https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110011
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, but 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 not 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]), and I did some tests with it. It appears that when -mfull-toc is provided, the cause is incorrect _Float128 constants MPFR_FLOAT128_MAX and -MPFR_FLOAT128_MAX, where one has #define MPFR_FLOAT128_MAX 0x1.ffffffffffffffffffffffffffffp+16383f128 Indeed, I added _Float128 m = MPFR_FLOAT128_MAX, n = -MPFR_FLOAT128_MAX; at the beginning of the mpfr_set_float128 function in src/set_float128.c, and did the following: 1. Compile MPFR (and the testsuite) using CFLAGS="-mcpu=power9 -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 = 5.96937875341074040910051755689516189e-4947 (gdb) print n $2 = 1.19416736664469867978830578385372193e-4946 Both are wrong. If I do the same test with CFLAGS="-mcpu=power9 -O0 -g" (i.e. without -mfull-toc), then I get (gdb) print m $1 = 1.18973149535723176508575932662800702e+4932 (gdb) print n $2 = -1.18973149535723176508575932662800702e+4932 These are the expected values. Unfortunately, I couldn't reproduce the issue with a simple C program.