[Webkit-unassigned] [Bug 150775] New: B3::LowerToAir should do copy propagation

bugzilla-daemon at webkit.org bugzilla-daemon at webkit.org
Sun Nov 1 09:19:09 PST 2015


https://bugs.webkit.org/show_bug.cgi?id=150775

            Bug ID: 150775
           Summary: B3::LowerToAir should do copy propagation
    Classification: Unclassified
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: All
                OS: All
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: JavaScriptCore
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: fpizlo at apple.com

For example, the Trunc value in B3 is really just an identity in Air.  Currently we emit a Move.  But, it would be more efficient to copy propagate: just set the tmp of the Trunc to the tmp of the input.

We could do similar things for ZExt and SExt.  For ZExt, we could record if the tmp for the input value was produced by an instruction that already zero-extends.  For SExt, we could record if the tmp for the input value was produced by an instruction that already sign-extends.  For both of them, we could also use an analysis of whether the input is non-negative.  If so, the we could copy-propagate through ZExt and SExt provided that the source either zero- or sign-extended.  For example, we could copy-prop through a SExt if the source zero-extended and the value is proved non-negative.

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