| Issue |
174220
|
| Summary |
`xor undef, undef` always evaluates to `0`
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
Veeloxfire
|
Tested on x64 clang trunk
```cpp
int foo_xor() {
int i;
int j;
return i ^ j;
}
```
During optimization this reaches `xor undef, undef`
`xor undef, undef` then "optimizes" to `0`
This is special cased to allow `a ^ a` to always be `0`:
https://github.com/llvm/llvm-project/blob/40155458d7660cf4d28894e2ec40010743172e44/llvm/lib/IR/ConstantFold.cpp#L628-L632
I am aware this is UB so the behaviour doesn't necessarily matter but it feels unreasonable that the 2 "different" `undef`'s here xor to `0` and we inject a concrete `0` that the optimizer has to work around
Not sure there is a way in llvm's internals to specify that 2 undef values are different so it may not be worth investigating but I thought I would raise
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