| Issue |
136646
|
| Summary |
[InstCombine] The sign bit of NaN is not preserved when folding `(X <= +/-0.0) ? (0.0 - X) : X to fabs(X)`
|
| Labels |
miscompilation,
llvm:instcombine
|
| Assignees |
dtcxzyw
|
| Reporter |
dtcxzyw
|
Reproducer: https://alive2.llvm.org/ce/z/2JaLU9
```
----------------------------------------
define half @src(half %x) {
#0:
%gtzero = fcmp ugt half %x, 0x0000
%negx = fsub half 0x0000, %x
%fabs = select i1 %gtzero, half %x, half %negx
ret half %fabs
}
=>
define half @src(half %x) nofree willreturn memory(none) {
#0:
%fabs = fabs half %x
ret half %fabs
}
Transformation doesn't verify!
ERROR: Value mismatch
Example:
half %x = #xfd00 (SNaN)
Source:
i1 %gtzero = #x1 (1)
half %negx = #xff00 (QNaN)
half %fabs = #xfd00 (SNaN)
Target:
half %fabs = #x7d00 (SNaN)
Source value: #xfd00 (SNaN)
Target value: #x7d00 (SNaN)
Summary:
0 correct transformations
1 incorrect transformations
0 failed-to-prove transformations
0 Alive2 errors
```
Related issues:
https://github.com/AliveToolkit/alive2/pull/1155
https://github.com/llvm/llvm-project/issues/59279
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