https://bugs.llvm.org/show_bug.cgi?id=49616
Bug ID: 49616
Summary: Failure to remove before/after reverse shuffles from
loop with decrementing iterator
Product: libraries
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: enhancement
Priority: P
Component: Scalar Optimizations
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected]
https://godbolt.org/z/dWTd9e
void loop_dec(unsigned short *p, int max, int n) {
for (int i = 0; i < (n & 31); i++) {
unsigned m = *--p;
*p = (unsigned short)(m >= max ? m-max : 0);
}
}
By decrementing the pointer in the loop we end up with loop bodies like this:
52:
%53 = phi i64 [ %41, %40 ], [ %64, %52 ]
%54 = sub i64 0, %53
%55 = getelementptr inbounds i16, i16* %51, i64 %54
%56 = bitcast i16* %55 to <4 x i16>*
%57 = load <4 x i16>, <4 x i16>* %56, align 2, !tbaa !2
%58 = shufflevector <4 x i16> %57, <4 x i16> poison, <4 x i32> <i32 3, i32 2,
i32 1, i32 0>
%59 = zext <4 x i16> %58 to <4 x i32>
%60 = call <4 x i32> @llvm.usub.sat.v4i32(<4 x i32> %59, <4 x i32> %50)
%61 = trunc <4 x i32> %60 to <4 x i16>
%62 = shufflevector <4 x i16> %61, <4 x i16> poison, <4 x i32> <i32 3, i32 2,
i32 1, i32 0>
%63 = bitcast i16* %55 to <4 x i16>*
store <4 x i16> %62, <4 x i16>* %63, align 2, !tbaa !2
%64 = add i64 %53, 4
%65 = icmp eq i64 %64, %45
br i1 %65, label %66, label %52, !llvm.loop !9
AFAICT we should be able to remove both these 'reverse' shufflevectors.
I'm not sure if we should be trying to fix this in InstCombine/VectorCombine or
inside the LoopVectorizer.
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