Various `ObligationForest` improvements #64500

nnethercote · 2019-09-16T06:40:33Z

These commits make the code both nicer and faster.

Those with type `usize` are now called `i`, those with type `NodeIndex` are called `index`.

This commit removes the custom index implementation of `NodeIndex`, which probably predates `newtype_index!`. As well as eliminating code, it improves the debugging experience, because the custom implementation had the property of being incremented by 1 (so it could use `NonZeroU32`), which was incredibly confusing if you didn't expect it. For some reason, I also had to remove an `unsafe` block marker from `from_u32_unchecked()` that the compiler said was now unnecessary.

These refer to code that no longer exists.

This makes the code a little faster, presumably because bounds checks aren't needed on `nodes` accesses. It requires making `scratch` a `RefCell`, which is not unreasonable.

It's more concise, more idiomatic, and measurably faster.

nnethercote · 2019-09-16T07:21:57Z

I will do a perf run.

@bors try

bors · 2019-09-16T07:22:09Z

⌛ Trying commit 4ecd94e with merge b903678...

@nikomatsakis

Various `ObligationForest` improvements These commits make the code both nicer and faster. r? @nikomatsakis

bors · 2019-09-16T10:23:24Z

☀️ Try build successful - checks-azure
Build commit: b903678

Centril · 2019-09-16T13:01:25Z

@rust-timer build b903678

rust-timer · 2019-09-16T13:01:27Z

Success: Queued b903678 with parent 3e3e06d, comparison URL.

rust-timer · 2019-09-16T14:55:08Z

Finished benchmarking try commit b903678, comparison URL.

nikomatsakis · 2019-09-16T20:56:23Z

src/librustc_data_structures/indexed_vec.rs

@@ -149,7 +149,7 @@ macro_rules! newtype_index {

            #[inline]
            $v const unsafe fn from_u32_unchecked(value: u32) -> Self {
-                unsafe { $type { private: value } }
+                $type { private: value }


it's not needed because the function itself is unsafe, so it's allowed to have unsafe operations in the body

Fair enough. The obvious follow-up question is "why didn't the compiler complain about this before?"

nikomatsakis · 2019-09-16T21:04:01Z

r=me once the benchmarking results are in.

Centril · 2019-09-16T21:06:14Z

@nikomatsakis the bench results are already in... :D

nikomatsakis · 2019-09-16T21:45:22Z

Oh, so they are.

@bors r+

bors · 2019-09-16T21:45:23Z

📌 Commit 4ecd94e has been approved by nikomatsakis

@nikomatsakis

…ikomatsakis Various `ObligationForest` improvements These commits make the code both nicer and faster. r? @nikomatsakis

@ghost

Rollup of 6 pull requests Successful merges: - #64085 (Tweak unsatisfied HRTB errors) - #64380 (Update bundled OpenSSL to 1.1.1d) - #64416 (Various refactorings to clean up nll diagnostics) - #64500 (Various `ObligationForest` improvements) - #64530 (Elide lifetimes in `Pin<&(mut) Self>`) - #64531 (Use shorthand syntax in the self parameter of methods of Pin) Failed merges: r? @ghost

nnethercote · 2019-09-17T03:32:37Z

Thank you for the fast review.

BTW, this code can be very hot and parts of it are very perf-sensitive to change. Here is a list of other ways I tried to speed up or clean up ObligationForest, but failed:

Inline and remove insert_into_error_cache again(); caused slow-downs even when that code wasn't executed, presumably due to different inlining?
Use std::usize::MAX to indicate nodes to be removed; caused slow-downs.
Use IndexVec<Node<O>> for nodes; caused slow-downs.
Use drain_filter() for dependents in apply_rewrites(); caused slow-downs.
Use drain_filter() in compress; couldn't get past the borrow checker.
Make process_obligation() iterate over new nodes added to nodes; caused infinite loops.
Split nodes in two, to reduce the number of bytes copied by the shuffling in compress(); didn't finish, felt too invasive and likely to cause slow-downs.
Use swap_and_remove() to reduce the number of bytes copied by the shuffling in `compress(); caused slow-downs.
Box everything in Node; caused slow-downs.
Box everything in Node except state: caused slow-downs

@nikomatsakis

More `ObligationForest` improvements Following on from #64500, these commits alsomake the code both nicer and faster. r? @nikomatsakis

sinkuu · 2019-09-19T01:09:55Z

src/librustc_data_structures/obligation_forest/mod.rs

-    scratch: Option<Vec<usize>>,
+    /// A scratch vector reused in various operations, to avoid allocating new
+    /// vectors.
+    scratch: RefCell<Vec<usize>>,


I believe this can be replaced with Cell, which is simpler and panic-free, as you are only calling .replace on this (i.e. not utilizing "Ref" functionality).

Thanks for the tip. I will do it in a follow-up.

@nikomatsakis

More `ObligationForest` improvements Following on from #64500, these commits alsomake the code both nicer and faster. r? @nikomatsakis

nnethercote added 11 commits Sep 16, 2019

Name index variables consistently.

04b1111

Those with type `usize` are now called `i`, those with type `NodeIndex` are called `index`.

Factor out repeated `self.nodes[i]` expressions.

1936e44

Remove out-of-date comments.

3fda957

These refer to code that no longer exists.

Fix some out-of-date names of things in comments.

ac061dc

Fix incorrect comment about contents of a `Node`.

6391ef4

Add comments about `waiting_cache`.

e2492b7

Use iterators in `error_at` and `process_cycle`.

6e48053

This makes the code a little faster, presumably because bounds checks aren't needed on `nodes` accesses. It requires making `scratch` a `RefCell`, which is not unreasonable.

Use `retain` for `waiting_cache` in `apply_rewrites()`.

f22bb2e

It's more concise, more idiomatic, and measurably faster.

Minor comment tweaks.

201afa6

Move `impl Node` just after `struct Node`.

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4ecd94e

rust-highfive assigned nikomatsakis Sep 16, 2019

rust-highfive added the S-waiting-on-review label Sep 16, 2019

nnethercote changed the title ~~Various `ObligationForest`~~ Various `ObligationForest` improvements Sep 16, 2019

bors added a commit that referenced this pull request Sep 16, 2019

Auto merge of #64500 - nnethercote:ObligForest-fixups, r=<try>

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b903678

Various `ObligationForest` improvements These commits make the code both nicer and faster. r? @nikomatsakis

nikomatsakis reviewed Sep 16, 2019

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nikomatsakis approved these changes Sep 16, 2019

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bors added S-waiting-on-bors and removed S-waiting-on-review labels Sep 16, 2019

Centril mentioned this pull request Sep 17, 2019

Rollup of 6 pull requests #64535

Merged

nnethercote deleted the nnethercote:ObligForest-fixups branch Sep 17, 2019

nnethercote mentioned this pull request Sep 17, 2019

More `ObligationForest` improvements #64545

Merged

sinkuu reviewed Sep 19, 2019

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rust-lang / rust

Various `ObligationForest` improvements #64500

Various `ObligationForest` improvements #64500

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Various `ObligationForest` improvements #64500

Various `ObligationForest` improvements #64500

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