T-compiler Meeting Agenda 2023-12-21

Announcements

Next week Rust stable release 1.75
Compiler Team Steering: P-high review at time:2023-12-22T10:00:00-05:00
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)

MCPs/FCPs

New MCPs (take a look, see if you like them!)
- “Add -Z direct-access-external-data” compiler-team#707 (Zulip)
Old MCPs (stale MCP might be closed as per MCP procedure)
- None at this time
Old MCPs (not seconded, take a look)
- “Require --known-broken-llvm flag for x.py to skip codegen tests on old LLVM” compiler-team#687 (Zulip)
  - concern codegen-tests-are-for-new-optimizations
- “Semantics of -Cinstrument-coverage=all” compiler-team#690 (Zulip) (last review activity: about 8 days ago)
- “Add hygiene attributes to compile expanded source code” compiler-team#692 (Zulip)
  - concern: added-complexity-to-frontend
- “New Tier-3 target: wasm32-wasi-preview2” compiler-team#694 (Zulip) (last review activity: about 2 days ago)
- “Smooth the renaming transition of wasm32-wasi” compiler-team#695 (Zulip) (last review activity: about 8 days ago)
- “Introduce perma-unstable wasm-c-abi flag” compiler-team#703 (Zulip) (last review activity: about 6 days ago)
- “Support patchable-function-entry” compiler-team#704 (Zulip) (last review activity: about 6 days ago)
Pending FCP requests (check your boxes!)
- “Add a new --build-id flag to rustc” compiler-team#635 (Zulip)
  - concern option-name
  - concern other-existing-options
- “Retire the mailing list and make all decisions on zulip” compiler-team#649 (Zulip)
  - concern automatic-sync
  - concern single-point-of-failure-via-stream-archival
- “Stabilize --json=unused-externs(-silent)” compiler-team#674 (Zulip)
  - concern lint-interactions
- “Support overriding warnings level for a specific lint via command line” rust#113307
  - concern nesting
- “Update Windows platform support” rust#115141
  - concern needs blog post
- “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)
  - concern needs-fcp
- “Report all lints, even if other errors already occurred.” compiler-team#633 (Zulip)
- “Stabilize --json=unused-externs(-silent)” compiler-team#674 (Zulip)
- “Use the MSVC provided Address Sanitizer runtime when building for MSVC” compiler-team#702 (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)
- “make soft_unstable show up in future breakage reports” rust#116274
Accepted MCPs
- “Add infrastructure to “compute the ABI of a Rust type, described as a C type”” compiler-team#672 (Zulip)
- “Support response files generated by ninja with @ninja:path syntax” compiler-team#684 (Zulip)
- “De-llvm some integer intrinsics that on the Rust side always use u32” compiler-team#693 (Zulip)
- “Consistent Handler naming” compiler-team#699 (Zulip)
- “Promote tier 3 riscv32 bare metal targets to tier 2” compiler-team#701 (Zulip)
Finalized FCPs (disposition merge)
- “Stabilize THIR unsafeck” rust#117673
- “Properly reject default on free const items” rust#117818
- “Make inductive cycles in coherence ambiguous always” rust#118649

WG checkins

@_WG-llvm by @nagisa and @Nikita Popov (previous checkin):

LLVM 18 is going to branch in about a month

Backport nominations

T-compiler stable / T-compiler beta

:beta: “Exhaustiveness: Improve complexity on some wide matches” rust#118796
- PR is being reviewed, fixes a (P-high) exhaustiveness check rust#118437 (comment)
- issue analysis and patch authored by @Nadrieril (big thanks!)
- nominated by @apiraino, could be nice to have it backported to beta after merge (issue report affects a real-world crate)

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

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

No new P-high regressions this time.

Unassigned P-high nightly regressions

No unassigned P-high nightly regressions this time.

Performance logs

triage logs for 2023-12-18

A lot of noise in the results this week; there was an lull in the noise recently, so our auto-inferred noise threshold went down, and thus five PR’s were artificially flagged this week (and three supposed improvements were just reverting to the mean). Beyond that, we had three nice improvements: the first to debug builds in #117962 (by ceasing emission of expensive+unused .debug_pubnames and .debug_pubtypes), a second to diesel and serde in #119048 (by avoiding some unnecessary work), and a third to several benchmarks in #117749 (by adding some caching of an internal compiler structure).

