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

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=22578

            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
RewriteExprTree.  

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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