[llvm-bugs] [Bug 44185] New: Can't remove shufflevector if input might be poison

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=44185

            Bug ID: 44185
           Summary: Can't remove shufflevector if input might be poison
           Product: libraries
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Keywords: miscompilation
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: nunoplopes at sapo.pt
                CC: efriedma at quicinc.com, llvm-bugs at lists.llvm.org,
                    regehr at cs.utah.edu, spatel+llvm at rotateright.com,
                    zhengyang at kuf.io

Test case from Transforms/InstCombine/insert-extract-shuffle.ll:

define <4 x float> @insert_not_undef_shuffle_translate_commute(float %x, <4 x
float> %y, <4 x float> %q) {
  %xv = insertelement <4 x float> %q, float %x, i32 2
  %r = shufflevector <4 x float> %y, <4 x float> %xv, <4 x i32> { 0, 6, 2,
undef }
  ret <4 x float> %r
}
=>
define <4 x float> @insert_not_undef_shuffle_translate_commute(float %x, <4 x
float> %y, <4 x float> %q) {
  %r = insertelement <4 x float> %y, float %x, i32 1
  ret <4 x float> %r
}
Transformation doesn't verify!
ERROR: Target is more poisonous than source

Example:
float %x = poison
<4 x float> %y = < poison, poison, poison, poison >
<4 x float> %q = < poison, poison, poison, poison >

Source:
<4 x float> %xv = < poison, poison, poison, poison >
<4 x float> %r = < poison, poison, poison, undef >

Target:
<4 x float> %r = < poison, poison, poison, poison >
Source value: < poison, poison, poison, undef >
Target value: < poison, poison, poison, poison >


This is with the semantics of LangRef, where an undef mask stops propagation of
poison. This implies this kind of shufflevectors can't be removed.

-- 
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/20191130/c064b241/attachment.html>