Triage done by @pnkfelix. Revision range: 57010939..bf9229a2

Summary:

(instructions:u)	mean	range	count
Regressions (primary)	0.3%	[0.2%, 0.4%]	9
Regressions (secondary)	0.4%	[0.2%, 0.9%]	21
Improvements (primary)	-1.8%	[-10.8%, -0.1%]	103
Improvements (secondary)	-1.2%	[-2.6%, -0.2%]	93
All (primary)	-1.6%	[-10.8%, 0.4%]	112

6 Regressions, 9 Improvements, 3 Mixed; 5 of them in rollups 67 artifact comparisons made in total

Regressions

rustc_passes: Enforce rustc::potential_query_instability lint #118870 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	13.5%	[0.5%, 44.5%]	8
Regressions (secondary)	-	-	0
Improvements (primary)	-	-	0
Improvements (secondary)	-	-	0
All (primary)	13.5%	[0.5%, 44.5%]	8

already marked as triaged by lqd; this PR did not regress things, the reported change is due to a periodic “slow s3 uploads on the collector”, i.e. instrument noise/bias.

fix –dry-run when the change-id warning is printed #118789 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	0.9%	[0.8%, 1.0%]	4
Regressions (secondary)	0.5%	[0.3%, 0.8%]	18
Improvements (primary)	-	-	0
Improvements (secondary)	-	-	0
All (primary)	0.9%	[0.8%, 1.0%]	4

already marked as triaged by lqd; this PR did not regress things. lqd hypothesized that the inferred noise threshold was lowered (due to a period of low noisiness?), and thus normal amounts of noise are tripping the threshold.

Rollup of 3 pull requests #118982 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	0.8%	[0.8%, 0.9%]	4
Regressions (secondary)	0.4%	[0.0%, 0.8%]	32
Improvements (primary)	-	-	0
Improvements (secondary)	-	-	0
All (primary)	0.8%	[0.8%, 0.9%]	4

already marked as triaged by lqd. We think this is another case where the inferred noise threshold was low and we are seeing regression reports as the noise ramps up again.

Rollup of 5 pull requests #119002 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	0.2%	[0.1%, 0.5%]	40
Regressions (secondary)	0.4%	[0.1%, 0.7%]	36
Improvements (primary)	-0.3%	[-0.3%, -0.3%]	1
Improvements (secondary)	-0.1%	[-0.1%, -0.1%]	2
All (primary)	0.2%	[-0.3%, 0.5%]	41

lqd identified PR #118396, “Collect lang items from AST, get rid of GenericBound::LangItemTrait” as the root cause for the regressions here.
not marking as triaged

More unescaping cleanups #118897 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	0.6%	[0.1%, 0.8%]	5
Regressions (secondary)	0.4%	[0.2%, 0.7%]	26
Improvements (primary)	-	-	0
Improvements (secondary)	-	-	0
All (primary)	0.6%	[0.1%, 0.8%]	5

lqd theorized this is another instance of noise threshold wriggling
pnkfelix agrees and marked as triaged.

coverage: Regression test for assert!(!false) #119011 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	0.7%	[0.7%, 0.8%]	4
Regressions (secondary)	0.5%	[0.3%, 0.6%]	2
Improvements (primary)	-	-	0
Improvements (secondary)	-	-	0
All (primary)	0.7%	[0.7%, 0.8%]	4

yet again, helloworld check is being flagged as regressing (in four scenarios), probably due to wriggling noise threshold.
pnkfelix marking as triaged

Improvements

Rollup of 8 pull requests #118838 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	-	-	0
Regressions (secondary)	-	-	0
Improvements (primary)	-	-	0
Improvements (secondary)	-0.9%	[-1.0%, -0.9%]	6
All (primary)	-	-	0

fix: stop emitting .debug_pubnames and .debug_pubtypes #117962 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	-	-	0
Regressions (secondary)	-	-	0
Improvements (primary)	-2.1%	[-5.2%, -0.3%]	24
Improvements (secondary)	-1.6%	[-3.1%, -0.2%]	61
All (primary)	-2.1%	[-5.2%, -0.3%]	24

nice improvement to debug builds, especially for ripgrep and serde_derive

Unbreak non-unix non-windows bootstrap #118894 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	-	-	0
Regressions (secondary)	-	-	0
Improvements (primary)	-10.4%	[-30.8%, -0.6%]	8
Improvements (secondary)	-	-	0
All (primary)	-10.4%	[-30.8%, -0.6%]	8

(don’t get excited; this is just a return-to-normal after the artificial regression flagged with PR #118870 above)

cache param env canonicalization #117749 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	-	-	0
Regressions (secondary)	-	-	0
Improvements (primary)	-2.0%	[-4.8%, -0.2%]	32
Improvements (secondary)	-1.3%	[-1.4%, -1.3%]	2
All (primary)	-2.0%	[-4.8%, -0.2%]	32

pretty solid improvement here to serde, diesel, bitmaps, hyper, image.

