https://llvm.org/bugs/show_bug.cgi?id=31116
Bug ID: 31116
Summary: SLP Vectorizer too aggressive with simple stores
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Scalar Optimizations
Assignee: unassignedb...@nondot.org
Reporter: ma...@braunis.de
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
In a situation like the following (modeled after what happens in a C++
constructor with two base classes), the SLP vectorizer seems too aggressive:
struct C {
char *BaseA;
char *BaseB;
};
extern char VTab[];
void initC(C *c) {
c->BaseA = VTab + 48;
c->BaseB = VTab + 16;
}
The SLP Vectorizer transforms:
%BaseA = getelementptr inbounds %struct.C, %struct.C* %c, i64 0, i32 0
store i8* getelementptr inbounds ([0 x i8], [0 x i8]* @VTab, i64 0, i64 48),
i8** %BaseA, align 8, !tbaa !2
%BaseB = getelementptr inbounds %struct.C, %struct.C* %c, i64 0, i32 1
store i8* getelementptr inbounds ([0 x i8], [0 x i8]* @VTab, i64 0, i64 16),
i8** %BaseB, align 8, !tbaa !7
into:
%BaseA = getelementptr inbounds %struct.C, %struct.C* %c, i64 0, i32 0
%BaseB = getelementptr inbounds %struct.C, %struct.C* %c, i64 0, i32 1
%0 = bitcast i8** %BaseA to <2 x i8*>*
store <2 x i8*> <i8* getelementptr inbounds ([0 x i8], [0 x i8]* @VTab, i64
0, i64 48), i8* getelementptr inbounds ([0 x i8], [0 x i8]* @VTab, i64 0, i64
16)>, <2 x i8*>* %0, align 8, !tbaa !2
ret void
which results in this code on X86:
movq _VTab@GOTPCREL(%rip), %rax
leaq 16(%rax), %rcx
movd %rcx, %xmm0
addq $48, %rax
movd %rax, %xmm1
punpcklqdq %xmm0, %xmm1 ## xmm1 = xmm1[0],xmm0[0]
movdqu %xmm1, (%rdi)
while it can be this if the SLP Vectorizer doesn't do anything:
movq _VTab@GOTPCREL(%rip), %rax
leaq 48(%rax), %rcx
movq %rcx, (%rdi)
addq $16, %rax
movq %rax, 8(%rdi)
--
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