<html>
<head>
<base href="https://bugs.llvm.org/">
</head>
<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Missed optimization: unnecessary move with division-by-multiplication"
href="https://bugs.llvm.org/show_bug.cgi?id=45868">45868</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Missed optimization: unnecessary move with division-by-multiplication
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>10.0
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<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>josephcsible@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>When Clang does division-by-multiplication, it seems to forget that
multiplication is commutative. It moves the numerator into %rax, but if it
moved the magic number there instead, then it wouldn't have to move the
numerator at all.
Input:
unsigned long f(unsigned long x) {
return x / 7;
}
Actual assembly:
f:
movabsq $2635249153387078803, %rcx
movq %rdi, %rax
mulq %rcx
subq %rdx, %rdi
shrq %rdi
leaq (%rdi,%rdx), %rax
shrq $2, %rax
retq
Desired assembly:
f:
movabsq $2635249153387078803, %rax
mulq %rdi
subq %rdx, %rdi
shrq %rdi
leaq (%rdi,%rdx), %rax
shrq $2, %rax
retq</pre>
</div>
</p>
<hr>
<span>You are receiving this mail because:</span>
<ul>
<li>You are on the CC list for the bug.</li>
</ul>
</body>
</html>