<html>
<head>
<base href="http://llvm.org/bugs/" />
</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 --- - Inefficient XOR calculation"
href="http://llvm.org/bugs/show_bug.cgi?id=16671">16671</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Inefficient XOR calculation
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>3.2
</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>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>nruslan_devel@yahoo.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>The backend seems to be not smart enough to generate good code for the example
below. The line '(a >> 63) ^ a' will do NOT operation if the operand is
negative. Also suppose it already preceeds by a line which makes sure that the
operand is non-negative, i.e. '(a >> 63) ^ a' should be optimized to just 'a'.
long long func(long long a)
{
if (a < 0) /* Condition, presumably somewhere before the code below */
return 0;
return (a >> 63 ) ^ a;
}
Clang produces the following code:
testq %rdi, %rdi
js .LBB0_1
movq %rdi, %rax
sarq $63, %rax
xorq %rdi, %rax
ret
.LBB0_1:
xorl %eax, %eax
ret
GCC produces optimized code:
xorl %eax, %eax
testq %rdi, %rdi
cmovns %rdi, %rax
ret
Similar inefficiency if we reverse condition, i.e. now it should simply do NOT
operation:
long long func(long long a)
{
if (a >= 0) /* Condition, presumably somewhere before the code below */
return 0;
return (a >> 63 ) ^ a;
}
Clang produces:
testq %rdi, %rdi
js .LBB0_2
xorl %eax, %eax
ret
.LBB0_2:
movq %rdi, %rax
sarq $63, %rax
xorq %rdi, %rax
ret
GCC produces:
movq %rdi, %rdx
xorl %eax, %eax
testq %rdi, %rdi
notq %rdx
cmovs %rdx, %rax
ret</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>