T-compiler Meeting Agenda 2024-01-11

Announcements

Big rework of the meetings calendar, thanks @davidtwco (Zulip topic)!
Compiler Team Planning meeting time:2024-01-12T16:00:00+01:00 ?
Reminder: if you see a PR/issue that seems like there might be legal implications due to copyright/IP/etc, please let us know (or at least message @davidtwco or @Wesley Wiser so we can pass it along).

Other WG meetings (calendar link)

Stable MIR Weekly time:2024-01-12T16:00:00+01:00
wg-macros Triage time:2024-01-11T17:00:00+01:00
Types Team Meeting time:2024-01-14T16:00:00+01:00
Types Team: Rotating Solver/Formality/Polonius Deep Dive time:2024-01-14T17:00:00+01:00

MCPs/FCPs

New MCPs (take a look, see if you like them!)
- No new proposals this time.
Old MCPs (stale MCP might be closed as per MCP procedure)
- None at this time
Old MCPs (not seconded, take a look)
- “Semantics of -Cinstrument-coverage=all” compiler-team#690 (Zulip) (last review activity: about 29 days ago)
- “Add hygiene attributes to compile expanded source code” compiler-team#692 (Zulip) (last review activity: about 29 days ago)
- “Introduce perma-unstable wasm-c-abi flag” compiler-team#703 (Zulip) (last review activity: about 27 days ago)
- “Support patchable-function-entry” compiler-team#704 (Zulip) (last review activity: about 27 days ago)
- “Add -Z direct-access-external-data” compiler-team#707 (Zulip) (last review activity: about 1 days ago)
Pending FCP requests (check your boxes!)
- “Add a new --build-id flag to rustc” compiler-team#635 (Zulip)
- “Retire the mailing list and make all decisions on zulip” compiler-team#649 (Zulip)
- “Stabilize --json=unused-externs(-silent)” compiler-team#674 (Zulip)
- “Support overriding warnings level for a specific lint via command line” rust#113307
- “Update Windows platform support” rust#115141
- “Stabilize Wasm target features that are in phase 4 and 5” rust#117457
- “Undeprecate lint unstable_features and make use of it in the compiler” rust#118639
Things in FCP (make sure you’re good with it)
- “Add a stable flag to enable/disable codegen UB checks” compiler-team#625 (Zulip)
- “Stabilize --json=unused-externs(-silent)” compiler-team#674 (Zulip)
- “New Tier-3 target: wasm32-wasi-preview2” compiler-team#694 (Zulip)
- “Smooth the renaming transition of wasm32-wasi” compiler-team#695 (Zulip)
- “Provide option to shorten symbol names by replacing them with a digest” compiler-team#705 (Zulip)
- “uplift some -Zverbose calls and rename to -Zverbose-internals” compiler-team#706 (Zulip)
- “Start using clippy on rustc” compiler-team#709 (Zulip)
- “Promote tier 3 *-pc-windows-gnullvm targets to tier 2” compiler-team#710 (Zulip)
- “Unstably support MIR-only rlibs” compiler-team#713 (Zulip)
- “Warn on references casting to bigger memory layout” rust#118983
- “const-eval interning: get rid of type-driven traversal” rust#119044
Accepted MCPs
- No new accepted proposals this time.
Finalized FCPs (disposition merge)
- No new finished FCP (disposition merge) this time.

WG checkins

None this week

Backport nominations

T-compiler beta / T-compiler stable

:beta: “[beta] Revert #117472: Stabilize C string literals” rust#119528
- Revert authored by Wesley, T-lang discussion

:beta: “Update LLVM submodule” rust#119802
- Not yet merged. Fixes a number of P-highs: Fixes #118867 #119160 #119179

No stable nominations for T-compiler this time.

T-types stable / T-types beta

No beta nominations for T-types this time.
No stable nominations for T-types this time.

