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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 - Poor code generated for __builtin_sub_overflow with constant RHS"
href="https://bugs.llvm.org/show_bug.cgi?id=40125">40125</a>
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<th>Summary</th>
<td>Poor code generated for __builtin_sub_overflow with constant RHS
</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>Windows NT
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>craig.topper@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>This generates some impressively bad code considering its equivalent to
checking if x is 0.
void bar();
void foo(unsigned x) {
unsigned y;
if (__builtin_sub_overflow(x, 1, &y))
bar();
}
X86 assembly
mov eax, edi
add rax, -1
movabs rcx, 8589934591
and rcx, rax
add edi, -1
cmp rcx, rdi
jne .LBB0_2
ret
.LBB0_2:
jmp bar() # TAILCALL
<a href="https://godbolt.org/z/0K1SuH">https://godbolt.org/z/0K1SuH</a></pre>
</div>
</p>
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