[llvm-bugs] [Bug 24426] New: load instruction erroneously removed by GVN

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=24426

            Bug ID: 24426
           Summary: load instruction erroneously removed by GVN
           Product: tools
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: opt
          Assignee: unassignedbugs at nondot.org
          Reporter: mikael.holmen at ericsson.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

I have the following program:

declare void @check(i8)

declare void @write(i8* %res)

define i8 @TEST1() {
  %buf = alloca [10 x i8]
  %_tmp30 = bitcast [10 x i8]* %buf to i8*

  call void @write(i8* %_tmp30)

  %_tmp33 = load i8, i8* %_tmp30
  call void @check(i8 %_tmp33)

  ret i8 0
}

and I run opt on it with
build-all/bin/opt -S -memcpyopt -mldst-motion -gvn -O3 ./f2.ll

and then the load is removed by GVN and %_tmp33 is replaced with undef.

GVN iteration: 0
GVN removed:   %_tmp33 = load i8, i8* %_tmp30


I notice that if I change TEST1 slightly, and rewrite it as

define i8 @TEST2() {
  %buf = alloca i8

  call void @write(i8* %buf)

  %_tmp33 = load i8, i8* %buf
  call void @check(i8 %_tmp33)

  ret i8 0
}

then the load is left intact

GVN iteration: 0
GVN: load i8 %_tmp33 is clobbered by   call void @write(i8* %buf.17)

In this case it seems to be the code

  if (llvm::PointerMayBeCapturedBefore(Object, /* ReturnCaptures */ true,
                                       /* StoreCaptures */ true, I, DT,
                                       /* include Object */ true,
                                       /* OrderedBasicBlock */ OBB))
    return MRI_ModRef;

in AliasAnalysis::callCapturesBefore that saves the load, which it doesn't in
the first case.

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