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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 - Single bit extract and add/sub could generate better code via carry bit"
href="https://bugs.llvm.org/show_bug.cgi?id=35908">35908</a>
</td>
</tr>
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<th>Summary</th>
<td>Single bit extract and add/sub could generate better code via carry bit
</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>
<tr>
<th>OS</th>
<td>All
</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>
</tr>
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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>dave@znu.io
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Consider the following code, which is a reduction derived from a client of
llvm::TrailingObjects.
void dummy(int arg);
void test_add(int x, int y, int z) { dummy(bool(z & (1 << 30)) + x + y); }
void test_sub(int x, int y, int z) { dummy(bool(z & (1 << 30)) - x - y); }
On pre-BEXTR CPUs, the following code is emitted:
"test_add"
shrl $30, %edx
andl $1, %edx
addl %esi, %edi
addl %edx, %edi
"test_sub"
shrl $30, %edx
andl $1, %edx
addl %esi, %edi
subl %edi, %edx
movl %edx, %edi
And on BEXTR capable CPUs:
"test_add"
movl $286, %eax # imm = 0x11E
bextrl %eax, %edx, %eax
addl %esi, %edi
addl %eax, %edi
"test_sub"
movl $286, %eax # imm = 0x11E
bextrl %eax, %edx, %eax
addl %esi, %edi
subl %edi, %eax
movl %eax, %edi
But in practice, pre-BEXTR CPUs can generate the best code
"test_add"
btl $30, %edx
adcl %esi, %edx
"test_sub"
btl $30, %edx
sbbl %esi, %edx</pre>
</div>
</p>
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