https://bugs.llvm.org/show_bug.cgi?id=44069
Bug ID: 44069
Summary: Infinite shuffle+extract legalization loop
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: unassignedb...@nondot.org
Reporter: aemer...@apple.com
CC: craig.top...@gmail.com, llvm-bugs@lists.llvm.org,
llvm-...@redking.me.uk, spatel+l...@rotateright.com
The following IR causes a hang with llc -O0 on x86:
define hidden void @foo() unnamed_addr #0 align 2 {
entry:
%0 = load <4 x float>, <4 x float>* undef, align 16
%vecext.i4849 = extractelement <4 x float> %0, i32 0
%1 = load <4 x i32>, <4 x i32>* null, align 16
%permil281 = shufflevector <4 x i32> %1, <4 x i32> undef, <4 x i32> <i32 1,
i32 1, i32 1, i32 1>
%2 = bitcast <4 x i32> %permil281 to <4 x float>
%vecext1.i4794 = extractelement <4 x float> %2, i32 0
%sub.i4795 = fsub float %vecext1.i4794, %vecext.i4849
%vecins.i4796 = insertelement <4 x float> %2, float %sub.i4795, i32 0
store <4 x float> %vecins.i4796, <4 x float>* undef, align 16
unreachable
}
attributes #0 = { "use-soft-float"="false" }
It seems that there's a combine in X86ISelLowering in
XFormVExtractWithShuffleIntoLoad() which is matching the load->shuffle->extract
element and then generating another shuffle->extractelement, which I assume is
intended for further optimization by DAGCombine. However, it just up in a
legalization loop on the extract.
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