| Issue |
144780
|
| Summary |
[llvm] Miscompilation of freeze instruction with llc -O1
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
8051Enthusiast
|
If my understanding of the freeze instruction is correct, the following code should always return a number that is of the form `n + 2 * k` where `k` can be anything.
```llvm
define i32 @preserve_parity(i32 %n) {
%2 = freeze i32 poison
%3 = add i32 %n, %2
%4 = add i32 %3, %2
ret i32 %4
}
```
However, for example on x86_64 `llc` (with optimization level at least 1) outputs the following code:
```asm
preserve_parity: # @preserve_parity
mov eax, edi
add eax, eax
add eax, eax
ret
```
Given that this always outputs an even number, but `n + 2 * k` is always odd for odd `n`, this function seems to behave incorrectly for odd n.
This does not only happen on x86_64, but a bunch of other backends too: https://godbolt.org/z/nzM4Mo98v.
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