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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Possibly missed optimization (performance and code size) when comparing floats as raw binary"
href="https://bugs.llvm.org/show_bug.cgi?id=36260">36260</a>
</td>
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<th>Summary</th>
<td>Possibly missed optimization (performance and code size) when comparing floats as raw binary
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
</tr>
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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>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>-New Bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>kobalicek.petr@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>The following code
------------------
#include <stdint.h>
#include <xmmintrin.h>
#include <emmintrin.h>
union F64 {
double d;
uint64_t u;
};
uint32_t compareBin(double a, double b) noexcept {
F64 a_ = { a };
F64 b_ = { b };
return a_.u == b_.u;
}
uint32_t compareDbl(double a, double b) noexcept {
return a == b;
}
Compiles to (-O2 -mavx2 -fno-math-errno):
-----------------------------------------
compareBin(double, double): # @compareBin(double, double)
vmovq rcx, xmm0
vmovq rdx, xmm1
xor eax, eax
cmp rcx, rdx
sete al
ret
compareDbl(double, double): # @compareDbl(double, double)
vcmpeqsd xmm0, xmm0, xmm1
vmovq rax, xmm0
and eax, 1
ret
I think the `compareBin` should use the same approach as `compareDbl`, which
would be:
compareBin(double, double):
vpcmpeqq xmm0, xmm0, xmm1
vmovq rax, xmm0
and eax, 1
ret</pre>
</div>
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