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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 --- - Duplicate loading of double constants"
href="http://llvm.org/bugs/show_bug.cgi?id=16938">16938</a>
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<tr>
<th>Summary</th>
<td>Duplicate loading of double constants
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
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<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>
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<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Common Code Generator Code
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>eltoder@gmail.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>
<p>
<div>
<pre>I found that in some cases llvm generates duplicate loads of double constants,
e.g.
$ cat t.c
double f(double* p, int n)
{
double s = 0;
if (n)
s += *p;
return s;
}
$ clang -S -O3 t.c -o -
...
f: # @f
.cfi_startproc
# BB#0:
xorps %xmm0, %xmm0
testl %esi, %esi
je .LBB0_2
# BB#1:
xorps %xmm0, %xmm0
addsd (%rdi), %xmm0
.LBB0_2:
ret
...
Note that there are 2 xorps instructions, the one in BB#1 being clearly
redundant
as it's dominated by the first one. Two xorps come from 2 FsFLD0SD generated by
instruction selection and never eliminated by machine passes. My guess would be
machine CSE should take care of it.
A variation of this case without indirection has the same problem, as well as a
related but separate issue: not commuting addps, resulting in an extra movps:
$ cat t.c
double f(double p, int n)
{
double s = 0;
if (n)
s += p;
return s;
}
$ clang -S -O3 t.c -o -
...
f: # @f
.cfi_startproc
# BB#0:
xorps %xmm1, %xmm1
testl %edi, %edi
je .LBB0_2
# BB#1:
xorps %xmm1, %xmm1
addsd %xmm1, %xmm0
movaps %xmm0, %xmm1
.LBB0_2:
movaps %xmm1, %xmm0
ret
...</pre>
</div>
</p>
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