[Webkit-unassigned] [Bug 162350] New: B3 should have a Depend operation

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=162350

            Bug ID: 162350
           Summary: B3 should have a Depend operation
    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

On ARM, there's a goofy way to force to loads to be dependent.  Say you have two loads:

a = *p
b = *q

You can make them dependent by doing:

a = *p
b = q[a ^ a]

Obviously, you can't say BitXor(a, a) since that would get strength-reduced away.  But we could introduce a Depend operation like this:

a = *p
b = *Depend(q, a)

On x86, this operation could get blown away just before lowering to Air.  On ARM, this operation would turn into the BitXor hack.

Intriguingly, this Depend operation would be useful even on x86 because it would let us express a load-load fence without pessimizing codegen too much.

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