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From: "lorien.lopez at unizar dot es" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug libgomp/110319] New: Performance slowdown using a pointer to perform a reduction vs. using a normal variable Date: Tue, 20 Jun 2023 11:55:59 +0000 [thread overview] Message-ID: <bug-110319-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110319 Bug ID: 110319 Summary: Performance slowdown using a pointer to perform a reduction vs. using a normal variable Product: gcc Version: 11.3.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: libgomp Assignee: unassigned at gcc dot gnu.org Reporter: lorien.lopez at unizar dot es CC: jakub at gcc dot gnu.org Target Milestone: --- When performing an OpenMP sum reduction into a regular variable, GCC uses a "lock cmpxchg" instruction. In contrast, when the reduction is performed into a pointer, it uses an OpenMP atomic region. The second version is several times slower in an Intel Skylake CPU. The original report can be found in Stack Overflow: https://stackoverflow.com/questions/76480632/performance-slowdown-using-a-pointer-to-perform-a-reduction-vs-using-a-normal-v
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