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* [Bug libgomp/110319] New: Performance slowdown using a pointer to perform a reduction vs. using a normal variable
@ 2023-06-20 11:55 lorien.lopez at unizar dot es
  0 siblings, 0 replies; only message in thread
From: lorien.lopez at unizar dot es @ 2023-06-20 11:55 UTC (permalink / raw)
  To: gcc-bugs

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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2023-06-20 11:55 [Bug libgomp/110319] New: Performance slowdown using a pointer to perform a reduction vs. using a normal variable lorien.lopez at unizar dot es

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