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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 - GCC does a better job with returning large structs"
href="https://bugs.llvm.org/show_bug.cgi?id=35134">35134</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>GCC does a better job with returning large structs
</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>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>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>jmuizelaar@mozilla.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>struct Foo {
int o[200];
};
struct Bar {
char p;
Foo f;
};
__attribute__((noinline))
Foo moo()
{
return {0};
}
void goo(Bar *f)
{
f->f = moo();
}
with -O3 -fno-exceptions -fomit-frame-pointer gives:
moo(): # @moo()
push rbx
mov rbx, rdi
xor esi, esi
mov edx, 800
call memset
mov rax, rbx
pop rbx
ret
goo(Bar*): # @goo(Bar*)
push r14
push rbx
sub rsp, 808
mov rbx, rdi
lea r14, [rsp + 8]
mov rdi, r14
call moo()
add rbx, 4
mov edx, 800
mov rdi, rbx
mov rsi, r14
call memcpy
add rsp, 808
pop rbx
pop r14
ret
in clang vs gcc's:
moo():
mov rdx, rdi
mov QWORD PTR [rdi], 0
mov QWORD PTR [rdi+792], 0
lea rdi, [rdi+8]
mov rcx, rdx
xor eax, eax
and rdi, -8
sub rcx, rdi
add ecx, 800
shr ecx, 3
rep stosq
mov rax, rdx
ret
goo(Bar*):
add rdi, 4
call moo()
rep ret
Notice the elided memcpy in goo()</pre>
</div>
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