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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - CGP generates inefficient branch for double selects"
href="https://llvm.org/bugs/show_bug.cgi?id=29095">29095</a>
</td>
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<tr>
<th>Summary</th>
<td>CGP generates inefficient branch for double selects
</td>
</tr>
<tr>
<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>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>LLVM Codegen
</td>
</tr>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>danielcdh@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>davidxl@google.com, llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>int*
rbsearch(int c, int *t, int n, int ne)
{
int *p;
int m;
while(n > 1) {
m = n >> 1;
p = t + m*ne;
if(__builtin_expect(c >= p[0], true)) {
t = p;
n = n-m;
} else
n = m;
}
return t;
}
For this test case, building with clang -O2, the generated x86 code is has
something like:
cmpl %edi, %r9d
jle .LBB0_3
# BB#2:
movl %r10d, %edx
jmp .LBB0_4
.p2align 4, 0x90
.LBB0_3:
subl %r10d, %edx
.LBB0_4:
cmpl %edi, %r9d
jg .LBB0_6
Note that the two cmpl instructions are redundant. The problem is gone if the
code is compiled with -mllvm -phi-node-folding-threshold=0
The issue is that CGP generates 2 conditional branches (with same condition)
for 2 select instructions.</pre>
</div>
</p>
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