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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - use the right data type when materializing an FP constant"
href="http://llvm.org/bugs/show_bug.cgi?id=17725">17725</a>
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<th>Summary</th>
<td>use the right data type when materializing an FP constant
</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>Backend: X86
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>kkhoo@perfwizard.com
</td>
</tr>
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>In the following code example, llvm is using the single-precision "xorps" to
materialize an FP double 0.0 constant.
Intel warns that there may be a performance penalty for mixing data types
("execution domains" in Intel-speak; see related <a class="bz_bug_link
bz_status_NEW "
title="NEW --- - match broadcast instruction to data type (int or FP)"
href="show_bug.cgi?id=17185">bug 17185</a>).
It would be better to use "xorpd" here to avoid that penalty:
$ cat addfp0.c
double foo(double x) {
return 0.0 + x;
}
$ ./clang -O3 -fomit-frame-pointer -o - -S addfp0.c
.section __TEXT,__text,regular,pure_instructions
.globl _foo
.align 4, 0x90
_foo: ## @foo
.cfi_startproc
## BB#0: ## %entry
xorps %xmm1, %xmm1 <---- use "xorpd" here
addsd %xmm0, %xmm1
movaps %xmm1, %xmm0
ret
.cfi_endproc
.subsections_via_symbols
$ ./clang -v
clang version 3.4 (trunk 193529) (llvm/trunk 193527)
Target: x86_64-apple-darwin11.4.2
Thread model: posix</pre>
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