PRs S-waiting-on-team

T-compiler

“large_assignments: Lint on specific large args passed to functions” rust#116520
- Seems there are some perf. regressions (in part already discussed). Maybe add a comment to help take a decision and unblock?
Other issues in progress or waiting on other teams

Issues of Note

Short Summary

P-critical

T-compiler

No P-critical issues for T-compiler this time.

T-types

No P-critical issues for T-types this time.

P-high regressions

P-high beta regressions

“🐛 Symbol multiply defined since nightly-2023-12-02 with custom builtins.” rust#118609
- Discussed also last week (Zulip)
- Wesley said
  
  Let’s revisit this next week. It looks like work is actively happening here so while I think a temporary revert is potentially a good idea, let’s give the author some time to propose a fix or argue why this should not be supported at all.

Unassigned P-high nightly regressions

No unassigned P-high nightly regressions this time.

Performance logs

triage logs for 2024-01-08

Not a particularly notable week. Large swings aren’t spurious but also are driven by changes in high-level behavior (diagnostics going from zero to one emission primarily), which causes a lot more work to happen. This isn’t really representative of the underlying rustc performance changing though.

Triage done by @simulacrum. Revision range: 67b6975051b83ef2bd28f06e8467470d570aceb3..76101eecbe9aa80753664bbe637ad06d1925f315

Summary:

(instructions:u)	mean	range	count
Regressions ❌(primary)	4.9%	[0.2%, 24.3%]	14
Regressions (secondary)	4.6%	[0.2%, 29.9%]	55
Improvements (primary)	-0.5%	[-1.5%, -0.2%]	61
Improvements (secondary)	-0.7%	[-1.0%, -0.4%]	14
All (primary)	0.5%	[-1.5%, 24.3%]	75

4 Regressions, 4 Improvements, 6 Mixed; 1 of them in rollups

33 artifact comparisons made in total

Regressions

rustc_lint: Enforce rustc::potential_query_instability lint #119251 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	0.2%	[0.2%, 0.3%]	2
Regressions (secondary)	0.3%	[0.2%, 0.4%]	8
Improvements (primary)	-	-	0
Improvements (secondary)	-	-	0
All (primary)	0.2%	[0.2%, 0.3%]	2

Minor change in just a few benchmarks. Not clear whether this is noise or not but the overall change is required for correctness.

Merge unused_tuple_struct_fields into dead_code #118297 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	0.6%	[0.3%, 0.8%]	7
Regressions (secondary)	8.3%	[0.2%, 30.4%]	28
Improvements (primary)	-	-	0
Improvements (secondary)	-	-	0
All (primary)	0.6%	[0.3%, 0.8%]	7

Regressions are related to this lint firing in a few benchmarks, which causes a good deal of lazy loading to actually happen in diagnostics infra.

Exhaustiveness: Statically enforce revealing of opaques #119329 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	-	-	0
Regressions (secondary)	1.9%	[1.7%, 2.0%]	6
Improvements (primary)	-	-	0
Improvements (secondary)	-	-	0
All (primary)	-	-	0

Only a match-stress regression, seems like an acceptable change for the correctness fix.

Inline a few utility functions around MIR #119459 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	0.3%	[0.3%, 0.4%]	3
Regressions (secondary)	-	-	0
Improvements (primary)	-	-	0
Improvements (secondary)	-	-	0
All (primary)	0.3%	[0.3%, 0.4%]	3

Potentially just noise. Overall impact is limited to just one benchmark and only incr-full.

