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<base href="https://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Large lookup table could be packed into 64-bit integer"
href="https://llvm.org/bugs/show_bug.cgi?id=30531">30531</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Large lookup table could be packed into 64-bit integer
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>3.8
</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>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>skvadrik@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Consider the following source code (1.c):
int f(char c)
{
switch (c) {
case 'A'...'Z':
case 'a'...'z':
return 1;
default:
return 0;
}
}
$ clang-3.8 -O2 -c 1.c -o 1.o && objdump -dr 1.o`
0000000000000000 <f>:
0: 83 c7 bf add $0xffffffbf,%edi
3: 83 ff 39 cmp $0x39,%edi
6: 77 0b ja 13 <f+0x13>
8: 48 63 c7 movslq %edi,%rax
b: 8b 04 85 00 00 00 00 mov 0x0(,%rax,4),%eax
e: R_X86_64_32S .rodata
12: c3 retq
13: 31 c0 xor %eax,%eax
15: c3 retq
Clang generates 232-byte lookup table in .rodata. For comparison, gcc packs
lookup table in one 64-bit integer:
$ gcc-5.4.0 -O2 -c 1.c -o 1.o && objdump -dr 1.o`
0000000000000000 <f>:
0: 8d 4f bf lea -0x41(%rdi),%ecx
3: 31 c0 xor %eax,%eax
5: 80 f9 39 cmp $0x39,%cl
8: 77 18 ja 22 <f+0x22>
a: b8 01 00 00 00 mov $0x1,%eax
f: 48 ba ff ff ff 03 ff movabs $0x3ffffff03ffffff,%rdx
16: ff ff 03
19: 48 d3 e0 shl %cl,%rax
1c: 48 85 d0 test %rdx,%rax
1f: 0f 95 c0 setne %al
22: 0f b6 c0 movzbl %al,%eax
25: c3 retq</pre>
</div>
</p>
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