[llvm-bugs] [Bug 40027] New: [X86] cmov conversion hurts binary search performance

via llvm-bugs llvm-bugs at lists.llvm.org
Fri Dec 14 09:45:51 PST 2018


            Bug ID: 40027
           Summary: [X86] cmov conversion hurts binary search performance
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: nikita.ppv at gmail.com
                CC: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, spatel+llvm at rotateright.com

Originally reported at: https://github.com/rust-lang/rust/issues/53823

The cmov conversion pass converts a cmov in Rust's binary search implementation
into a branch. As binary search branches are badly predicted, this
significantly hurts performance. For small data sets, which are not dominated
by cache performance, the slowdown is around 2-3x.

https://gist.github.com/nikic/13e907d7097f74a92d082fcc61bc212c has the LLVM IR
for the function, as well as generated assembly with cmov conversion enabled
(default) and disabled (-x86-cmov-converter=false).

It would be great if the cmov conversion heuristics could be adjusted in some
way to avoid converting this case.

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