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From: "vincent-gcc at vinc17 dot net" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/110011] New: -mfull-toc yields incorrect _Float128
 constants on power9
Date: Sun, 28 May 2023 00:32:35 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110011

            Bug ID: 110011
           Summary: -mfull-toc yields incorrect _Float128 constants on
                    power9
           Product: gcc
           Version: 8.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincent-gcc at vinc17 dot net
  Target Milestone: ---

Note: I selected version 8.3.1, because this is what I had for my tests, bu=
t at
least 13.1.0 is still affected (see below).

We got a report of a GNU MPFR failure on an IBM POWER9 machine:
https://sympa.inria.fr/sympa/arc/mpfr/2023-05/msg00000.html

With additional information given in a private discussion, Matthew R. Wilson
found that the failure came from the use of -mfull-toc in CFLAGS (this is n=
ot
visible in the report mentioned above), and he could reproduce the behavior
with GCC 13.1.0, 12.2.0, as well as the Debian-provided GCC 10.2.1. As he
noticed, -mfull-toc is documented to be the default, so that this shouldn't
have any effect.

I could reproduce the failure on gcc135 at the Compile Farm with
/opt/at12.0/bin/gcc
(gcc (GCC) 8.3.1 20190304 (Advance-Toolchain-at12.0) [revision 269374]), an=
d I
did some tests with it.

It appears that when -mfull-toc is provided, the cause is incorrect _Float1=
28
constants MPFR_FLOAT128_MAX and -MPFR_FLOAT128_MAX, where one has

#define MPFR_FLOAT128_MAX 0x1.ffffffffffffffffffffffffffffp+16383f128

Indeed, I added

  _Float128 m =3D MPFR_FLOAT128_MAX, n =3D -MPFR_FLOAT128_MAX;

at the beginning of the mpfr_set_float128 function in src/set_float128.c, a=
nd
did the following:

1. Compile MPFR (and the testsuite) using
   CFLAGS=3D"-mcpu=3Dpower9 -O0 -g -mfull-toc"
2. In the "tests" directory, gdb ./tset_float128
3. Add a breakpoint on mpfr_set_float128, then run, next, and print values:

(gdb) print m
$1 =3D 5.96937875341074040910051755689516189e-4947
(gdb) print n
$2 =3D 1.19416736664469867978830578385372193e-4946

Both are wrong.

If I do the same test with CFLAGS=3D"-mcpu=3Dpower9 -O0 -g" (i.e. without
-mfull-toc), then I get

(gdb) print m
$1 =3D 1.18973149535723176508575932662800702e+4932
(gdb) print n
$2 =3D -1.18973149535723176508575932662800702e+4932

These are the expected values.

Unfortunately, I couldn't reproduce the issue with a simple C program.=