Add a != check to ChunkedBitSet::union.

[nnethercote]

It's a big speed win for cranelift-codegen-0.119.0.

r? @Zalathar

[nnethercote]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 701ca72 (701ca7229fdda54e3b2aaf5d347786d78565ec43, parent: 2300c2aef7dbc2a7bbbeaa9894d07d459abd9bc6)

[Zalathar]

For future work, we could hypothetically get even more clever, and try to start the bitwise_changes check from where the equality check failed (for suitable operators).

But that might not be worth the extra implementation complexity (or might even reduce the benefit of the equality check).

[rust-timer]

Finished benchmarking commit (701ca72): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-4.7%	[-9.6%, -1.3%]	6
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	1
All ❌✅ (primary)	-4.7%	[-9.6%, -1.3%]	6

Max RSS (memory usage)

Results (primary 2.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.0%	[2.0%, 2.0%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.0%	[2.0%, 2.0%]	1

Cycles

Results (primary -3.3%, secondary 2.7%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.7%	[2.7%, 2.7%]	1
Improvements ✅ (primary)	-3.3%	[-3.5%, -3.1%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.3%	[-3.5%, -3.1%]	2

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 472.507s -> 474.412s (0.40%)
Artifact size: 388.12 MiB -> 388.13 MiB (0.00%)

[nnethercote]

For future work, we could hypothetically get even more clever, and try to start the bitwise_changes check from where the equality check failed (for suitable operators).

But that might not be worth the extra implementation complexity (or might even reduce the benefit of the equality check).

Interesting idea! I tried it out, with code like this:

                    let i = match iter::zip(
                        &self_chunk_words[0..num_words],
                        &other_chunk_words[0..num_words],
                    )
                    .position(|(a, b)| a != b)
                    {
                        Some(i) => i,
                        None => continue, // all elements equal, nothing more to do
                    };

                    let op = |a, b| a | b;
                    if !bitwise_changes(
                        &self_chunk_words[i..num_words],
                        &other_chunk_words[i..num_words],
                        op,
                    ) {
                        continue;
                    }
                    
                    // ... do the mutation ...

And it ended up being a significant slowdown. I think because that iter::zip/position loop is much less optimized than simple slice comparison, which would use something like memcmp.

[nnethercote]

I have update the code a little to include some refactoring.

[Zalathar]

Looks good, thanks!

@bors r+

[bors]

📌 Commit ad8b2e0 has been approved by Zalathar

It is now in the queue for this repository.

[rust-log-analyzer]

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

...........................................        (143/143)

======== tests/rustdoc-gui/globals.goml ========

[ERROR] `tests/rustdoc-gui/globals.goml` line 14: The following errors happened: [Property named `"searchIndex"` doesn't exist]: for command `assert-window-property-false: {"searchIndex": null}`

======== tests/rustdoc-gui/search-result-display.goml ========

[WARNING] `tests/rustdoc-gui/search-result-display.goml` line 39: Delta is 0 for "x", maybe try to use `compare-elements-position` instead?