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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 - Missed optimization: optimizing set of if statements"
href="https://bugs.llvm.org/show_bug.cgi?id=38344">38344</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Missed optimization: optimizing set of if statements
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</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>LLVM Codegen
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>zamazan4ik@tut.by
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>clang(trunk) with -O3 -std=c++17 for this code:
unsigned int foo(unsigned int x)
{
if(x % 2 == 0)
{
return x * 2;
}
if(x % 4 == 0)
{
return x * 4;
}
if(x % 8 == 0)
{
return x * 8;
}
if(x % 16 == 0)
{
return x * 16;
}
if(x % 32 == 0)
{
return x * 32;
}
return 100;
}
generates this:
foo(unsigned int): # @foo(unsigned int)
mov ecx, 1
test dil, 1
je .LBB0_7
test dil, 3
je .LBB0_2
test dil, 7
je .LBB0_4
test dil, 15
je .LBB0_6
mov ecx, edi
shl ecx, 5
test dil, 31
mov eax, 100
cmove eax, ecx
ret
.LBB0_2:
mov ecx, 2
.LBB0_7:
shl edi, cl
mov eax, edi
ret
.LBB0_4:
mov ecx, 3
shl edi, cl
mov eax, edi
ret
.LBB0_6:
mov ecx, 4
shl edi, cl
mov eax, edi
ret
As you see, generated code is suboptimal: here we can leave only first 'if'
statement and otherwise return 100.</pre>
</div>
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