RFR (S): 8067014: LinearScan::is_sorted significantly slows down fastdebug builds' performance

[Aleksey Shipilev]

Hi Filipp,

On 02/18/2016 06:31 PM, Filipp Zhinkin wrote:
> I've looked at how frequently misses are actually occur and
> how far false positives are from the interval we're looking for.
> 
> Also I've tried to implement interval_cmp so that it returns 0
> if difference between interval "from" values is below some threshold:
> http://cr.openjdk.java.net/~fzhinkin/8067014/webrev.02/stats.txt
> 
> All those misses with distance greater than 64 came from
> javax.swing.plaf.synth.SynthStyle::populateDefaultValues [1].

Okay, let me recap: your data seems to say that current binary search
misses in 25% of the cases, and around 95% of the misses are within 8
intervals from the false value, right? And almost all 99.99% are within
32 intervals?

This does not seem bad, I was afraid of orders of magnitude larger values.

> I've also looked to another possible slowness sources and
> we spend about 10% of time in LinearScan's verify_intervals method
> which checks that every two intervals don't simultaneously intersect
> and share the same register [2].
> 
> I don't see a way to significantly speed up such verification,
> but I've slightly improved performance by rearranging some expressions.

Yes, those re-arrangements make sense.

> Here is an updated webrev:
> http://cr.openjdk.java.net/~fzhinkin/8067014/webrev.02/

Looks good to me (not a Reviewer).

> Also, unless CommentedAssembly flag is explicitly turned off,
> we're generating comments for stubs even if we're not going to print it out.
> Avoiding comments generation in such case will speed up compilation a bit more,
> but I think it would be better to deal with it in a separate RFE.
> Difference in code emission time is about 30% when CommentedAssembly is off
> (~ 40s w/ CommentedAssembly, ~ 25s w/o CommentedAssembly).

Oh wow. Let's fix that! Of course, separate RFE is fine.

Cheers,
-Aleksey

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