[llvm-bugs] [Bug 35226] New: Significant performance regression with r291800 ("Tune bypassing of slow division for Intel CPUs")

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https://bugs.llvm.org/show_bug.cgi?id=35226

            Bug ID: 35226
           Summary: Significant performance regression with r291800 ("Tune
                    bypassing of slow division for Intel CPUs")
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: atdt at google.com
                CC: llvm-bugs at lists.llvm.org

rL291800 (https://reviews.llvm.org/rL291800) took an optimization for lowering
64-bit division to 32-bit and enabled it on all Intel big cores, starting with
Sandy Bridge. This change is associated with a significant regression in an
internal, search-related benchmark, when compiled for -march=haswell.

In the differential revision (https://reviews.llvm.org/D28196), the reviewer
pointed out that the fact that the latency/throughput of 64-bit division falls
along a range suggests that this optimization may already be done in hardware.
Do we know whether this is true or this is true? Additionally, is it possible
that improvements in the latency and throughput of division and remainder
operations on recent big core Intel CPUs render this optimization obsolete?

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