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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 --- - better 387 code for float x == (int)x"
href="http://llvm.org/bugs/show_bug.cgi?id=20086">20086</a>
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<tr>
<th>Summary</th>
<td>better 387 code for float x == (int)x
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>jay.foad@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 get this with clang 3.5.0 (trunk) at r211342:
$ clang -mno-sse -O3 -S -o - -x c - <<<"int f(float x) { return x == (int)x; }"
...
f:
flds 8(%rsp)
fnstcw -10(%rsp)
movw -10(%rsp), %ax
movw $3199, -10(%rsp) # imm = 0xC7F
fldcw -10(%rsp)
movw %ax, -10(%rsp)
fistl -8(%rsp)
fldcw -10(%rsp)
movl -8(%rsp), %eax
movl %eax, -4(%rsp)
fildl -4(%rsp)
fucompi %st(1)
fstp %st(0)
setnp %al
sete %cl
andb %al, %cl
movzbl %cl, %eax
retq
All the fnstcw and fldcw stuff is unnecessary, because when we convert x to
int, it doesn't matter what rounding mode we use; all that matters is whether
it is already an exact int value or not.
I think the x == (int)x pattern is fairly common, and it would be nice if clang
generated better 387 code for it.
On the corresponding GCC bug:
<a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61563">https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61563</a>
Joseph pointed out that for other combinations of floating point and integer
types (e.g. double and int) it's not completely safe to use a different
rounding mode, because it might affect whether the conversion to int raises an
"invalid" exception.</pre>
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