Optimize skip_slice by recording serialized length

[Gankra]

For @jrmuizel to test.

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This change is 

[glennw]

@gankro do we want to review / merge this?

[glennw]

@gankro do we want to review / merge this?

[Gankra]

Still need to do perf eval, gotten distracted the last few days.

[jrmuizel]

I tried performance testing this. I couldn't get solid numbers. WebRender seems to have some weird bimodal timing behaviour that makes measuring header.

[glennw]

@gankro I can try to do some profiling with this tomorrow, if that would be helpful?

[glennw]

I did some simple profiling on one of our benchmarks, with the following:

vblank_mode=0 ../target/release/wrench -r show benchmarks/text-rendering.yaml

This uses -r to replay the display list every frame, and vblank_mode=0 disables vsync on Intel GPUs. The benchmark draws 64 reasonably long text runs of varying colors and font sizes, so it contains a lot of glyphs in the display list.

Without: ~325 fps.
With: ~385 fps.

So, at least in this test case, it's a very significant win. It'd be interesting to see if others can reproduce similar results.

With this patch, this is what the profile graph looks look:

The two areas I've marked with a red dashed line are time spent inside deserialization functions / iterators, so hopefully there's still quite a lot of wins to be found there. There's certainly a couple of other big targets in that profile I can look at performance-wise in the future too.

[Gankra]

Wow that's really surprising since text is one of the slices that shouldn't actually need this optimization (with a Sufficiently Smart Compiler).

Seems pretty convincing to me it's a good win. Let's ship it.

[glennw]

@bors-servo r+

[bors-servo]

📌 Commit 13bfa65 has been approved by glennw

[bors-servo]

⌛ Testing commit 13bfa65 with merge 517ef53...

[bors-servo]

☀️ Test successful - status-appveyor, status-travis
Approved by: glennw
Pushing 517ef53 to master...

[jrmuizel]

serde-rs/serde#855 is probably the next place where we're going to get deserialization wins. @gankro will look at it when he gets a chance.