[llvm-bugs] [Bug 42198] New: incorrect instcombine fold for icmp sgt

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42198

            Bug ID: 42198
           Summary: incorrect instcombine fold for icmp sgt
           Product: libraries
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: nunoplopes at sapo.pt
                CC: lebedev.ri at gmail.com, llvm-bugs at lists.llvm.org,
                    regehr at cs.utah.edu

The following instcombine rewrite seems incorrect
(test/Transforms/InstCombine/canonicalize-constant-low-bit-mask-and-icmp-sgt-to-icmp-sgt.ll):


define i1 @cv0_GOOD(i8 %y) {
%0:
  %x = call i8 @gen8()
  %tmp0 = lshr i8 255, %y
  %tmp1 = and i8 %tmp0, %x
  %ret = icmp sgt i8 %x, %tmp1
  ret i1 %ret
}
=>
define i1 @cv0_GOOD(i8 %y) {
%0:
  %x = call i8 @gen8()
  %tmp0 = lshr i8 255, %y
  %1 = icmp sgt i8 %x, %tmp0
  ret i1 %1
}
Transformation doesn't verify!
ERROR: Value mismatch

Example:
i8 %y = #x00 (0)

Source:
i8 %x = #x7f (127, -129)
i8 %tmp0 = #xff (255, -1)
i8 %tmp1 = #x7f (127, -129)
i1 %ret = #x0 (0)

Target:
i8 %x = #x7f (127, -129)
i8 %tmp0 = #xff (255, -1)
i1 %1 = #x1 (1)
Source value: #x0 (0)
Target value: #x1 (1)

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