[llvm-bugs] [Bug 43879] New: InstCombine incorrectly optimizes `select`s with conditionally undef values

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43879

            Bug ID: 43879
           Summary: InstCombine incorrectly optimizes `select`s with
                    conditionally undef values
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: george.burgess.iv at gmail.com
                CC: llvm-bugs at lists.llvm.org

Found while trying to bootstrap clang-tidy.

The intent of the repro is to call @bar() once and return false if `%B` is
true. Otherwise, it should loop calling @bar() forever. (The non-reduced code
isn't an infloop, but the infloop doesn't appear to make a difference here)

[ub] ~/l/t/gbiv_repro [ 20ms ] ((6082a062…)) ~> git rev-parse HEAD
6082a062a76d0bc9c9c641f5600569bafdf4c338
[ub] ~/l/t/gbiv_repro [ 5ms ] ((6082a062…)) ~> cat repro.ll
declare void @bar()

define dso_local i1 @_Z3foob(i1 zeroext %B) local_unnamed_addr #0 {
entry:
  br label %loop

loop:
  %x = phi i1 [ undef, %entry ], [ %foo, %loop.end ]
  call void @bar()
  br label %loop.end

loop.end:
  %foo = select i1 %B, i1 false, i1 %x
  br i1 %B, label %return, label %loop

return:
  ret i1 %foo
}
[ub] ~/l/t/gbiv_repro [ 2ms ] ((6082a062…)) ~> ../build/release/bin/opt
-instcombine < repro.ll -S
; ModuleID = '<stdin>'
source_filename = "<stdin>"

declare void @bar()

define dso_local i1 @_Z3foob(i1 zeroext %B) local_unnamed_addr {
entry:
  br label %loop

loop:                                             ; preds = %loop.end, %entry
  call void @bar()
  br label %loop.end

loop.end:                                         ; preds = %loop
  br i1 %B, label %return, label %loop

return:                                           ; preds = %loop.end
  ret i1 undef
}


AIUI, select should block undef from entering the program if %B is true, so the
`ret i1 undef` shouldn't be possible.

-- 
You are receiving this mail because:
You are on the CC list for the bug.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-bugs/attachments/20191101/5bdeb2f0/attachment.html>