[Webkit-unassigned] [Bug 152198] New: B3->Air compare-branch fusion should fuse even if the result of the comparison is used more than once

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=152198

            Bug ID: 152198
           Summary: B3->Air compare-branch fusion should fuse even if the
                    result of the comparison is used more than once
    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

This code:

    cmp
    setXX
    movzx -> put result in %res
    test %res, %res
    jnz
    ...
    test %res, %res
    jnz

is always worse than this code:

    cmp
    jXX
    ...
    cmp
    jXX

But our instruction selector emits the worse code because it's afraid of emitting the compare instruction more than once.  It's also afraid of duplicating loads.  The solution is:

- Don't be afraid of duplicating comparisons if you're emitting a branch, since for a shared comparison result, the branch will have to do a second comparison anyway.
- When we decide to match a shared comparison we could simply disable load fusion.

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