Speed up `plug_leaks` #36917

nnethercote · 2016-10-03T01:18:34Z

Profiling shows that plug_leaks and the functions it calls are hot on some benchmarks. It's very common that skol_map is empty in this function, and we can specialize plug_leaks in that case for some big speed-ups.

The PR has two commits. I'm fairly confident that the first one is correct -- I traced through the code to confirm that the fold_regions and pop_skolemized calls are no-ops when skol_map is empty, and I also temporarily added an assertion to check that result ends up having the same value as value in that case. This commit is responsible for most of the improvement.

I'm less confident about the second commit. The call to resolve_type_vars_is_possible can change value when skol_map is empty... but testing suggests that it doesn't matter if the call is
omitted.

So, please check both patches carefully, especially the second one!

Here are the speed-ups for the first commit alone.

stage1 compiler (built with old rustc, using glibc malloc), doing debug builds:

futures-rs-test  4.710s vs  4.538s --> 1.038x faster (variance: 1.009x, 1.005x)
issue-32062-equ  0.415s vs  0.368s --> 1.129x faster (variance: 1.009x, 1.010x)
issue-32278-big  1.884s vs  1.808s --> 1.042x faster (variance: 1.020x, 1.017x)
jld-day15-parse  1.907s vs  1.668s --> 1.143x faster (variance: 1.011x, 1.007x)
piston-image-0. 13.024s vs 12.421s --> 1.049x faster (variance: 1.004x, 1.012x)
rust-encoding-0  3.335s vs  3.276s --> 1.018x faster (variance: 1.021x, 1.028x)

stage2 compiler (built with new rustc, using jemalloc), doing debug builds:

futures-rs-test  4.167s vs  4.065s --> 1.025x faster (variance: 1.006x, 1.018x)
issue-32062-equ  0.383s vs  0.343s --> 1.118x faster (variance: 1.012x, 1.016x)
issue-32278-big  1.680s vs  1.621s --> 1.036x faster (variance: 1.007x, 1.007x)
jld-day15-parse  1.671s vs  1.478s --> 1.131x faster (variance: 1.016x, 1.004x)
piston-image-0. 11.336s vs 10.852s --> 1.045x faster (variance: 1.003x, 1.006x)
rust-encoding-0  3.036s vs  2.971s --> 1.022x faster (variance: 1.030x, 1.032x)

I've omitted the benchmarks for which the change was negligible.

And here are the speed-ups for the first and second commit in combination.

stage1 compiler (built with old rustc, using glibc malloc), doing debug
builds:

futures-rs-test  4.684s vs  4.498s --> 1.041x faster (variance: 1.012x, 1.012x)
issue-32062-equ  0.413s vs  0.355s --> 1.162x faster (variance: 1.019x, 1.006x)
issue-32278-big  1.869s vs  1.763s --> 1.060x faster (variance: 1.013x, 1.018x)
jld-day15-parse  1.900s vs  1.602s --> 1.186x faster (variance: 1.010x, 1.003x)
piston-image-0. 12.907s vs 12.352s --> 1.045x faster (variance: 1.005x, 1.006x)
rust-encoding-0  3.254s vs  3.248s --> 1.002x faster (variance: 1.063x, 1.045x)

stage2 compiler (built with new rustc, using jemalloc), doing debug builds:

futures-rs-test  4.183s vs  4.046s --> 1.034x faster (variance: 1.007x, 1.004x)
issue-32062-equ  0.380s vs  0.340s --> 1.117x faster (variance: 1.020x, 1.003x)
issue-32278-big  1.671s vs  1.616s --> 1.034x faster (variance: 1.031x, 1.012x)
jld-day15-parse  1.661s vs  1.417s --> 1.172x faster (variance: 1.013x, 1.005x)
piston-image-0. 11.347s vs 10.841s --> 1.047x faster (variance: 1.007x, 1.010x)
rust-encoding-0  3.050s vs  3.000s --> 1.017x faster (variance: 1.016x, 1.012x)

@eddyb: git blame suggests that you should review this. Thanks!

rust-highfive · 2016-10-03T01:18:46Z

r? @nikomatsakis

(rust_highfive has picked a reviewer for you, use r? to override)

nnethercote · 2016-10-03T01:19:48Z

BTW, I used Valgrind's "DHAT" profilter to find this. It told me that lots of malloc calls were happening in and beneath plug_leaks.

nnethercote · 2016-10-03T01:22:51Z

r? @eddyb

eddyb · 2016-10-03T02:02:30Z

src/librustc/infer/higher_ranked/mod.rs

-        debug!("plug_leaks: result={:?}",
-               result);
+        if skol_map.is_empty() {
+            result = value;


I think you can use early return to avoid indenting the entire function to the right.

