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From: "t3suzuki at ucsd dot edu" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/107660] New: Running binaries compiled with g++11 or later
 produces different results than g++ version 10 or earlier
Date: Sat, 12 Nov 2022 03:28:20 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 12.2.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107660

            Bug ID: 107660
           Summary: Running binaries compiled with g++11 or later produces
                    different results than g++ version 10 or earlier
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: t3suzuki at ucsd dot edu
  Target Milestone: ---

[gcc version]
$ ./g++ -v
Using built-in specs.
COLLECT_GCC=3D./g++
COLLECT_LTO_WRAPPER=3D/home/tomoya-s/gcc-12.2.0/install/libexec/gcc/x86_64-=
pc-linux-gnu/12.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ./configure --prefix=3D/home/tomoya-s/gcc-12.2.0/install
--disable-multilib
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.2.0 (GCC)

[the command line that triggers the bug]
% git clone https://github.com/sbeamer/gapbs.git
% cd gapbs
% make bfs
% ./bfs -g 10 -n 1

[the output by using g++-12.2.0 (or g++-11) ]
Generate Time:       0.00282
Build Time:          0.00044
Graph has 1024 nodes and 10496 undirected edges for degree: 10
Source:                  204
    i                0.00001
   td          2     0.00000
   td         59     0.00000
    e                0.00000
   bu        711     0.00003
   bu        123     0.00001
   bu          0     0.00001
    c                0.00000
Trial Time:          0.00017
Average Time:        0.00017

[the output by using g++-10 or earlier or clang++]
Generate Time:       0.00797
Build Time:          0.00102
Graph has 1024 nodes and 10496 undirected edges for degree: 10
Source:                  204
    i                0.00001
   td         15     0.00001
    e                0.00000
   bu        615     0.00004
   bu        264     0.00001
   bu          1     0.00001
    c                0.00000
   td          0     0.00000
Trial Time:          0.00017
Average Time:        0.00017

Of course, the execution time is different.

We expect the numbers 2 and 59 and 711 after td and bu to be the same.
This is because the input and the process itself are the same, and there is=
 no
randomness inside the process (random seed is fixed).

Note that this program uses OpenMP, but even if we disable it, the results =
will
be different depending on the g++ version as well.=