect results from floating point computations on x86 when optimized

[Bug ada/110564] New: Incorrect results from floating point computations on x86 when optimized

[drh at sqlite dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110564

            Bug ID: 110564
           Summary: Incorrect results from floating point computations on
                    x86 when optimized
           Product: gcc
           Version: 9.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: ada
          Assignee: unassigned at gcc dot gnu.org
          Reporter: drh at sqlite dot org
                CC: dkm at gcc dot gnu.org
  Target Milestone: ---

Created attachment 55484
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55484&action=edit
bug.c - compile and run to demonstrate the problem

Simplified test case attached.  Incorrect floating point result on Linux x86
when compiling and running as follows:

    gcc bug.c -m32 -O1 && ./a.out

The problem goes away under any one or more of the following conditions:

  *  Omit the -m32 flag (to generate x86_64 code)
  *  Omit the -O flag or use -O0
  *  Add the -ffloat-store option
  *  Uncomment both of the "printf()" statements in the test case

Please advise if I am doing something wrong (undefined or unspecified behavior)
so that I might fix it.

SQLite bug report at https://sqlite.org/forum/forumpost/ee7278611394034c.  A
work-around has been checked into SQLite at
https://sqlite.org/src/info/5d9e9364808793d6.  Please advise if there is a
better work-around.

[Bug ada/110564] Incorrect results from floating point computations on x86 when optimized

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110564

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Either use -fexcess-precision=standard or -std=c99 .

Basically this is a dup of bug 323.

*** This bug has been marked as a duplicate of bug 323 ***

[Bug target/110564] Incorrect results from floating point computations on x86 when optimized

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110564

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
>undefined or unspecified behavior

Neither. Basically GCC uses excess-precision for the fpu as supplied by x87
(80bit IEEE FPU).  Anyways read bug 323 and all of the specifications around FP
math and more.

[Bug target/110564] Incorrect results from floating point computations on x86 when optimized

[ebotcazou at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110564

Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ebotcazou at gcc dot gnu.org

--- Comment #3 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
Yes, everybody should use -mfpmath=sse these days on x86.