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[Bug tree-optimization/59967] [4.8/4.9 Regression] Performance regression from 4.7.x to 4.8.x (loop not unrolled)
- From: "chbreitkopf at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: Thu, 03 Apr 2014 06:18:30 +0000
- Subject: [Bug tree-optimization/59967] [4.8/4.9 Regression] Performance regression from 4.7.x to 4.8.x (loop not unrolled)
- Auto-submitted: auto-generated
- References: <bug-59967-4 at http dot gcc dot gnu dot org/bugzilla/>
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59967
--- Comment #3 from Christoph Breitkopf <chbreitkopf at gmail dot com> ---
It's this conditional in the inner loop. The expression becomes constant only
if both loops are unrolled (i and j are the loop counters):
if (1<<j & i)
ck0[j] += s;
This condition is probably not perfectly predictable even by modern branch
predictors, so I assume this is the main reason for the regression.
Chris