Bug 83234

Summary: DFG InstanceOf should not uselessly speculate cell
Product: WebKit Reporter: Filip Pizlo <fpizlo>
Component: JavaScriptCoreAssignee: Nobody <webkit-unassigned>
Status: RESOLVED FIXED    
Severity: Normal    
Priority: P2    
Version: 528+ (Nightly build)   
Hardware: All   
OS: All   
Attachments:
Description Flags
the patch oliver: review+

Filip Pizlo
Reported 2012-04-04 16:31:50 PDT
A useless speculation is one that: 1) Is performed on a value that dies at the point of speculation. 2) Handling the case where the speculation would fail is trivial and takes fewer instructions than an OSR exit trampoline (i.e. <12 bytes on x86). InstanceOf has a SpeculateCell optimization, which is probably great most of the time, but there are times when you do something like: if (o.f instanceof p) The temporary result of o.f dies immediately. Handling the case where it's not a cell is super easy: just return false. So there's little use in speculating cell here.
Attachments
the patch (1.80 KB, patch)
2012-04-04 16:34 PDT, Filip Pizlo 		oliver: review+
DetailsFormatted DiffDiff
View All Add attachment proposed patch, testcase, etc.
Filip Pizlo
Comment 1 2012-04-04 16:34:55 PDT
Created attachment 135717 [details] the patch
Filip Pizlo
Comment 2 2012-04-04 17:59:24 PDT
Landed in http://trac.webkit.org/changeset/113269
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