| Issue |
123550
|
| Summary |
[Indvars] Miscompile: SSA value incorrectly simplified
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
Nirhar
|
One can find the incorrect transformation here: https://godbolt.org/z/T9qMhT7rb
And the proof of incorrect transformation here: https://alive2.llvm.org/ce/z/PXjb5J
For the record, I describe the problem here. Given the IR:
```
define i32 @widget() {
bb:
br label %bb1
bb1: ; preds = %bb4, %bb
%phi = phi i32 [ -173, %bb ], [ %sub, %bb4 ]
%phi2 = phi i32 [ 1, %bb ], [ %add, %bb4 ]
%srem = srem i32 729259140, %phi
%trunc = trunc i32 %phi2 to i8
%urem = urem i8 -83, %trunc
%zext = zext i8 %urem to i32
%sub = sub nsw i32 0, %zext
br label %bb4
bb4: ; preds = %bb1
%add = add nuw nsw i32 %phi2, 1
%icmp = icmp eq i32 %sub, 0
br i1 %icmp, label %bb5, label %bb1
bb5: ; preds = %bb4
%phi6 = phi i32 [ %srem, %bb4 ]
ret i32 %phi6
}
```
which returns `srem i32 729259140, -173` which is `130`. The above IR under the `indvars` pass transforms to:
```
define i32 @widget() {
br label %bb1
bb1: ; preds = %bb4, %bb
br label %bb4
bb4: ; preds = %bb1
br i1 true, label %bb5, label %bb1
bb5: ; preds = %bb4
ret i32 729259140
}
```
which returns `729259140` which is the wrong answer.
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