[LLVMbugs] [Bug 8908] New: call ModRefInfo not sensitive to ordering

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=8908

           Summary: call ModRefInfo not sensitive to ordering
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: nlewycky at google.com
                CC: llvmbugs at cs.uiuc.edu


Created an attachment (id=5960)
 --> (http://llvm.org/bugs/attachment.cgi?id=5960)
.ll testcase (run opt -basicaa -gvn)

This is a breakout bug from bug 8886 comment 2. The problem is that we have in
pseudo:

  %x = alloca
  store @CONSTANT, %x
  call @external1()
  %DEAD = load %x
  call @external2(%x)

and the load isn't being eliminated by GVN. MemDep does its backwards scan but
when it asks BasicAA about the call to external1, BasicAA looks at the alloca's
address leaking into external2 and bails returning ModRef.

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