This commit avoids the `fold_regions` call in `plug_leaks` when `skol_map` is empty, which is the common case. This gives speed-ups of up to 1.14x on some of the rustc-benchmarks.

This commit avoids the `resolve_type_vars_if_possible` call in `plug_leaks` when `skol_map` is empty, which is the common case. It also changes the signature of `plug_leaks` slightly to avoid the need for a `clone` of `value`. These changes give speed-ups of up a few percent on some of the rustc-benchmarks.

eddyb · 2016-10-03T02:43:02Z

@bors r+

bors · 2016-10-03T02:43:03Z

📌 Commit 3779971 has been approved by eddyb

arielb1 · 2016-10-03T12:01:00Z

We should also be doing this for leak_check.

nnethercote · 2016-10-03T22:33:14Z

We should also be doing this for leak_check.

leak_check doesn't show up nearly as high as plug_leaks in profiles, but it does show up. I'll do it in a follow-up. Thank you for the suggestion.

arielb1 · 2016-10-04T08:35:44Z

leak-check done by #36931.

nnethercote · 2016-10-04T09:45:33Z

leak-check done by #36931.

Huh. I have a local patch to add an early return to leak_check. It produces a measurable speed-up of a couple of percent on some of the rustc-benchmarks, but it doesn't look anything like the commits in #36931.

@eddyb

…eddyb Speed up `plug_leaks` Profiling shows that `plug_leaks` and the functions it calls are hot on some benchmarks. It's very common that `skol_map` is empty in this function, and we can specialize `plug_leaks` in that case for some big speed-ups. The PR has two commits. I'm fairly confident that the first one is correct -- I traced through the code to confirm that the `fold_regions` and `pop_skolemized` calls are no-ops when `skol_map` is empty, and I also temporarily added an assertion to check that `result` ends up having the same value as `value` in that case. This commit is responsible for most of the improvement. I'm less confident about the second commit. The call to `resolve_type_vars_is_possible` can change `value` when `skol_map` is empty... but testing suggests that it doesn't matter if the call is omitted. So, please check both patches carefully, especially the second one! Here are the speed-ups for the first commit alone. stage1 compiler (built with old rustc, using glibc malloc), doing debug builds: ``` futures-rs-test 4.710s vs 4.538s --> 1.038x faster (variance: 1.009x, 1.005x) issue-32062-equ 0.415s vs 0.368s --> 1.129x faster (variance: 1.009x, 1.010x) issue-32278-big 1.884s vs 1.808s --> 1.042x faster (variance: 1.020x, 1.017x) jld-day15-parse 1.907s vs 1.668s --> 1.143x faster (variance: 1.011x, 1.007x) piston-image-0. 13.024s vs 12.421s --> 1.049x faster (variance: 1.004x, 1.012x) rust-encoding-0 3.335s vs 3.276s --> 1.018x faster (variance: 1.021x, 1.028x) ``` stage2 compiler (built with new rustc, using jemalloc), doing debug builds: ``` futures-rs-test 4.167s vs 4.065s --> 1.025x faster (variance: 1.006x, 1.018x) issue-32062-equ 0.383s vs 0.343s --> 1.118x faster (variance: 1.012x, 1.016x) issue-32278-big 1.680s vs 1.621s --> 1.036x faster (variance: 1.007x, 1.007x) jld-day15-parse 1.671s vs 1.478s --> 1.131x faster (variance: 1.016x, 1.004x) piston-image-0. 11.336s vs 10.852s --> 1.045x faster (variance: 1.003x, 1.006x) rust-encoding-0 3.036s vs 2.971s --> 1.022x faster (variance: 1.030x, 1.032x) ``` I've omitted the benchmarks for which the change was negligible. And here are the speed-ups for the first and second commit in combination. stage1 compiler (built with old rustc, using glibc malloc), doing debug builds: ``` futures-rs-test 4.684s vs 4.498s --> 1.041x faster (variance: 1.012x, 1.012x) issue-32062-equ 0.413s vs 0.355s --> 1.162x faster (variance: 1.019x, 1.006x) issue-32278-big 1.869s vs 1.763s --> 1.060x faster (variance: 1.013x, 1.018x) jld-day15-parse 1.900s vs 1.602s --> 1.186x faster (variance: 1.010x, 1.003x) piston-image-0. 12.907s vs 12.352s --> 1.045x faster (variance: 1.005x, 1.006x) rust-encoding-0 3.254s vs 3.248s --> 1.002x faster (variance: 1.063x, 1.045x) ``` stage2 compiler (built with new rustc, using jemalloc), doing debug builds: ``` futures-rs-test 4.183s vs 4.046s --> 1.034x faster (variance: 1.007x, 1.004x) issue-32062-equ 0.380s vs 0.340s --> 1.117x faster (variance: 1.020x, 1.003x) issue-32278-big 1.671s vs 1.616s --> 1.034x faster (variance: 1.031x, 1.012x) jld-day15-parse 1.661s vs 1.417s --> 1.172x faster (variance: 1.013x, 1.005x) piston-image-0. 11.347s vs 10.841s --> 1.047x faster (variance: 1.007x, 1.010x) rust-encoding-0 3.050s vs 3.000s --> 1.017x faster (variance: 1.016x, 1.012x) ``` @eddyb: `git blame` suggests that you should review this. Thanks!

