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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Field ordering still causes extra memcpy"
href="https://bugs.llvm.org/show_bug.cgi?id=40574">40574</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Field ordering still 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>
<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>Even with <a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED FIXED - Field ordering causes extra memcpy"
href="show_bug.cgi?id=39844">bug 39844</a> fixed we can still get extra memcpys depending on field
ordering:
#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;
SV b;
};
template<class T>
struct Allocation {
T *vec;
void init(T s) {
*vec = s;
}
};
void bar(Allocation<L> a, double g) {
L s = { SV::make(), SV::make() };
a.init(s);
}
produces
bar(Allocation<L>, double): # @bar(Allocation<L>, double)
subq $680, %rsp # imm = 0x2A8
xorps %xmm0, %xmm0
movaps %xmm0, (%rsp)
movaps %xmm0, 336(%rsp)
movq %rsp, %rsi
movl $672, %edx # imm = 0x2A0
callq memcpy
addq $680, %rsp # imm = 0x2A8
retq
but moving capacity to the end gives:
bar(Allocation<L>, double): # @bar(Allocation<L>, double)
movq $0, (%rdi)
xorps %xmm0, %xmm0
movups %xmm0, 328(%rdi)
movq $0, 664(%rdi)
retq</pre>
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