[LLVMbugs] [Bug 22578] New: [Reassociate] Use localized Changed flag to avoid calls to RewriteExprTree

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Fri Feb 13 09:35:35 PST 2015


            Bug ID: 22578
           Summary: [Reassociate] Use localized Changed flag to avoid
                    calls to RewriteExprTree
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: mcrosier at codeaurora.org
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

The Reassociation pass uses a global flag to determine if something in the IR
has changed.  The flag should be maintained per expression to avoid calls to

If the expression hasn't changed in some non-trivial way, then the rewriting is
unnecessary.  Rewriting an unchanged expressions increases compile-time, drops
unsigned/signed wrap flags and generally results in a completely different
expression topology.  I'm seeing that the rewriting generally produces the
longest critical path, which is especially bad for FP code.

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