Remove OneThread

[Zoxc]

This removes OneThread by switching incr_comp_session over to RwLock.

[rustbot]

r? @cjgillot

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

[petrochenkov]

It would be good to have the motivation written out explicitly for changes like this.

Why was OneThread used previously? Why can RwLock be used now instead? What changed?

[Zoxc]

Both OneThread and RwLock are fine for incr_comp_session, but the other uses of OneThread are removed, so switching incr_comp_session over to RwLock allows its removal.

[cjgillot]

This OneThread encodes that only the main thread is supposed to change the directory. In practice, only 2 functions are allowed to do that, called directly from rustc_interface.
Can we do better and move incr_comp_session out of Session?
Perhaps into rustc_interface::queries::Queries?

[bjorn3]

That would be nice. More mutable state has been moved out of Session recently and I have a PR pending to remove another bit of mutable state in Session.

[bjorn3]

One thing however is that I would also like to be able to finalize the incremental session from within a codegen backend. The (experimental) jit mode of cg_clif never returns control back to rustc_interface after codegen has ended. It directly runs the program and exits. This means incremental compilation is currently broken with the jit mode. For the regular jit mode it would technically be possible to move execution to CodegenBackend::link using global mutable state in a thread local, but that won't work for the (even more experimental) lazy jit mode which needs the TyCtxt to remain available all the way till the end to allow codegening new functions on the fly.

[bors]

☔ The latest upstream changes (presumably #115964) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #119767) made this pull request unmergeable. Please resolve the merge conflicts.

[Zoxc]

@rustbot ready

This OneThread encodes that only the main thread is supposed to change the directory

That's not actually an invariant though.

[cjgillot]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 63446d0 with merge c8e6073...

[bors]

☀️ Try build successful - checks-actions
Build commit: c8e6073 (c8e607340856c8692c43cb00c4ebfd09a78c98a9)

[bors]

☀️ Try build successful - checks-actions
Build commit: c8e6073 (c8e607340856c8692c43cb00c4ebfd09a78c98a9)

[rust-timer]

Finished benchmarking commit (c8e6073): comparison URL.

Overall result: no relevant changes - no action needed

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.

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

Instruction count

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

Max RSS (memory usage)

Results

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)	-	-	0
Improvements ✅ (secondary)	-2.2%	[-4.4%, -1.1%]	3
All ❌✅ (primary)	-	-	0

Cycles

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

Binary size

Results

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)	0.5%	[0.5%, 0.5%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.5%	[0.5%, 0.5%]	1

Bootstrap: 663.584s -> 663.307s (-0.04%)
Artifact size: 308.31 MiB -> 308.32 MiB (0.01%)

[cjgillot]

@bors r+

[bors]

📌 Commit 63446d0 has been approved by cjgillot

It is now in the queue for this repository.

[bors]

⌛ Testing commit 63446d0 with merge 6745c60...

[bors]

☀️ Test successful - checks-actions
Approved by: cjgillot
Pushing 6745c60 to master...

[rust-timer]

Finished benchmarking commit (6745c60): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.7%	[-1.7%, -1.7%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results

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)	1.3%	[1.3%, 1.3%]	1
Improvements ✅ (primary)	-2.4%	[-2.4%, -2.4%]	1
Improvements ✅ (secondary)	-2.8%	[-4.3%, -1.5%]	3
All ❌✅ (primary)	-2.4%	[-2.4%, -2.4%]	1

Cycles

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

Binary size

Results

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)	0.5%	[0.5%, 0.5%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.5%	[0.5%, 0.5%]	1

Bootstrap: 665.464s -> 666.038s (0.09%)
Artifact size: 308.33 MiB -> 308.34 MiB (0.00%)