-Ztrait-solver=next to -Znext-solver #118937 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	-	-	0
Regressions (secondary)	-	-	0
Improvements (primary)	-0.9%	[-1.0%, -0.9%]	4
Improvements (secondary)	-0.5%	[-0.8%, -0.3%]	21
All (primary)	-0.9%	[-1.0%, -0.9%]	4

(just noise)

Use the LLVM option NoTrapAfterNoreturn #110494 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	-	-	0
Regressions (secondary)	-	-	0
Improvements (primary)	-0.9%	[-0.9%, -0.8%]	4
Improvements (secondary)	-0.5%	[-0.8%, -0.3%]	25
All (primary)	-0.9%	[-0.9%, -0.8%]	4

(just noise)

Add support for --env on tracked_env::var #118830 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	-	-	0
Regressions (secondary)	-	-	0
Improvements (primary)	-0.7%	[-0.8%, -0.7%]	3
Improvements (secondary)	-0.5%	[-0.7%, -0.3%]	7
All (primary)	-0.7%	[-0.8%, -0.7%]	3

(just noise)

don’t fold ParamEnv in register_predicate_obligation #119048 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	-	-	0
Regressions (secondary)	-	-	0
Improvements (primary)	-2.1%	[-7.3%, -0.2%]	23
Improvements (secondary)	-0.3%	[-0.3%, -0.3%]	1
All (primary)	-2.1%	[-7.3%, -0.2%]	23

nice improvement to diesel and serde.

resolve: Replace visibility table in resolver outputs with query feeding #118657 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	-	-	0
Regressions (secondary)	-	-	0
Improvements (primary)	-0.3%	[-0.5%, -0.1%]	70
Improvements (secondary)	-0.6%	[-1.2%, -0.1%]	42
All (primary)	-0.3%	[-0.5%, -0.1%]	70

Mixed

codegen_llvm: set DW_AT_accessibility #115165 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	0.2%	[0.2%, 0.3%]	4
Regressions (secondary)	-	-	0
Improvements (primary)	-0.7%	[-0.7%, -0.6%]	4
Improvements (secondary)	-0.4%	[-0.7%, -0.2%]	22
All (primary)	-0.2%	[-0.7%, 0.3%]	8

the improvements are helloworld noise; the regressions are minor.

Rollup of 6 pull requests #119009 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	0.2%	[0.2%, 0.2%]	1
Regressions (secondary)	0.4%	[0.2%, 1.2%]	8
Improvements (primary)	-1.3%	[-1.9%, -0.9%]	8
Improvements (secondary)	-	-	0
All (primary)	-1.1%	[-1.9%, 0.2%]	9

The single tiny regression to exa doc full does not warrant spending time trying to tease apart this rollup, IMO.
marked as triaged

Rollup of 5 pull requests #119069 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	-	-	0
Regressions (secondary)	0.3%	[0.3%, 0.3%]	2
Improvements (primary)	-0.2%	[-0.3%, -0.1%]	12
Improvements (secondary)	-0.3%	[-0.5%, -0.2%]	13
All (primary)	-0.2%	[-0.3%, -0.1%]	12

The two minor regressions to secondary benchmarks do not warrant spending time trying to tease apart this rollup, IMO.
marked as triaged

Nominated Issues

T-compiler

No I-compiler-nominated issues this time.

RFC

No I-compiler-nominated RFCs this time.

Oldest PRs waiting for review

T-compiler

“tidy watcher” rust#114209 (last review activity: 2 months ago)
- anyone wants to adopt this puppet? Seems it’s bouncing a bit around
“Try Carbon’s new hashing strategy in the compiler” rust#117079 (last review activity: about 58 days ago)
- cc: @bjorn3 (IIUC)
“mark ty::Const::Error when meet unsupport ty for const generic params” rust#117176 (last review activity: about 56 days ago)
- cc: @michael goulet (compiler-errors) (for some questions but review is assigned to @Wesley Wiser)
“Make #![allow_internal_unstable(..)] work with stmt_expr_attributes” rust#117420 (last review activity: about 51 days ago)
- cc: @wesley wiser

Next week’s WG checkins

@WG-mir-opt by @oli (on leave, other WG member?)
@_T-rust-analyzer by @Lukas Wirth

Next meetings' agenda draft: hackmd link