| Issue |
203104
|
| Summary |
[LoopInterchange] Miscompile when outer loop reduction has extra use in inner loop
|
| Labels |
miscompilation
|
| Assignees |
|
| Reporter |
nikic
|
https://llvm.godbolt.org/z/P48nz4sfb
```llvm
; Stray use of the outer reduction PHI (%sum) inside the inner loop body.
; %sum holds the accumulated value at the start of outer iteration i.
; After interchange, %sum becomes the inner-loop accumulator and takes
; completely different values at each (i,j) point.
@A = global [4 x [4 x i32]] zeroinitializer
@B = global [4 x [4 x i32]] zeroinitializer
define i32 @f(ptr noalias %A, ptr noalias %B) {
entry:
br label %outer.header
outer.header:
%i = phi i64 [ 0, %entry ], [ %i.next, %outer.latch ]
%sum = phi i32 [ 0, %entry ], [ %sum.lcssa, %outer.latch ]
br label %inner.header
inner.header:
%j = phi i64 [ 0, %outer.header ], [ %j.next, %inner.latch ]
%sum.inner = phi i32 [ %sum, %outer.header ], [ %sum.next, %inner.latch ]
%gepA = getelementptr inbounds [4 x [4 x i32]], ptr %A, i64 0, i64 %j, i64 %i
%a = load i32, ptr %gepA
%sum.next = add i32 %sum.inner, %a
%gepB = getelementptr inbounds [4 x [4 x i32]], ptr %B, i64 0, i64 %j, i64 %i
store i32 %sum, ptr %gepB ; stray use of outer reduction PHI
br label %inner.latch
inner.latch:
%j.next = add nuw nsw i64 %j, 1
%jcmp = icmp slt i64 %j.next, 4
br i1 %jcmp, label %inner.header, label %outer.latch
outer.latch:
%sum.lcssa = phi i32 [ %sum.next, %inner.latch ]
%i.next = add nuw nsw i64 %i, 1
%icmp = icmp slt i64 %i.next, 4
br i1 %icmp, label %outer.header, label %exit
exit:
%res = phi i32 [ %sum.lcssa, %outer.latch ]
ret i32 %res
}
```
Disclosure: The test case is AI generated.
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