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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 - Inline memcpy/memmove/memset with unknown size but bounded by derefenceable info"
href="https://bugs.llvm.org/show_bug.cgi?id=43888">43888</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Inline memcpy/memmove/memset with unknown size but bounded by derefenceable info
</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>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>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define C 8
int d[C];
int s[C];
__attribute__((noinline))
void old(int *d, int *s, int n) {
memcpy(d, s, n*sizeof(int));
}
__attribute__((noinline))
void new(int *d, int *s, int n) {
switch(n) {
case 8: memcpy(d, s, n*sizeof(int)); break;
case 7: memcpy(d, s, n*sizeof(int)); break;
case 6: memcpy(d, s, n*sizeof(int)); break;
case 5: memcpy(d, s, n*sizeof(int)); break;
case 4: memcpy(d, s, n*sizeof(int)); break;
case 3: memcpy(d, s, n*sizeof(int)); break;
case 2: memcpy(d, s, n*sizeof(int)); break;
case 1: memcpy(d, s, n*sizeof(int)); break;
case 0: break;
}
}
int main(void) {
for (int i = 0; i < 1000000000; ++i) {
new(d, s, C);
}
}
Idea:
Analyze memcpy's dst an src buffers - for example: infer that sizeof(dst) is 8
ints, sizeof(dst) is 8 ints. Replace small memcpy with jump table with inlined
memcpys (see "new" function).
Performance improvements:
For code above:
time ./old.out
real 0m2,535s
user 0m2,534s
sys 0m0,000s
time ./new.out
real 0m1,981s
user 0m1,981s
sys 0m0,000s
Or C = 8 ints and copy 6 ints.
time ./old.out
real 0m3,123s
user 0m3,123s
sys 0m0,000s
time ./new.out
real 0m1,979s
user 0m1,973s
sys 0m0,004s</pre>
</div>
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