| Issue |
203972
|
| Summary |
[RISC-V][GlobalISel] Miscompilation where a bitwise-and of a value with its own negation is dropped, returning only the negation
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
sairam2661
|
**Fuzzer Generated Test**
**Reproducer**
1. SelectionDAG - https://godbolt.org/z/jTTfefPr3
2. GlobalISel - https://godbolt.org/z/14Y9Y73zn
**Test Commit**
[efb038f38f23ee201ac872ae98668c2ef922f0fa](https://github.com/llvm/llvm-project/commit/efb038f38f23ee201ac872ae98668c2ef922f0fa)
**Description**
Since `mul nuw i32 x, -1` equals `-x`, the function is the lowest-set-bit idiom `x & -x`, but GlobalISel drops the `and` and returns just `-x`
**Steps to reproduce**
- Minimized test case, `input.ll`
```
define i32 @f(i32 %x) {
%neg = mul nuw i32 %x, -1
%r = and i32 %x, %neg
ret i32 %r
}
```
```
llc -mtriple=riscv64 -mattr=+m,+b input.ll # SelectionDAG
llc -mtriple=riscv64 -mattr=+m,+b -global-isel input.ll # GlobalISel
```
**Output**
```
; SelectionDAG (x & -x)
f:
negw a1, a0
and a0, a0, a1
ret
; GlobalISel (drops the `and`, returns -x)
f:
negw a0, a0
ret
```
With `x = 1` (`mul nuw 1, -1` = `0xFFFFFFFF`, poison-free): source `1 & -1 = 1`, GlobalISel returns `0xFFFFFFFF` (`-1`)
**CC**: @regehr
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