It looks like Shootout-C++ improved after that commit, if I'm reading these results correctly: http://llvm.org/perf/db_default/v4/nts/87812
> On 2016-Jul-17, at 15:16, Eric Fiselier <e...@efcs.ca> wrote: > > FYI, > > I recommitted the previously removed optimization in r275734. I believe it > should fix the LNT performance regression. The benchmarks I used to test this > change can be found in D22240 (https://reviews.llvm.org/D22240#3266819f) > > /Eric > > On Sun, Jul 17, 2016 at 3:09 PM, Eric Fiselier <e...@efcs.ca> wrote: > > > On Sun, Jul 17, 2016 at 2:54 PM, Arthur O'Dwyer <arthur.j.odw...@gmail.com> > wrote: > Given that this patch is basically Chandler's talk from CppCon 2015 > (https://www.youtube.com/watch?v=nXaxk27zwlk), I'm surprised that the commit > message isn't explicitly mentioning that; and surprised that Chandler himself > isn't weighing in on either the "this is a good idea" or "this is a bad idea" > side. > > > The commit message should have been clearer. Hopefully Chandler will weigh in > one way or another. > > > IMHO, if replacing "%" with "fastmod" in general-purpose code like this were > a good idea, > (A) libc++ should introduce a helper function __fastmod(m,n) for the purpose, > not repeat the same patch everywhere there's currently a "%" operator; and/or > (B) someone with authority over the Clang x86 backend > (*cough*Chandler*cough*) should look into improving the codegen for "%" by > auto-detecting when it might make sense to use this heuristic. > > The alleged existence of performance regressions on this patch as it stands > seems like evidence for doing (B), IMHO, even if it takes longer. > > The performance regressions are almost certainly related to the optimization > I removed/replaced in this patch. Previously when __hash_table::find was > walking the bucket elements it would only re-computer the constrained hash > if the unconstrained hash didn't match that of the element being searched > for. I removed this optimization in this patch with the intent of > re-implementing it in the very near future (it was only in-tree for about a > week). > > What I'm curious about if this performance regression was cause by (1) the > removal of this 2 week old optimization or (2) the implementation of "fast > mod". I suspect it's because of (1). > > I'm writing more benchmarks as we speak to figure this out. > > > > It's also counterintuitive to me that (__h < __bc) would be true any > significant fraction of the time, on a 64-bit platform. Does this happen > because __bc is often astronomically high, or because __h is often > astronomically low (presumably due to bad hash functions, such as "always > hash to constant 0")? > > It happens due to bad hash functions. Primarily because integral types use > the identify hash function. > > > my $.02, > Arthur > > > On Sun, Jul 17, 2016 at 1:11 PM, Eric Fiselier via cfe-commits > <cfe-commits@lists.llvm.org> wrote: > Hi Duncan, > > It's possibly expected. It depends on what operation it's performing. I > expected a bit of a performance drop in some cases but I have a plan to fix > those. > Do you have a link to LNT? > > On Wed, Jul 13, 2016 at 6:41 PM, Duncan P. N. Exon Smith > <dexonsm...@apple.com> wrote: > Hmm. I implied there were other regressions, but I just finished scanning > them. Shootout-C++/hash2 is the only major one. The others were small, and > only at -O0. > > > On 2016-Jul-13, at 17:38, Duncan P. N. Exon Smith via cfe-commits > > <cfe-commits@lists.llvm.org> wrote: > > > > We saw mixed results from this on LNT, including some major regressions. > > For example, on x86_64, SingleSource/Benchmarks/Shootout-C++/hash2 > > regressed 18.5% at -O3 and over 20% at -Os. > > > > Is this expected? > > ^ Still interested in an answer, though ;). > > > > >> On 2016-Jul-11, at 15:02, Eric Fiselier via cfe-commits > >> <cfe-commits@lists.llvm.org> wrote: > >> > >> Author: ericwf > >> Date: Mon Jul 11 17:02:02 2016 > >> New Revision: 275114 > >> > >> URL: http://llvm.org/viewvc/llvm-project?rev=275114&view=rev > >> Log: > >> Don't compute modulus of hash if it is smaller than the bucket count. > >> > >> This cleans up a previous optimization attempt in hash, and results in > >> additional performance improvements over that previous attempt. > >> Additionally > >> this new optimization does not hinder the power of 2 bucket count > >> optimization. > >> > >> Modified: > >> libcxx/trunk/include/__hash_table > >> > >> Modified: libcxx/trunk/include/__hash_table > >> URL: > >> http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/__hash_table?rev=275114&r1=275113&r2=275114&view=diff > >> ============================================================================== > >> --- libcxx/trunk/include/__hash_table (original) > >> +++ libcxx/trunk/include/__hash_table Mon Jul 11 17:02:02 2016 > >> @@ -90,7 +90,8 @@ inline _LIBCPP_INLINE_VISIBILITY > >> size_t > >> __constrain_hash(size_t __h, size_t __bc) > >> { > >> - return !(__bc & (__bc - 1)) ? __h & (__bc - 1) : __h % __bc; > >> + return !(__bc & (__bc - 1)) ? __h & (__bc - 1) : > >> + (__h < __bc ? __h : __h % __bc); > >> } > >> > >> inline _LIBCPP_INLINE_VISIBILITY > >> @@ -2201,8 +2202,7 @@ __hash_table<_Tp, _Hash, _Equal, _Alloc> > >> if (__nd != nullptr) > >> { > >> for (__nd = __nd->__next_; __nd != nullptr && > >> - (__hash == __nd->__hash_ > >> - || __constrain_hash(__nd->__hash_, __bc) == __chash); > >> + __constrain_hash(__nd->__hash_, __bc) == __chash; > >> __nd = > >> __nd->__next_) > >> { > >> if ((__nd->__hash_ == __hash) && key_eq()(__nd->__value_, > >> __k)) > >> @@ -2231,8 +2231,7 @@ __hash_table<_Tp, _Hash, _Equal, _Alloc> > >> if (__nd != nullptr) > >> { > >> for (__nd = __nd->__next_; __nd != nullptr && > >> - (__hash == __nd->__hash_ > >> - || __constrain_hash(__nd->__hash_, __bc) == __chash); > >> + __constrain_hash(__nd->__hash_, __bc) == __chash; > >> __nd = > >> __nd->__next_) > >> { > >> if ((__nd->__hash_ == __hash) && key_eq()(__nd->__value_, > >> __k)) > >> > >> > >> _______________________________________________ > >> cfe-commits mailing list > >> cfe-commits@lists.llvm.org > >> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits > > > > _______________________________________________ > > cfe-commits mailing list > > cfe-commits@lists.llvm.org > > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits > > > > _______________________________________________ > cfe-commits mailing list > cfe-commits@lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits > > > > _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
