Date: Fri, 10 Aug 2012 21:18:03 +0400 From: Solar Designer <solar@...nwall.com> To: musl@...ts.openwall.com Subject: Re: crypt_blowfish integration, optimization On Thu, Aug 09, 2012 at 06:32:59PM -0400, Rich Felker wrote: > On Fri, Aug 10, 2012 at 02:21:03AM +0400, Solar Designer wrote: > > Hmm, for me "gcc -Q -O2 --help=optimizers" and ditto for -O3 both show > > "disabled" for -funroll-loops. Why was the loop unrolled for you? > > Not sure. I've found -Q --help=optimizers completely unreliable in the > past though. It only reports minimal differences between -Os, -O2, and > -O3, and trying to start with -O3 and reproduce -Os by just changing > the options that are different does not give effects even remotely > similar to -Os. Frankly, this matches my experience. OK, -Q --help=optimizers is unreliable. But is -O3 supposed to include -funroll-loops now? Does it? Or did you get loop unrolling done for some other reason? I think this needs to be understood by us. > > As discussed, the problem with avoiding such hand-unrolls is that the > > compiler doesn't know just which loops are most important to unroll. > > My experience has been that it tends to make good decisions overall, Yes, good decisions overall - like measured in terms of geometric mean or median for performance change across many functions (I wrote a script called relbench that reports such measurements for JtR builds) - but sometimes poor decisions for individual performance-critical functions. So hand-unrolling in those special cases helps. > and that if somebody is using -Os, they really want smallest size, not > performance. Maybe, however: So far, -Os was often providing good performance as well, on par with -O2. IIRC, in the relbench tests mentioned above, it was 92% of -O2 on gcc 4.6 on x86_64 for the geometric mean across about 150 separate benchmark results, but in some cases -Os code was actually faster than -O2. So someone using -Os may want nearly optimal code that is also slightly smaller. If for some function we get a more than ~8% hit with -Os vs. -O3 (or whatever does the unrolling), this means that the function could use some hand-optimization to fix that. Alexander
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