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From: "hkzhang455 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/111672] New: Inappropriate function splitting during
 pass_split_functions
Date: Tue, 03 Oct 2023 09:41:45 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D111672

            Bug ID: 111672
           Summary: Inappropriate function splitting during
                    pass_split_functions
           Product: gcc
           Version: 12.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hkzhang455 at gmail dot com
  Target Milestone: ---

Created attachment 56034
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D56034&action=3Dedit
example C file that can trigger inappropriate function splitting

When the GCC compiler performs function splitting optimization, the shorter=
 and
closer path is split into a new function, while the remaining more complex =
and
expensive code is retained, resulting in the complexity of the original
function being increased after the split, and the split new function only
performs simple operations (such as 'printf()').

You can compile the source code file I put in the attachment with the follo=
wing
command, and look at the gimple corresponding to the generated fnsplit to f=
ind
the phenomenon I described.

  gcc test.c -O3 -flto -fdump-tree-fnsplit -Wall -Wextra

Of course, this is only sample code, so the resulting executable does not
reflect the efficiency gap due to the problem of inline. But in more complex
code, efficiency decreases.

Hardware: 12th Gen Intel(R) Core(TM) i9-12900KF
System: Ubuntu 22.04
Output of `gcc -v`:

Using built-in specs.
COLLECT_GCC=3Dxxx/install/bin/gcc
COLLECT_LTO_WRAPPER=3Dxxx/install/libexec/gcc/x86_64-pc-linux-gnu/12.3.0/lt=
o-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../configure --prefix=3Dxxx/install --enable-threads=3Dpos=
ix
--disable-checking --disable-multilib --disable-bootstrap
--enable-languages=3Dc,c++,lto
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 12.3.0 (GCC)=