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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Avoid generating branch in branchless code"
href="https://bugs.llvm.org/show_bug.cgi?id=41110">41110</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Avoid generating branch in branchless 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>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>struct edge {
int from;
int to;
};
int edge_to_rank(struct edge e, size_t n) {
return e.from < e.to ? e.to - 1 : e.from - 1 + n / 2;
}
Clang -O3:
edge_to_rank(edge, unsigned long): # @edge_to_rank(edge, unsigned long)
mov rax, rdi
movabs rcx, -4294967296
mov rdx, rdi
shr rdx, 32
cmp eax, edx
jge .LBB0_2
add rax, rcx
sar rax, 32
ret
.LBB0_2:
shl rax, 32
add rax, rcx
sar rax, 32
shr rsi
add rax, rsi
ret
GCC -O3:
edge_to_rank(edge, unsigned long):
mov rax, rdi
sar rax, 32
cmp edi, eax
jge .L2
dec eax
ret
.L2:
shr rsi
lea eax, [rdi-1+rsi]
ret
ICC -O3
edge_to_rank(edge, unsigned long):
mov rax, rdi #9.21
movsxd rdx, edi #9.48
shr rsi, 1 #9.56
shr rax, 32 #9.21
movsxd rcx, eax #9.35
dec rcx #9.35
cmp edi, eax #9.21
lea r8, QWORD PTR [-1+rdx+rsi] #9.56
cmovl r8, rcx #9.21
mov eax, r8d #9.21
ret
I think Clang trunk generates supoptimal code. GCC's output is small and nice
(maybe three op LEA should not be used), but with a branch...
ICC generates the code I would expect - branchless using cmov.
<a href="https://godbolt.org/z/z6uzsn">https://godbolt.org/z/z6uzsn</a></pre>
</div>
</p>
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