[llvm-dev] LLVM is getting faster, April edition
Daniel Berlin via llvm-dev
llvm-dev at lists.llvm.org
Mon Apr 17 21:41:03 PDT 2017
Is it just renaming you need, or general updating?
The RewriteUse call there is one that inserts phis, so i'm not sure
renaming alone will help.
On Thu, Apr 13, 2017 at 2:48 PM, Davide Italiano <davide at freebsd.org> wrote:
> On Thu, Apr 13, 2017 at 1:52 PM, Davide Italiano <davide at freebsd.org>
> > On Tue, Apr 11, 2017 at 7:52 PM, Mikhail Zolotukhin via llvm-dev
> > <llvm-dev at lists.llvm.org> wrote:
> >> Hi,
> >> It's been a while since I sent the last compile time report , where
> >> was shown that LLVM was getting slower over time. But now I'm happy to
> >> some good news: finally, LLVM is getting faster, not slower :)
> > Thanks a lot for the update, this is very good to hear :)
> >> *** Current status ***
> >> Many areas of LLVM have been examined and improved since then:
> >> SCEV, APInt implementation, and that resulted in almost 10% improvement
> >> compared to January compiler. I remeasured compile time data for CTMark
> >> tests and annotated the biggest changes, the graphs for Os and O0-g are
> >> attached below. Thick black line represents geomean, colored thin lines
> >> represent individual tests. The data is normalized on the first
> revision in
> >> the range (which is ~Jun, 2015).
> >> *** Future work ***
> >> There are still plenty of opportunities to make LLVM faster. Here is a
> >> of some ideas that can further help compile-time:
> >> - KnownBits Cache. InstCombine and other passes use known bits, which
> >> happens to be pretty expensive. Hal posted a patch  that implements a
> >> cache for known bits, but there are still some issues to fix there.
> >> - SCEV. Some parts of SCEV still need to be improved. For instance,
> >> createAddRecFromPHI function seems to be very inefficient: it can
> >> many expensive traversals over entire function/loop nest, and most of
> >> are probably redundant.
> >> - Forming LCSSA. PR31851 reports that the current implementation of
> >> forming can be expensive. A WIP patch  should address the problem,
> >> probably there are more to be improved here.
> > https://reviews.llvm.org/rL300255 is a first step towards the goal.
> > For some large tests, LCSSA is still slow, and that's due to a lot of
> > time spent in the updater. I'll try to fix that one next.
> > --
> > Davide
> To clarify, here's the profiler output
> I think there's not much we can do (easily) for `getExitBlocks()` as
> we already cache the call for each loop in `fromLCSSAfoInstructions`
> but switching to a faster renamer should help. Dan, do you think it's
> possible to move your O(def + use) renamer out of PredicateInfo to a
> common file and use that here?
> "There are no solved problems; there are only problems that are more
> or less solved" -- Henri Poincare
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