https://bugs.llvm.org/show_bug.cgi?id=49875
Bug ID: 49875
Summary: Miscompile with LoopLoadElim due to wrong store
forwarding
Product: new-bugs
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: enhancement
Priority: P
Component: new bugs
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected]
Run: opt -loop-load-elim -S on the following test:
----------------------------------------------------------
define void @test(i32** %arg) {
bb:
%tmp = load i32*, i32** %arg, align 8
br label %bb1
bb1: ; preds = %bb1, %bb
%tmp2 = phi i64 [ %tmp7, %bb1 ], [ 1, %bb ]
%tmp3 = add nsw i64 %tmp2, -1
%tmp4 = getelementptr i32, i32* %tmp, i64 %tmp3
%tmp5 = load i32, i32* %tmp4, align 4
%tmp6 = getelementptr i32, i32* %tmp, i64 %tmp2
store i32 %tmp5, i32* %tmp6, align 4
%tmp7 = add nuw nsw i64 %tmp2, 1
%tmp8 = icmp ugt i64 %tmp2, 2
br i1 %tmp8, label %bb9, label %bb1
bb9: ; preds = %bb1
ret void
}
----------------------------------------------------------
The result will be like:
----------------------------------------------------------
define void @test(i32** %arg) {
bb:
%tmp = load i32*, i32** %arg, align 8
%load_initial = load i32, i32* %tmp, align 4
br label %bb1
bb1: ; preds = %bb1, %bb
%store_forwarded = phi i32 [ %load_initial, %bb ], [ %store_forwarded, %bb1 ]
%tmp2 = phi i64 [ %tmp7, %bb1 ], [ 1, %bb ]
%tmp3 = add nsw i64 %tmp2, -1
%tmp4 = getelementptr i32, i32* %tmp, i64 %tmp3
%tmp5 = load i32, i32* %tmp4, align 4
%tmp6 = getelementptr i32, i32* %tmp, i64 %tmp2
store i32 %store_forwarded, i32* %tmp6, align 4
%tmp7 = add nuw nsw i64 %tmp2, 1
%tmp8 = icmp ugt i64 %tmp2, 2
br i1 %tmp8, label %bb9, label %bb1
bb9: ; preds = %bb1
ret void
}
----------------------------------------------------------
This is a miscompile for the following reason: in the initial example we were
making something like arr[i] = arr[i - 1], and each stored value was different
(it was loaded from memory by inductive offset). After the transform, the
stored value %store_forwarded does not change.
It looks like a messy update of backedge value for %store_forwarded.
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