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<base href="https://llvm.org/bugs/" />
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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 --- - [ppc] redundant memory store and load for vector::push_back"
href="https://llvm.org/bugs/show_bug.cgi?id=30556">30556</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>[ppc] redundant memory store and load for vector::push_back
</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>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Backend: PowerPC
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>carrot@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>When compile following source code with options
-m64 -O2 -mvsx -mcpu=power8 -std=c++11
#include <cstdint>
#include <vector>
#include <algorithm>
using std::vector;
typedef int64_t int64;
typedef uint64_t uint64;
struct O {
int64 f1;
int f2;
int f3;
};
class B {
public:
bool Add(int64 d, vector<uint64> p) {
const int s = f5.size();
O o;
o.f1 = d; // *
o.f2 = s; // *
o.f3 = p.size(); // *
f4.push_back(o); // *
return true;
}
int size() {
return f4.size();
}
vector<O> f4;
vector<uint64> f5;
};
vector<int64> foo();
int bar(vector<uint64>* y) {
auto h = foo();
B b;
std::for_each(h.rbegin(), h.rend(), [&](const int64& hit) {
b.Add(hit, *y);
});
return b.size();
}
LLVM generates following instructions for statements marked with *
...
.LBB0_8: #
%_ZNSt6vectorImSaImEEC2ERKS1_.exit.i.i
# in Loop: Header=BB0_2 Depth=1
ld 3, 128(31)
ld 4, 120(31)
ld 5, 112(31)
rldicl 6, 26, 61, 3
std 23, 168(31) // o.f1 = d;
stw 6, 180(31) // o.f3 = p.size();
sub 4, 3, 4
ld 3, 104(31)
rldicl 4, 4, 61, 3
stw 4, 176(31) // o.f2 = s;
cmpld 3, 5 // vector full?
beq 0, .LBB0_10
# BB#9: # %if.then.i.i.i.i1
# in Loop: Header=BB0_2 Depth=1
lxvd2x 0, 0, 28 // copy object o
stxvd2x 0, 0, 3 // to vector f4
ori 2, 2, 0
ld 3, 104(31)
addi 3, 3, 16
std 3, 104(31)
b .LBB0_11
...
Note that the several st instructions construct the local object o, lxvd2x
immediately load it into register, it is very slow due to store forwarding. At
the same time, the value of o are still in register r23,r6,r4, we can directly
store these registers to vector.
Or even better, it is not needed to construct the local object o in BB8, it is
only required in slow path and before calling
_ZNSt6vectorI1OSaIS0_EE19_M_emplace_back_auxIJRKS0_EEEvDpOT_.</pre>
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