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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 --- - [SLP vectorization] scalar loads not combined into vector load"
href="http://llvm.org/bugs/show_bug.cgi?id=19657">19657</a>
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<th>Summary</th>
<td>[SLP vectorization] scalar loads not combined into vector load
</td>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
</tr>
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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>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Loop Optimizer
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
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<th>Reporter</th>
<td>spatel+llvm@rotateright.com
</td>
</tr>
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
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<th>Classification</th>
<td>Unclassified
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</tr></table>
<p>
<div>
<pre>Using:
$ ./clang -v
clang version 3.5.0 (207996)
Target: x86_64-apple-darwin13.1.0
Thread model: posix
On the following C code that multiplies and adds 4 double-precision array
elements:
$ cat vmul.c
void foo(double *x) {
x[0] = x[0] * x[0] + x[0];
x[1] = x[1] * x[1] + x[1];
x[2] = x[2] * x[2] + x[2];
x[3] = x[3] * x[3] + x[3];
}
The generated code contains scalar loads for x[2] and x[3], but all other
operations are vectorized. It would be better in size and speed to optimize the
scalar loads into a single vector load:
$ ./clang -O2 -S -o - vmul.c -march=btver2
...
vmovupd (%rdi), %xmm0 <---- vector load: good
vmulpd %xmm0, %xmm0, %xmm1
vaddpd %xmm1, %xmm0, %xmm0
vmovupd %xmm0, (%rdi)
vmovsd 16(%rdi), %xmm0 <---- scalar load: bad
vmovhpd 24(%rdi), %xmm0, %xmm0 <---- scalar load: bad
vmulpd %xmm0, %xmm0, %xmm1
vaddpd %xmm1, %xmm0, %xmm0
vmovupd %xmm0, 16(%rdi)
popq %rbp
retq</pre>
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