[loops] Should "unswitch" trivial conditions #1079
Description
| Bugzilla Link | 707 |
| Resolution | FIXED |
| Resolved on | Feb 22, 2010 12:45 |
| Version | 1.0 |
| OS | All |
Extended Description
Consider this simple case:
void test(int N, int* P) {
int i, j;
for (j = 0; j < N; ++j)
for (i = 0; i < N; ++i)
P[i+j] = 0;
}
The inner loop does a comparison of N against 0 to see if it should enter the inner loop. This test is
invariant in the outer loop (and even shared) so it is hoisted. Despite this, we still do the comparison
each time through the outer loop. If the comparison can't succeed, and if the outer loop does no other
work, we should do a trivial unswitch on that value.
This is particularly important if the outer loop has many more iterations than the inner loop, such as in
173.applu and other programs.
-Chris