[llvm-dev] LLVM is getting faster, April edition (original) (raw)

Davide Italiano via llvm-dev llvm-dev at lists.llvm.org
Thu Apr 13 13:52:55 PDT 2017

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 [1], where it was shown that LLVM was getting slower over time. But now I'm happy to bring 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: InstCombine, 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 list of some ideas that can further help compile-time: - KnownBits Cache. InstCombine and other passes use known bits, which often happens to be pretty expensive. Hal posted a patch [2] 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 perform many expensive traversals over entire function/loop nest, and most of them are probably redundant. - Forming LCSSA. PR31851 reports that the current implementation of LCSSA forming can be expensive. A WIP patch [3] should address the problem, but 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

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