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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Missed small switch optimizations"
href="https://bugs.llvm.org/show_bug.cgi?id=37368">37368</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Missed small switch optimizations
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</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>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Hello,
When we have small "switch"-es with two or three cases, clang could generate a
smarter code.
Example:
char * a(int e) {
switch (e) {
case 0:
return "0";
default:
return "default";
}
}
Clang's output:
a: # @a
push rbp
mov rbp, rsp
mov dword ptr [rbp - 12], edi
mov edi, dword ptr [rbp - 12]
test edi, edi
jne .LBB0_2
jmp .LBB0_1
.LBB0_1:
movabs rax, offset .L.str
mov qword ptr [rbp - 8], rax
jmp .LBB0_3
.LBB0_2:
movabs rax, offset .L.str.1
mov qword ptr [rbp - 8], rax
.LBB0_3:
mov rax, qword ptr [rbp - 8]
pop rbp
ret
Better code (GCC does it):
a:
test edi, edi
mov edx, OFFSET FLAT:.LC1
mov eax, OFFSET FLAT:.LC0
cmovne rax, rdx
ret
Also for this example:
char * a(int e) {
switch (e) {
case 0:
return "0";
case 1:
return "1";
default:
return "default";
}
}
A better code can be generated like:
a:
mov eax, OFFSET FLAT:.LC1
test edi, edi
je .L1
cmp edi, 1
mov eax, OFFSET FLAT:.LC2
mov edx, OFFSET FLAT:.LC0
cmovne rax, rdx
.L1:
ret
Thanks</pre>
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