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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Arithmetic shift optimization for signed numbers"
href="https://bugs.llvm.org/show_bug.cgi?id=36760">36760</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Arithmetic shift optimization for signed numbers
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>6.0
</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>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>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>This was reported a while ago, but seems clang/llvm still does not optimize
this case well. (For the examples below -O2 option is used)
1. Consider the following example:
long long func(long long a)
{
if (a < 0)
return 0;
return (a >> 63 ) ^ a;
}
Basically, since a is already checked to be non-negative, (a >> 63) must be 0.
Therefore, the shift operation should be optimized out.
clang/llvm 6.0 generates the following:
movq %rdi, %rcx
sarq $63, %rcx
xorq %rdi, %rcx
xorl %eax, %eax
testq %rdi, %rdi
cmovnsq %rcx, %rax
retq
whereas gcc can optimize it well:
testq %rdi, %rdi
movl $0, %eax
cmovns %rdi, %rax
ret
2. Another example is when we reverse the condition:
long long func(long long a)
{
if (a >= 0)
return 0;
return (a >> 63 ) ^ a;
}
In this case, again, the shift is unnecessary since (a >> 63) are all 1's.
Therefore, return statement is basically ~a.
clang/llvm:
movq %rdi, %rcx
sarq $63, %rcx
xorq %rdi, %rcx
xorl %eax, %eax
testq %rdi, %rdi
cmovsq %rcx, %rax
retq
gcc:
movq %rdi, %rax
testq %rdi, %rdi
movl $0, %edx
notq %rax
cmovns %rdx, %rax
ret</pre>
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