[LLVMbugs] [Bug 9939] New: Loop strength reduction breaks this test

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=9939

           Summary: Loop strength reduction breaks this test
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Common Code Generator Code
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: rafael.espindola at gmail.com
                CC: llvmbugs at cs.uiuc.edu


Created an attachment (id=6604)
 --> (http://llvm.org/bugs/attachment.cgi?id=6604)
testcase

Loop strength reduction is breaking firefox's test_jsctypes.js.

I reduced the problem to the attached testcase, but haven't been able to find
the bug itself yet. Looks like what it gets wrong is the compensation added
when the loop condition is converted from 

        cmpq    %r9, %rdx
        jne     .LBB0_4
to

        addq    $-2, %rcx
        jne     .LBB0_4

I checked that building with " -Xclang -mllvm -Xclang -disable-lsr" added
causes the original test to pass.

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