Improvements

Separate immediate and in-memory ScalarPair representation #118991 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	0.6%	[0.6%, 0.6%]	1
Regressions (secondary)	-	-	0
Improvements (primary)	-1.0%	[-1.5%, -0.6%]	13
Improvements (secondary)	-0.2%	[-0.2%, -0.2%]	1
All (primary)	-0.9%	[-1.5%, 0.6%]	14

rustc_span: Optimize syntax context comparisons #119531 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	-	-	0
Regressions (secondary)	-	-	0
Improvements (primary)	-0.6%	[-0.8%, -0.4%]	5
Improvements (secondary)	-	-	0
All (primary)	-0.6%	[-0.8%, -0.4%]	5

Exhaustiveness: remove Matrix.wildcard_row #119667 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	-	-	0
Regressions (secondary)	-	-	0
Improvements (primary)	-	-	0
Improvements (secondary)	-2.8%	[-3.0%, -2.6%]	6
All (primary)	-	-	0

macro_rules: Add an expansion-local cache to span marker #119693 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	-	-	0
Regressions (secondary)	-	-	0
Improvements (primary)	-1.4%	[-20.5%, -0.2%]	80
Improvements (secondary)	-0.8%	[-1.9%, -0.3%]	16
All (primary)	-1.4%	[-20.5%, -0.2%]	80

Largest improvements here are recovery from a spurious regression in previous PR, but this is still a good win even aside from that.

Mixed

Reorder check_item_type diagnostics so they occur next to the corresponding check_well_formed diagnostics #117213 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	0.2%	[0.2%, 0.4%]	7
Regressions (secondary)	0.8%	[0.2%, 2.3%]	5
Improvements (primary)	-0.3%	[-0.3%, -0.3%]	2
Improvements (secondary)	-	-	0
All (primary)	0.1%	[-0.3%, 0.4%]	9

Stabilize THIR unsafeck #117673 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	-	-	0
Regressions (secondary)	0.3%	[0.2%, 0.4%]	2
Improvements (primary)	-0.4%	[-0.9%, -0.2%]	39
Improvements (secondary)	-0.6%	[-1.1%, -0.4%]	9
All (primary)	-0.4%	[-0.9%, -0.2%]	39

Improvements outweigh regressions.

Replace a number of FxHashMaps/Sets with stable-iteration-order alternatives #119192 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	-	-	0
Regressions (secondary)	0.3%	[0.1%, 0.5%]	7
Improvements (primary)	-0.4%	[-0.6%, -0.2%]	2
Improvements (secondary)	-	-	0
All (primary)	-0.4%	[-0.6%, -0.2%]	2

Correctness fix, acceptable regressions.

Rollup of 9 pull requests #119662 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	0.2%	[0.2%, 0.2%]	1
Regressions (secondary)	0.7%	[0.2%, 1.3%]	13
Improvements (primary)	-0.4%	[-0.4%, -0.2%]	6
Improvements (secondary)	-	-	0
All (primary)	-0.3%	[-0.4%, 0.2%]	7

tt-muncher is the primary significant regression, and appears to be significantly beyond the noise level for that benchmark. Investigation is ongoing.

mark vec::IntoIter pointers as !nonnull #114205 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	0.7%	[0.6%, 0.8%]	2
Regressions (secondary)	-	-	0
Improvements (primary)	-0.4%	[-0.4%, -0.4%]	1
Improvements (secondary)	-	-	0
All (primary)	0.4%	[-0.4%, 0.8%]	3

Likely slightly more work for LLVM.

Nominated Issues

T-compiler

“Environment variable sandboxing” rust#118372
- @Guillaume Gomez (imperio) asks on Zulip about the best conduct to stabilize a --env flag and its liasion with rfc#2794
- That RFC is been worked by @_Jeremy Fitzhardinge and was already scrutinized during a T-compiler meeting (see comment)

RFC

No I-compiler-nominated RFCs this time.

Oldest PRs waiting for review

T-compiler

“Create the previous dep graph index on a background thread” rust#116375 (last review activity: 3 months ago)
- cc: cjgillot
“offset: allow zero-byte offset on arbitrary pointers” rust#117329
- cc: @nikita Popov (maybe)
“Save liveness results for DestinationPropagation” rust#115291
- cc @Jak{e,ob} Degen (can you comment if your review is final?)

Next week’s WG checkins

@_WG-mir-opt by @oli
@_T-rust-analyzer by @Lukas Wirth

Next meetings' agenda draft: hackmd link