https://llvm.org/bugs/show_bug.cgi?id=28128
Bug ID: 28128
Summary: auto vectorizer makes code run slower
Product: clang
Version: 3.8
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: -New Bugs
Assignee: unassignedclangb...@nondot.org
Reporter: albertnet...@gmail.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
```
#include <stdlib.h>
#include <stdio.h>
#include <time.h>
void tik()
{
static clock_t start, end;
static int flag = 0;
static float elapsed_time;
if (flag == 0) {
start = clock();
} else {
end = clock();
elapsed_time = (float)(end - start) / (float)CLOCKS_PER_SEC;
printf("Elapsed time: %f seconds\n", elapsed_time);
}
flag = 1 - flag;
}
typedef unsigned int uint;
static uint i_max = 64 * 1024 * 1024;
void test()
{
static uint acc[2] = {1, 1};
tik();
for (uint i = 0; i < i_max; ++i) {
acc[0] *= 3;
}
tik();
acc[0] = acc[1] = 1;
tik();
for (uint i = 0; i < i_max; ++i) {
acc[0] *= 3;
acc[1] *= 3;
}
tik();
acc[0] = acc[1] = 1;
tik();
#pragma clang loop vectorize(disable)
for (uint i = 0; i < i_max; ++i) {
acc[0] *= 3;
acc[1] *= 3;
}
tik();
acc[0] = acc[1] = 1;
tik();
for (uint i = 0; i < i_max; ++i) {
acc[0] *= 3;
}
for (uint i = 0; i < i_max; ++i) {
acc[1] *= 3;
}
tik();
}
int main (int argc, char** argv) {
test();
return 0;
}
```
Output:
```
$ clang -O vector.c; ./a.out
Elapsed time: 0.003569 seconds
Elapsed time: 0.011365 seconds
Elapsed time: 0.008421 seconds
Elapsed time: 0.005950 seconds
```
I was expecting the second case run ~2x slower than the first one, for it's
doing twice much work. The forth case's runtime is more reasonable.
ENV:
clang: 3.8.1-svn271127-1~exp1 (branches/release_38)
OS: xubuntu 14.04
CPU: Intel(R) Core(TM) i7-4600U CPU @ 2.10GHz
--
You are receiving this mail because:
You are on the CC list for the bug.
_______________________________________________
llvm-bugs mailing list
llvm-bugs@lists.llvm.org
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-bugs
