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