[Bug target/115071] New: performance regression, x86, between gcc-14 and gcc-13 using -O3 and _Pragma("GCC unroll 4") on skylake

[colin.king at intel dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115071

            Bug ID: 115071
           Summary: performance regression, x86, between gcc-14 and gcc-13
                    using -O3 and _Pragma("GCC unroll 4") on skylake
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: colin.king at intel dot com
  Target Milestone: ---

Created attachment 58191
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=58191&action=edit
reproducer source code

I'm seeing a ~15% performance regression in gcc-14 compared to gcc-13, using
gcc on Ubuntu 24.04:

Versions:
gcc version 13.2.0 (Ubuntu 13.2.0-23ubuntu4) 
gcc version 14.0.1 20240412 (experimental) [master r14-9935-g67e1433a94f]
(Ubuntu 14-20240412-0ubuntu1) 

cking@skylake:~$ gcc-13 reproducer-bitonicsort.c -O2
cking@skylake:~$ ./a.out 
duration: 5.71 seconds, count = 1119566602

cking@skylake:~$ gcc-14 reproducer-bitonicsort.c -O2
cking@skylake:~$ ./a.out 
duration: 6.56 seconds, count = 1119566602

The original issue appeared when regression testing stress-ng bitonic sorting
stressor [1]. I've managed to extract the attached reproducer from the original
code (see attached).

Salient point to focus on:

1. The issue is also dependant on the use of _Pragma("GCC unroll 4")
2. The issue is also dependant on the use of __attribute__((optimize("-O3")))
by use of the OPTIMIZE3 macro in the example.

Attached are the reproducer C source and disassembled object code. 

References:
[1] https://github.com/ColinIanKing/stress-ng/blob/master/stress-bitonicsort.c