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Fix exponential projection complexity on nested types #48296

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merged 4 commits into from
Feb 25, 2018
Merged

Fix exponential projection complexity on nested types #48296

merged 4 commits into from
Feb 25, 2018

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ishitatsuyuki
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@ishitatsuyuki ishitatsuyuki commented Feb 17, 2018 •
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This implements solution 1 from #38528 (comment).

The code quality was extremely poor, but it has improved during review.

Blocking issues:

  • we probably don't want a quadratic deduplication for obligations.
  • is there an alternative to deduplication?

Based on #48315.

Needs changelog. Noticable improvement on compile time is expected.

Fix #38528
Close #39684
Close #43757

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r? @nikomatsakis

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@rust-highfive rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Feb 17, 2018
@ishitatsuyuki ishitatsuyuki changed the title [wip] (PoC) Fix exponential blowup on nested types [wip] (PoC) Fix exponential projection complexity on nested types Feb 17, 2018
This was referenced Feb 17, 2018
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ishitatsuyuki commented Feb 17, 2018
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Here are some self-reminders.

src/librustc/traits/project.rs Outdated
@@ -818,55 +822,74 @@ fn project_type<'cx, 'gcx, 'tcx>(
&obligation_trait_ref,
&mut candidates);

let decide_commit = |candidates: &mut ProjectionTyCandidateSet<'tcx>| {
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I have no way but to reorder this before the error handling. It looks non intuitive.

src/librustc/traits/project.rs Outdated
});
debug!("resulting candidate set: {:?}", candidates.vec);
if candidates.vec.len() != 1 {
candidates.ambiguous = true;
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This is nearly a hack.

src/librustc/traits/project.rs Outdated
}
};

match vtable {
match vtable.clone() {
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This copy can be avoided.

@kennytm kennytm added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Feb 17, 2018
@ishitatsuyuki ishitatsuyuki changed the title [wip] (PoC) Fix exponential projection complexity on nested types [wip] Fix exponential projection complexity on nested types Feb 18, 2018
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The other part of the blowup (typeck related) is yet to be solved, but I think the code is ready for a round of review. I have removed whitespace changes and clarified the description.

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@kennytm A reminder for you to change the review labels.

@kennytm kennytm added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Feb 18, 2018
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kennytm commented Feb 18, 2018

@bors try

Let’s do a perf check while we’re waiting for review :)

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bors commented Feb 18, 2018

⌛ Trying commit 3d81698 with merge e35a3273278915e65cf22237f934456a55a438b6...

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kennytm commented Feb 18, 2018

@ishitatsuyuki please remove the [wip] from the PR title if it is ready for review.

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Well, I'm currently tackling another part of the exponential issue (so this is WIP), but I need comments on improving the code organization.

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bors commented Feb 18, 2018

☀️ Test successful - status-travis
State: approved= try=True

@ishitatsuyuki ishitatsuyuki changed the title [wip] Fix exponential projection complexity on nested types Fix exponential projection complexity on nested types Feb 18, 2018
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By deduplicating the obligations the issue is completely resolved. I'm not sure if this approach is the best way though, and I copied the dumb deduplication becuase I didn't want to implement Ord everywhere. I will fix them later.

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@kennytm I need another try.

@ishitatsuyuki ishitatsuyuki mentioned this pull request Feb 18, 2018
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@Mark-Simulacrum Mark-Simulacrum added the relnotes Marks issues that should be documented in the release notes of the next release. label Feb 18, 2018
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@bors try

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bors commented Feb 18, 2018

⌛ Trying commit 4a40c55 with merge 4ac2425...

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@bors r+

@ishitatsuyuki -- it'd be great to add at least one of these exponential blow-up cases to the perf website so we can monitor for regressions. Perhaps @Mark-Simulacrum can walk you through it.

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bors commented Feb 22, 2018

📌 Commit 5a2bec9 has been approved by nikomatsakis

@bors bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Feb 22, 2018
Manishearth added a commit to Manishearth/rust that referenced this pull request Feb 24, 2018
@Manishearth
Rollup merge of rust-lang#48296 - ishitatsuyuki:exp-unblow, r=nikomat…
58af0c7 
…sakis

Fix exponential projection complexity on nested types

This implements solution 1 from rust-lang#38528 (comment).

The code quality is currently extremely poor, but we can improve them during review.

Blocking issues:

- we probably don't want a quadratic deduplication for obligations.
- is there an alternative to deduplication?

Based on rust-lang#48315.

Needs changelog. Noticable improvement on compile time is expected.

Fix rust-lang#38528
Close rust-lang#39684
Close rust-lang#43757
@Manishearth Manishearth mentioned this pull request Feb 24, 2018
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bors added a commit that referenced this pull request Feb 25, 2018
@bors
Auto merge of #48520 - Manishearth:rollup, r=Manishearth
026339e 
Rollup of 15 pull requests

- Successful merges: #47689, #48110, #48197, #48296, #48386, #48392, #48404, #48415, #48441, #48448, #48452, #48481, #48490, #48499, #48503
- Failed merges:
@bors bors merged commit 5a2bec9 into rust-lang:master Feb 25, 2018
@ishitatsuyuki ishitatsuyuki mentioned this pull request Feb 26, 2018
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@Antti Antti mentioned this pull request Feb 26, 2018
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@ishitatsuyuki ishitatsuyuki mentioned this pull request Feb 26, 2018
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@ishitatsuyuki ishitatsuyuki deleted the exp-unblow branch March 1, 2018 08:56
@ishitatsuyuki ishitatsuyuki mentioned this pull request Mar 1, 2018
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@ishitatsuyuki ishitatsuyuki mentioned this pull request Sep 29, 2020
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bors pushed a commit to rust-lang-ci/rust that referenced this pull request May 21, 2021
@ishitatsuyuki
Revert rust-lang#43546
a1815d7 
This effectively reverts rust-lang#43546 as it seems that it does affect performance more than the PR has anticipated. Follow-up changes from rust-lang#44269 are also reverted.

This also removes the deduplication code from rust-lang#48296 as duplications were primarily coming from rust-lang#43546 and after removing that code it probably doesn't worth paying the cost of using a hash map.
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