nunoplopes wrote:
Here's a simple-ish example:
```llvm
; Transforms/InstCombine/add.ll
define i5 @zext_sext_not(i4 %x) {
%zx = zext i4 %x to i5
%notx = xor i4 %x, 15
%snotx = sext i4 %notx to i5
%r = add i5 %zx, %snotx
ret i5 %r
}
=>
define i5 @zext_sext_not(i4 %x) {
%zx = zext i4 %x to i5
%notx = xor i4 %x, 15
%snotx = sext i4 %notx to i5
%r = or disjoint i5 %zx, %snotx
ret i5 %r
}
Transformation doesn't verify! (unsound)
ERROR: Target is more poisonous than source
Example:
i4 %x = undef
Source:
i5 %zx = #x00 (0) [based on undef value]
i4 %notx = #xf (15, -1) [based on undef value]
i5 %snotx = #x1f (31, -1)
i5 %r = #x1f (31, -1)
Target:
i5 %zx = #x0f (15)
i4 %notx = #xf (15, -1)
i5 %snotx = #x1f (31, -1)
i5 %r = poison
Source value: #x1f (31, -1)
Target value: poison
```
Essentially the code doesn't take into consideration that a same register may
have different values when it is undef.
https://github.com/llvm/llvm-project/pull/72912
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