[llvm-bugs] [Bug 46940] New: Wrong code generated from instcombine

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=46940

            Bug ID: 46940
           Summary: Wrong code generated from instcombine
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: kazu at google.com
                CC: llvm-bugs at lists.llvm.org

I'm seeing what appears to be miscompilation after:

https://github.com/llvm/llvm-project/commit/00472067c34ccbceb2fad4b905524f3c780bb7d5

Consider:

target triple = "x86_64-grtev4-linux-gnu"

define i1 @foo(i32 %arg) {
entry:
  %cmp1 = icmp eq i32 %arg, 37
  br i1 %cmp1, label %bb_then, label %bb_else

bb_then:
  call void @bar()
  br label %bb_exit

bb_else:
  %cmp2 = icmp slt i32 %arg, 17
  br label %bb_exit

bb_exit:
  %phi1 = phi i1 [ true, %bb_else ], [ false, %bb_then ]
  %phi2 = phi i1 [ %cmp2, %bb_else ], [ undef, %bb_then ]
  %and1 = and i1 %phi2, %phi1
  ret i1 %and1
}

declare void @bar()

Note that for every control flow, the return value is well defined.

arg == 37 -> return false
arg <  17 -> return true
otherwise -> return false

Then

$ ./build/bin/opt -instcombine -S ~/undef2.ll -o ~/undef2-opt.ll

modifies bb_exit like so:

bb_exit:                                          ; preds = %bb_else, %bb_then
  %phi2 = phi i1 [ %cmp2, %bb_else ], [ undef, %bb_then ]
  ret i1 %phi2

Notice undef in the phi node.

FWIW, immediately after SimplifyUsingControlFlow and replaceInstUsesWith
run on phi1, we have:

bb_exit:                                          ; preds = %bb_else, %bb_then
  %phi1 = phi i1 [ true, %bb_else ], [ false, %bb_then ]
  %phi2 = phi i1 [ %cmp2, %bb_else ], [ undef, %bb_then ]
  %0 = xor i1 %cmp1, true
  %and1 = and i1 %phi2, %0
  ret i1 %and1

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