Introduce ByteSymbol

[nnethercote]

r? @ghost

[nnethercote]

@bors try @rust-timer queue

[bors]

⌛ Trying commit e83f532 with merge f661a16...

[bors]

☀️ Try build successful - checks-actions
Build commit: f661a16 (f661a16cf599bf46f7b251ef3f5caba62f9ce4ac)

[rust-timer]

Finished benchmarking commit (f661a16): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

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

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.2%, 0.2%]	1
Regressions ❌ (secondary)	0.5%	[0.5%, 0.5%]	2
Improvements ✅ (primary)	-3.0%	[-3.0%, -3.0%]	1
Improvements ✅ (secondary)	-0.4%	[-0.5%, -0.3%]	2
All ❌✅ (primary)	-1.4%	[-3.0%, 0.2%]	2

Max RSS (memory usage)

Results (primary -1.9%, secondary -1.7%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.2%	[0.4%, 6.7%]	8
Improvements ✅ (primary)	-1.9%	[-2.2%, -1.6%]	2
Improvements ✅ (secondary)	-2.5%	[-7.9%, -0.4%]	35
All ❌✅ (primary)	-1.9%	[-2.2%, -1.6%]	2

Cycles

Results (primary -2.4%, secondary 0.3%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	1.0%	[0.5%, 2.9%]	11
Improvements ✅ (primary)	-2.4%	[-3.1%, -1.7%]	2
Improvements ✅ (secondary)	-0.8%	[-1.3%, -0.4%]	7
All ❌✅ (primary)	-2.4%	[-3.1%, -1.7%]	2

Binary size

Results (primary -1.1%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.1%	[-1.1%, -1.1%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.1%	[-1.1%, -1.1%]	1

Bootstrap: 775.84s -> 774.376s (-0.19%)
Artifact size: 372.26 MiB -> 372.24 MiB (-0.00%)

[nnethercote]

The motivation here: introduce ByteSymbol, to eliminate the Arcs in ast::LitKind, to make ast::LitKind impl Copy, to avoid the need to arena-allocate ast::LitKind in HIR.

It works, and the max-rss results (and page faults) are down for the literal-heavy benchmarks: deep-vector, tuple-stress, coercions, ucd. (And it's difficult to get max-rss down in general.) But it has no real effect on speed. So I'm not sure if it's worth the extra code.

[petrochenkov]

cc #65818 (comment)

Upd: but maybe most of the (non-backend) improvements we already done in #103812.

[petrochenkov]

This is a real issue because people want to include relatively large binary blobs into their programs, but I'm not sure we have a relevant benchmark in the perf suite.

It makes sense to benchmark this change on something specifically binary-blob-oriented.

[petrochenkov]

I actually wanted to share the interner between string symbols and byte symbols.
The interner shouldn't care whether the bytes it stores or looks up are UTF-8 or not.
Symbol and ByteSymbol would then both be wrappers around an InternerIndex, but Symbol would give a guarantee that the bytes it points to are UTF-8, and ByteSymbol would give no such guarantee, and Symbol(index = 123) and ByteSymbol(index = 123) would actually share the storage if the bytes are UTF-8.

#65818 (comment) suggested making Symbol and ByteSymbol symbols dynamically discernible from each other (e.g. the former counts from 0, and the latter from u32::MAX), but maybe just having two static types would be enough.

Not sure if the same sharing can be done for metadata encoding/decoding tables.

[petrochenkov]

I think this generally makes sense, the byte array interner is not large and is mostly a copypaste of the string interner, so not much new complexity.
@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[nnethercote]

I benchmarked this program:

pub const BYTES: &[u8] = include_bytes!("/bin/containerd");

fn main() {
    println!("len = {}", BYTES.len());
}

On my Linux box /bin/containerd is a 48MB file.

hyperfine results, before and after:

Benchmark 1: $RUSTC1 b2.rs
  Time (mean ± σ):     414.3 ms ±  27.7 ms    [User: 183.2 ms, System: 294.1 ms]
  Range (min … max):   366.9 ms … 461.0 ms    10 runs
 
Benchmark 1: $RUSTC3 b2.rs
  Time (mean ± σ):     443.1 ms ±  26.2 ms    [User: 191.3 ms, System: 315.1 ms]
  Range (min … max):   413.7 ms … 500.2 ms    10 runs

I.e. this PR makes things a little slower. Probably because of the cost of hashing the binary blob to convert it into a ByteSymbol.

[petrochenkov]

Seems reasonable, big binary blobs are unlikely to be identical, so not much sense to try deduplicating them.

[nnethercote]

Surprising it's only a little slower: Cachegrind says my example program takes 39 million instrs to compile normally and 67 million instructions with this PR's changes. The differences being all in hashing and some memcpy.

Edit: if I put the benchmark into rustc-perf and measure it, for a check full build I get a 58% wall-time increase from 0.15s to 0.23s.