Bug 143532

Summary: DFG::IntegerCheckCombiningPhase's wrap-around check shouldn't trigger C++ undef behavior on wrap-around
Product: WebKit Reporter: Filip Pizlo <fpizlo>
Component: JavaScriptCoreAssignee: Filip Pizlo <fpizlo>
Status: RESOLVED FIXED    
Severity: Normal CC: barraclough, benjamin, ggaren, mark.lam, mhahnenb, mmirman, msaboff, nrotem, oliver, saam, sam
Priority: P2    
Version: 528+ (Nightly build)   
Hardware: All   
OS: All   
Attachments:
Description Flags
the patch barraclough: review+

Filip Pizlo
Reported 2015-04-08 12:48:06 PDT
Oh the irony! We were protecting an optimization that only worked if there was no wrap-around in JavaScript. But the C++ code had wrap-around, which is undef in C++. So, if the compiler was smart enough, our compiler would think that there never was wrap-around.
Attachments
the patch (2.40 KB, patch)
2015-04-08 12:50 PDT, Filip Pizlo 		barraclough: review+
DetailsFormatted DiffDiff
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Filip Pizlo
Comment 1 2015-04-08 12:50:13 PDT
Created attachment 250376 [details] the patch
Filip Pizlo
Comment 2 2015-04-08 13:24:02 PDT
Landed in http://trac.webkit.org/changeset/182562
Darin Adler
Comment 3 2015-04-10 09:36:33 PDT
Comment on attachment 250376 [details] the patch View in context: https://bugs.webkit.org/attachment.cgi?id=250376&action=review > Source/JavaScriptCore/dfg/DFGIntegerCheckCombiningPhase.cpp:367 > + uint32_t unsignedDifference = maxBound - minBound; > + return !(unsignedDifference >> 31); Could also have written: int32_t difference = maxBound - minBound; return difference >= 0;
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