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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 copy to reg before cmp"
href="https://bugs.llvm.org/show_bug.cgi?id=42189">42189</a>
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<tr>
<th>Summary</th>
<td>Avoid copy to reg before cmp
</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>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Backend: X86
</td>
</tr>
<tr>
<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, spatel+llvm@rotateright.com
</td>
</tr></table>
<p>
<div>
<pre>int g();
int f();
int shortmax(short c)
{
if (c == 32767)
return g();
return f();
}
Clang:
shortmax(short):
movzx eax, di
cmp eax, 32767
jne .LBB0_2
jmp g()
.LBB0_2:
jmp f()
But shouldn't we just use cmp di, 32767?
And I am just looking what -Os produces, and this is really interesting...
shortmax(short):
movzx eax, di
cmp eax, 32767
jne f() // neat
jmp g()
So for this pattern
if (x) call fn1();
else call fn2();
Possibly -O2/O3 should do this tranformation too?
Maybe ideal code (what -O3 should produce) for my example is:
shortmax(short):
cmp di, 32767
jne f()
jmp g()
?</pre>
</div>
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