[llvm-bugs] [Bug 38366] New: GlobalISel: Lightweight CSE

via llvm-bugs llvm-bugs at lists.llvm.org
Mon Jul 30 06:26:34 PDT 2018


            Bug ID: 38366
           Summary: GlobalISel: Lightweight CSE
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: GlobalISel
          Assignee: unassignedbugs at nondot.org
          Reporter: aemerson at apple.com
                CC: llvm-bugs at lists.llvm.org

>From a theoretical perspective, having a self contained CSE pass that operates
as a single phase in the pipeline is attractive for the simplicity and
elegance. However, we know empirically that this is expensive in compile time.
Not only does the CSE pass itself take a non-negligible time to run, but having
it as a late pass can result in the non-CSE’d code from the IRTranslator
onwards surviving for a long time, taking up time in analysis at each stage of
compilation. We believe running a light weight CSE early is a win. SelectionDAG
currently does CSE by default when building the DAG, and this is something we
could explore as part of a custom IRBuilder.

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