| Issue |
204219
|
| Summary |
[GlobalISel] Miscompilation where a negation is dropped
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
sairam2661
|
**Fuzzer Generated Test**
**Reproducer**
1. SelectionDAG - https://godbolt.org/z/YMY116ovr
2. GlobalISel - https://godbolt.org/z/ojcooKEMY
**Test Commit**
[efb038f38f23ee201ac872ae98668c2ef922f0fa](https://github.com/llvm/llvm-project/commit/efb038f38f23ee201ac872ae98668c2ef922f0fa)
**Description**
Since `sub 0, (xor x, -1)` equals `x + 1`, the function is `x & (x + 1)`, but GlobalISel drops the negation and folds it to `andn x, x` (`x & ~x`, always 0)
**Steps to reproduce**
- Minimized test case, `input.ll`
```llvm
define i32 @f(i32 %g) {
%not = xor i32 %g, -1
%neg = sub i32 0, %not
%and = and i32 %g, %neg
ret i32 %and
}
```
```bash
llc -mtriple=riscv64 -mattr=+m,+b input.ll # SelectionDAG
llc -mtriple=riscv64 -mattr=+m,+b -global-isel input.ll # GlobalISel
```
**Output**
; SelectionDAG (x & (x + 1))
```asm
f:
addiw a1, a0, 1
and a0, a0, a1
ret
```
; GlobalISel (x & ~x, always 0)
```asm
f:
andn a0, a0, a0
ret
```
With `g = 2`: source `2 & 3 = 2`, GlobalISel returns `0`
**CC**: @regehr
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