[llvm-dev] LLVM is getting faster, April edition

[Davide Italiano via llvm-dev]

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