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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Field ordering causes extra memcpy"
href="https://bugs.llvm.org/show_bug.cgi?id=39844">39844</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Field ordering causes extra memcpy
</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>
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<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>The following program:
#include <stdlib.h>
struct SV {
size_t capacity;
size_t disc;
size_t data[40];
static SV make() {
SV ret;
ret.capacity = 0;
ret.disc = 0;
return ret;
}
};
struct L {
SV a;
};
template<class T>
struct Allocation {
T *vec;
void init(T s) {
*vec = s;
}
};
void bar(Allocation<L> a, double g) {
L s = { SV::make() };
a.init(s);
}
produces:
bar(Allocation<L>, double): # @bar(Allocation<L>, double)
sub rsp, 344
xorps xmm0, xmm0
movaps xmmword ptr [rsp], xmm0
mov rsi, rsp
mov edx, 336
call memcpy
add rsp, 344
ret
Moving the capacity field from the beginning of the struct to the end gives the
more desirable:
bar(Allocation<L>, double): # @bar(Allocation<L>, double)
mov qword ptr [rdi], 0
mov qword ptr [rdi + 328], 0
retf</pre>
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