[llvm-bugs] [Bug 31243] New: SLP causes a miscompile

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=31243

            Bug ID: 31243
           Summary: SLP causes a miscompile
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: michael.v.zolotukhin at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Created attachment 17705
  --> https://llvm.org/bugs/attachment.cgi?id=17705&action=edit
Reproducer

Reproducer (slp_bug.ll is attached):

> llc -O0 slp_bug.ll   -o good.s
> clang -m32 -o good good.s

> opt -slp-vectorizer slp_bug.ll -o after_slp.ll -S
> llc -O0 after_slp.ll -o bad.s
> clang -m32 -o bad bad.s

> ./good
> echo $?
106

> ./bad
> echo $?
0

The bug is that we sign-extend values, which should be zero-extended instead.
For instance, here:
  %10 = zext <4 x i8> %9 to <4 x i32>        ; Original value is zero-extended
  %11 = trunc <4 x i32> %10 to <4 x i8>      ; Now we truncate it...
  %12 = extractelement <4 x i8> %11, i32 0   ; extract first element...
  %13 = sext i8 %12 to i32                   ; and sign-extend it.
  ...

The issue seems to be introduced in r258929.

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