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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 - Avoid generation of __lshrti3 libcall for cold code"
href="https://bugs.llvm.org/show_bug.cgi?id=47897">47897</a>
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<tr>
<th>Summary</th>
<td>Avoid generation of __lshrti3 libcall for cold code
</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>
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<th>OS</th>
<td>Linux
</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>
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<th>Component</th>
<td>Backend: X86
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, pengfei.wang@intel.com, spatel+llvm@rotateright.com
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<p>
<div>
<pre>int a, b, c;
__attribute__((__cold__)) void d() { c = (unsigned __int128)a >> b; }
Clang -O3:
d(): # @d()
push rax
movsxd rdi, dword ptr [rip + a]
mov rsi, rdi
sar rsi, 63
movzx edx, byte ptr [rip + b]
call __lshrti3
mov dword ptr [rip + c], eax
pop rax
ret
a:
.long 0 # 0x0
b:
.long 0 # 0x0
c:
.long 0 # 0x0
While Clang 11/GCC generates:
d(): # @d()
movsxd rax, dword ptr [rip + a]
mov rdx, rax
sar rdx, 63
mov cl, byte ptr [rip + b]
shrd rax, rdx, cl
shr rdx, cl
test cl, 64
cmove rdx, rax
mov dword ptr [rip + c], edx
ret
a:
.long 0 # 0x0
b:
.long 0 # 0x0
c:
.long 0 # 0x0
<a href="https://godbolt.org/z/Kqf3EW">https://godbolt.org/z/Kqf3EW</a></pre>
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