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<base href="https://bugs.llvm.org/">
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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 - Potential code size reduction optimization: reusing function tails"
href="https://bugs.llvm.org/show_bug.cgi?id=33763">33763</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Potential code size reduction optimization: reusing function tails
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</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>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>jpakkane@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Suppose you have code that looks like this:
int func1();
int func2();
int func3();
int func4();
int func5() {
int i = 0;
i+=func2();
i+=func3();
i+=func4();
return i+func1();
}
int func6() {
int i=1;
i+=func3();
i+=func3();
i+=func4();
return i+func1();
}
It gets compiled to the following assembly when using -Os:
func5():
pushq %rbx
call func2()
movl %eax, %ebx
call func3()
addl %eax, %ebx
call func4()
addl %eax, %ebx
call func1()
addl %ebx, %eax
popq %rbx
ret
func6():
pushq %rbx
call func3()
leal 1(%rax), %ebx
call func3()
addl %eax, %ebx
call func4()
addl %eax, %ebx
call func1()
addl %ebx, %eax
popq %rbx
ret
However the ends of the two functions are identical. This could be compiled
into the following which is functionally the same but takes less space:
func5():
pushq %rbx
call func2()
movl %eax, %ebx
common_tail:
call func3()
addl %eax, %ebx
call func4()
addl %eax, %ebx
call func1()
addl %ebx, %eax
popq %rbx
ret
func6():
pushq %rbx
call func3()
leal 1(%rax), %ebx
jmp common_tail
Testing with compiler explorer says that neither GCC, Clang, MSVC nor ICC do
this optimization but some embedded compilers do.</pre>
</div>
</p>
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