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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 - Missed optimization to use the carry flag after subtracting"
href="https://bugs.llvm.org/show_bug.cgi?id=46809">46809</a>
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<th>Summary</th>
<td>Missed optimization to use the carry flag after subtracting
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>10.0
</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>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Backend: X86
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>josephcsible@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
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<p>
<div>
<pre>Consider these two C functions:
unsigned f1(unsigned x, unsigned y) {
unsigned z = x - y;
if (x < y) {
z += 100;
}
return z;
}
unsigned f2(unsigned x, unsigned y) {
unsigned z;
if (__builtin_usub_overflow(x, y, &z)) {
z += 100;
}
return z;
}
At -O3 or -Os, they both generate the same assembly:
movl %edi, %eax
movl %edi, %ecx
subl %esi, %ecx
addl $100, %ecx
subl %esi, %eax
cmovbl %ecx, %eax
retq
<a href="https://godbolt.org/z/MevEM9">https://godbolt.org/z/MevEM9</a>
That's suboptimal. They should have instead used lea to save a move, save a
duplicate subtraction, and avoid clobbering the carry flag, like this:
subl %esi, %edi
leal 100(%rdi), %eax
cmovael %edi, %eax
retq</pre>
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</p>
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