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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - 128 bit arithmetic --- inefficient logic test"
href="https://bugs.llvm.org/show_bug.cgi?id=50197">50197</a>
</td>
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<tr>
<th>Summary</th>
<td>128 bit arithmetic --- inefficient logic test
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<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>
<tr>
<th>Component</th>
<td>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>zero@smallinteger.com
</td>
</tr>
<tr>
<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=24821" name="attach_24821" title="Sample code">attachment 24821</a> <a href="attachment.cgi?id=24821&action=edit" title="Sample code">[details]</a></span>
Sample code
See attached code, compiled with -O2. The condition inside the while () test
is unnecessarily evaluated with a 128 bit shift instruction,
square: # @square
mov rax, rdi
.LBB0_1: # =>This Inner Loop Header: Depth=1
add rax, 1
adc rsi, 0
mov rcx, rsi
shld rcx, rax, 4
mov rdx, rsi
shr rdx, 60
or rdx, rcx
jne .LBB0_1
ret
even though a 64 bit shift instruction suffices. However, changing || to | in
the logic condition yields the more efficient code below.
square: # @square
mov rax, rdi
.LBB0_1: # =>This Inner Loop Header: Depth=1
add rax, 1
adc rsi, 0
mov rcx, rax
shr rcx, 60
or rcx, rsi
jne .LBB0_1
ret
Found with clang-10 on Ubuntu 20.04 LTS, verified for clang 10, 11, and trunk
using godbolt. Note that gcc -O2 handles both of these cases emitting the more
efficient code.</pre>
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