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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Infinite shuffle+extract legalization loop"
href="https://bugs.llvm.org/show_bug.cgi?id=44069">44069</a>
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<th>Summary</th>
<td>Infinite shuffle+extract legalization loop
</td>
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<tr>
<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Backend: X86
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>aemerson@apple.com
</td>
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<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
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<pre>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.</pre>
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