[llvm-bugs] [Bug 42699] New: EmitGEPOffset() incorrectly adds NUW to multiplications

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42699

            Bug ID: 42699
           Summary: EmitGEPOffset() incorrectly adds NUW to
                    multiplications
           Product: libraries
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Transformation Utilities
          Assignee: unassignedbugs at nondot.org
          Reporter: nunoplopes at sapo.pt
                CC: llvm-bugs at lists.llvm.org, regehr at cs.utah.edu,
                    sanjoy at playingwithpointers.com

The following transformation in test/Transforms/InstCombine/sub.ll exposes a
bug in EmitGEPOffset() that dates back to 2011.
It assumes that 'gep inbounds' guarantees that %i * 4 doesn't overflow
unsigned, which isn't true since %i could be negative.
Changing nuw to nsw makes it correct.

Alive's report:

define i64 @test30(* %foo, i64 %i, i64 %j) {
  %bit = bitcast * %foo to *
  %gep1 = gep inbounds * %bit, 4 x i64 %i
  %gep2 = gep inbounds * %foo, 1 x i64 %j
  %cast1 = ptrtoint * %gep1 to i64
  %cast2 = ptrtoint * %gep2 to i64
  %sub = sub i64 %cast1, %cast2
  ret i64 %sub
}
=>
define i64 @test30(* %foo, i64 %i, i64 %j) {
  %gep1.idx = shl nuw i64 %i, 2
  %1 = sub i64 %gep1.idx, %j
  ret i64 %1
}
Transformation doesn't verify!
ERROR: Target is more poisonous than source

Example:
* %foo = pointer(non-local, block_id=0, offset=8)
i64 %i = -1
i64 %j = 0

Source:
* %bit = pointer(non-local, block_id=0, offset=8)
* %gep1 = pointer(non-local, block_id=0, offset=4)
* %gep2 = pointer(non-local, block_id=0, offset=8)
i64 %sub = -4

Target:
i64 %gep1.idx = poison
i64 %1 = poison

Source value: -4
Target value: poison

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