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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - LLVM does not properly produce vectorized zextload instructions"
href="http://llvm.org/bugs/show_bug.cgi?id=20443">20443</a>
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<th>Summary</th>
<td>LLVM does not properly produce vectorized zextload instructions
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Backend: X86
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>kree@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
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<div>
<pre>Created <span class=""><a href="attachment.cgi?id=12823" name="attach_12823" title="Example .ll file deomonstrating problem with pmovzxbd">attachment 12823</a> <a href="attachment.cgi?id=12823&action=edit" title="Example .ll file deomonstrating problem with pmovzxbd">[details]</a></span>
Example .ll file deomonstrating problem with pmovzxbd
Hello,
I believe that LLVM is not producing the optimal machine code in the case of a
zextload command on vectorized types. The basic problem I have is that when I
have a pair of instructions like:
%6 = load <4 x i8>* %5, align 16
%7 = zext <4 x i8> %6 to <4 x i32>
LLVM produces the following assembly code:
pmovzxbd (%rdi), %xmm0
pand .LCPI0_0(%rip), %xmm0
The pand here is unnecessary since pmovzxbd already ensures that the zext has
been done. I have attached a file "sse_pmovzx_test.ll" that demonstrates this
issue.
In the scalar case things work just fine, so if the above .ll code used scalar
types, we would get a single assembly line of the form:
movzbl (%rdi), %eax
It seems like the vector SSE operations are not matching against the zextload
pattern, and it looks like this could be intentional. In DAGCombiner.cpp I
found this comment:
// fold (zext (load x)) -> (zext (truncate (zextload x)))
// None of the supported targets knows how to perform load and vector_zext
// on vectors in one instruction. We only perform this transformation on
// scalars.
if (ISD::isNON_EXTLoad(N0.getNode()) && !VT.isVector() &&
ISD::isUNINDEXEDLoad(N0.getNode()) &&
((!LegalOperations && !cast<LoadSDNode>(N0)->isVolatile()) ||
TLI.isLoadExtLegal(ISD::ZEXTLOAD, N0.getValueType()))) {
So my question is what would it take to allow the combiner to make this
transformation for vector types? Is this comment out of date, since we now have
pmovzx** in SSE4.1 and vpmovzx** in AVX2?</pre>
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