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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 - std::clamp generates suboptimal assembly for primitive types"
href="https://bugs.llvm.org/show_bug.cgi?id=49909">49909</a>
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<th>Summary</th>
<td>std::clamp generates suboptimal assembly for primitive types
</td>
</tr>
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<th>Product</th>
<td>libc++
</td>
</tr>
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Windows NT
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>All Bugs
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>vittorio.romeo@outlook.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, mclow.lists@gmail.com
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<p>
<div>
<pre>`std::clamp` generates poor assembly compared to hand-written counterpart for
primitive types like `float`, even with `-Ofast -ffast-math`:
stdclamp(float, float, float): # @stdclamp(float,
float, float)
movss dword ptr [rsp - 4], xmm0
movss dword ptr [rsp - 8], xmm1
movss dword ptr [rsp - 12], xmm2
ucomiss xmm2, xmm0
lea rax, [rsp - 12]
lea rcx, [rsp - 4]
cmovb rcx, rax
ucomiss xmm0, xmm1
lea rax, [rsp - 8]
cmovae rax, rcx
movss xmm0, dword ptr [rax] # xmm0 = mem[0],zero,zero,zero
ret
myclamp(float, float, float): # @myclamp(float, float,
float)
maxss xmm0, xmm1
minss xmm0, xmm2
ret
Live example:
<a href="https://godbolt.org/z/5oxvocevK">https://godbolt.org/z/5oxvocevK</a>
More information on:
<a href="https://secret.club/2021/04/09/std-clamp.html">https://secret.club/2021/04/09/std-clamp.html</a></pre>
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