| Issue |
204182
|
| Summary |
[GlobalISel] Miscompilation where an i31 no-wrap negation inside a min chain is lowered incorrectly
|
| Labels |
|
| Assignees |
|
| Reporter |
sairam2661
|
**Fuzzer Generated Test**
**Reproducer**
1. SelectionDAG - https://godbolt.org/z/T7Y5ajG5Y
2. GlobalISel - https://godbolt.org/z/9ns9WG1P4
**Test Commit**
[efb038f38f23ee201ac872ae98668c2ef922f0fa](https://github.com/llvm/llvm-project/commit/efb038f38f23ee201ac872ae98668c2ef922f0fa)
**Description**
Since the `nuw` flag forces `sext(sub nuw i31 0, trunc %sel)` to 0, the function just returns `0`, but GlobalISel keeps the term and mis-lowers the i31 negate to a 32-bit `negw`, returning `-umin(a,y)`
**Steps to reproduce**
- Minimized test case, `input.ll`
```llvm
define i32 @f(i32 %a, i32 %y) {
%cmp1 = icmp ult i32 %a, %y
%sel = select i1 %cmp1, i32 %a, i32 %y
%1 = trunc i32 %sel to i31
%2 = sub nuw i31 0, %1
%3 = sext i31 %2 to i32
ret i32 %3
}
```
```bash
llc -mtriple=riscv64 -mattr=+m,+b input.ll # SelectionDAG
llc -mtriple=riscv64 -mattr=+m,+b -global-isel input.ll # GlobalISel
```
**Output**
; SelectionDAG (returns 0)
```asm
f: # @f
li a0, 0
ret
```
; GlobalISel (-umin(a,y))
```asm
f: # @f
sext.w a0, a0
sext.w a1, a1
minu a0, a0, a1
negw a0, a0
ret
```
**CC**: @regehr
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