Rollup of 12 pull requests - Successful merges: #36798, #36878, #36902, #36903, #36908, #36916, #36917, #36921, #36928, #36938, #36941, #36951 - Failed merges:

arielb1 · 2016-10-04T11:50:42Z

Huh. I have a local patch to add an early return to leak_check. It produces a measurable speed-up of a couple of percent on some of the rustc-benchmarks, but it doesn't look anything like the commits in #36931.

How do you run the rustc benchmarks? I've been measuring bootstrap times. My patch short-cuts the fast path to leak_check at the callsites.

nnethercote · 2016-10-04T22:24:38Z

@arielb1: I updated my local repo to get #36931 and the speed-ups from my local patch have now evaporated, so I guess its effect overlapped with the effect of your commit. My patch was simpler, though, adding just a if skol_map.is_empty() { return Ok(()); } near the start of leak_check. I don't know the code well enough to tell if that's functionally equivalent to your commit.

As for running the benchmarks, I compared two stage1 compilers that were both configured with --enable-optimize --enable-debuginfo. I used the compare.py script for this, see https://internals.rust-lang.org/t/recent-rustc-benchmarks-improvements/4153 for instructions.

arielb1 · 2016-10-04T22:25:50Z

It should be functionally equivalent. Is it performance-equivalent (my patch skips a bunch of unneeded setup code)? [checking]

arielb1 · 2016-10-04T23:00:29Z

The leak_check patch seems to be slower. Will investigate more tomorrow.

nnethercote · 2016-10-08T21:19:20Z

Good news -- the improvement for jld-day15-parser showed up clearly on perf.rust-lang.org.

It's the big drop on the RHS on Oct 4. The time dropped from 1.402 to 1.199, which is a 1.169x speed-up (a.k.a a 14.5% reduction in execution time).

The other benchmarks mentioned in the first comment also saw speedups similar to what I measured locally, which is good.

rust-highfive assigned nikomatsakis Oct 3, 2016

rust-highfive assigned eddyb and unassigned nikomatsakis Oct 3, 2016

nnethercote mentioned this pull request Oct 3, 2016

compile-time: massive regression during 2015 #25069

Closed

eddyb suggested changes Oct 3, 2016

View reviewed changes

nnethercote added 2 commits October 3, 2016 13:32

Optimize plug_leaks.

1fece3d

This commit avoids the `fold_regions` call in `plug_leaks` when `skol_map` is empty, which is the common case. This gives speed-ups of up to 1.14x on some of the rustc-benchmarks.

nnethercote force-pushed the speed-up-plug_leaks branch from 6016e8c to 3779971 Compare October 3, 2016 02:42

eddyb approved these changes Oct 3, 2016

View reviewed changes

bluss mentioned this pull request Oct 3, 2016

Improve compile time of parser combinators such as Combine #36918

Closed

brson added the relnotes Marks issues that should be documented in the release notes of the next release. label Oct 4, 2016

Manishearth mentioned this pull request Oct 4, 2016

Rollup of 12 pull requests #36953

Merged

bors added a commit that referenced this pull request Oct 4, 2016

Auto merge of #36953 - Manishearth:rollup, r=Manishearth

4a9af01

Rollup of 12 pull requests - Successful merges: #36798, #36878, #36902, #36903, #36908, #36916, #36917, #36921, #36928, #36938, #36941, #36951 - Failed merges:

bors merged commit 3779971 into rust-lang:master Oct 4, 2016

nnethercote deleted the speed-up-plug_leaks branch October 4, 2016 21:52

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Speed up plug_leaks #36917

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Speed up `plug_leaks` #36917

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