T-compiler Meeting Agenda 2023-09-07

Announcements

Rust stable 1.72.1 is planned for Sept. 14th (Zulip thread)
Compiler Team Steering Meeting - Review of open T-compiler RFCs (issue) at time:2023-09-08T10:00:00-04:00
Types team meeting at time:2023-09-11T10:00:00-04:00
[Types] Rotating new solver / formality / polonius deep dive at time:2023-09-11T11:00:00-04:00
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!)
- “Allow overriding default codegen backend on a per-target basis” compiler-team#670
- “Add infrastructure to “compute the ABI of a Rust type, described as a C type”” compiler-team#672
Old MCPs (stale MCP might be closed as per MCP procedure)
- None at this time
Old MCPs (not seconded, take a look)
- “Consistently use “region” terminology in later stages of the compiler” compiler-team#634 (last review activity: 3 months ago)
- “Add a new --build-id flag to rustc” compiler-team#635 (last review activity: 3 months ago)
- “Simplify and improve explicitness of the check-cfg syntax” compiler-team#636 (last review activity: 2 months ago)
- “Add support for visionOS targets” compiler-team#642 (last review activity: about 5 days ago)
- “Add illumos Tier3 targets” compiler-team#644 (last review activity: about 44 days ago)
- “Promote loongarch64-unknown-none* to Tier 2” compiler-team#664(last review activity: about 20 days ago)
- “Match the behavior of strip and split-debuginfo across platforms” compiler-team#669(last review activity: about 2 days ago)
Pending FCP requests (check your boxes!)
- “Retire the mailing list and make all decisions on zulip” compiler_team#649 (Zulip)
- “Tracking issue for dyn upcasting coercion” rust#65991
- “stabilize combining +bundle and +whole-archive link modifiers” rust#113301
- “Support overriding warnings level for a specific lint via command line” rust#113307
Things in FCP (make sure you’re good with it)
- “Add a blanket flag to enable/disable codegen UB checks” compiler_team#625 (Zulip)
- “Add support for Zephyr OS” compiler_team#629 (Zulip)
- “Report all lints, even if other errors already occurred.” compiler_team#633 (Zulip)
- “Add option to pass environment variables” compiler_team#653 (Zulip)
- “Add tidy rule against issue-[0-9]+.rs tests” compiler_team#658 (Zulip)
- “Add builtin:$path option to the -L flag” compiler_team#659 (Zulip)
- “Support larger sizes in rmeta tables and incr comp alloc offsets” compiler_team#666 (Zulip)
- “Make unknown/renamed/removed lints passed via command line respect lint levels” compiler_team#667 (Zulip)
- “Condemn asmjs-unknown-emscripten to push a boulder up a hill for eternity” compiler_team#668 (Zulip)
- “Create a binary-size working group” compiler_team#671 (Zulip)
- “Stabilize PATH option for --print KIND=PATH” rust#114183
Accepted MCPs
- No new accepted proposals this time.
Finalized FCPs (disposition merge)
- No new finished FCP (disposition merge) this time.

WG checkins

@_WG-rustc-dev-guide by @Santiago Pastorino and @Yuki Okushi|217081 (latest checkin):
Checkin text

Backport nominations

T-compiler stable / T-compiler beta

:beta: [1.73.0-beta.4] “fix version for abi_thiscall to 1.73.0, which was forgotten to change when stabilized and (later) moved to beta” rust#115446
- updates the abi_thiscall version

:beta: [1.73.0-beta.4] “Don’t require Drop for [PhantomData<T>; N] where N and T are generic, if T requires Drop” rust#115527
- patch authored by @oli
- fixes #115403 and #115410, probably P-high regressions

:beta: [1.73.0-beta.4] “implied bounds: do not ICE on unconstrained region vars” rust#115559
- patch authored by @lcnr
- fixes #112832 (unclear the priority evaluation, comment)
- also nominated for stable backport

:beta: “Remove assert that checks type equality” rust#115215
- still open and being reviewed. nominated by @apiraino since already candidate for backports (comment)

:stable: [1.72.1] “implied bounds: do not ICE on unconstrained region vars” rust#115559

PRs S-waiting-on-team

T-compiler

“Add new simpler and more explicit syntax for check-cfg” rust#111072
- MCP compiler-team#636 is not seconded (not a lot of discussion either)
- This PR probably needs a look from someone other than @Urgau / @Vadim Petrochenkov
- issue was discussed a while ago in a prev. meeting
Other issues in progress or waiting on other teams

Issues of Note

Short Summary

P-critical

Note: Rust stable 1.72.1 is planned for Sept. 14th

T-compiler

“ICE: unexpected initial operand type.” rust#114858
- Originated in #113108 (comment)
- Should be addressed by #115215, authored by @Onur (onur-ozkan), being reviewed by @Michael Goulet (compiler-errors) (PR likely candidate for a backport)
“ScalarReplacementOfAggregates causes rustc to emit invalid LLVM debuginfo (and sometimes rustc segfaults)” rust#115113
- addressed in #115113, just waiting for the next beta (comment)
“Failed assert 2 != 2 after update to LLVM 17” rust#115385
- regressed in the latest LLVM17 upgrade
- fixed by upstream llvm-project#65195 (comment)

T-types

“RPIT hidden types can be ill-formed” rust#114728
- hits some real world crates (comment)
- should be addressed by #114470 by @Michael Goulet (compiler-errors), subject to RFC t-type consensus
- a parallel effort to fix in #114933 by @aliemjay, being reviewed by @lcnr (suggests waiting for this PR, comment)

T-rustdoc

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

P-high regressions

P-high beta regressions

No P-high beta regressions this time.

Unassigned P-high nightly regressions

No unassigned P-high nightly regressions this time.

Performance logs

triage logs for 2023-09-05

A lot of spurious noise this week from a few benchmarks (bitmaps-3.1.0, libc, and even cargo-0.60.0 at a few points). Beyond that, we had a few small improvements associated with the trait system and with parallel-rustc.

Triage done by @pnkfelix. Revision range: cedbe5c7..15e52b05

Summary:

(instructions:u)	mean	range	count
Regressions (primary)	0.6%	[0.2%, 1.2%]	11
Regressions (secondary)	1.0%	[0.2%, 2.0%]	40
Improvements (primary)	-1.9%	[-10.2%, -0.3%]	69
Improvements (secondary)	-1.5%	[-3.7%, -0.4%]	26
All (primary)	-1.5%	[-10.2%, 1.2%]	80

4 Regressions, 7 Improvements, 8 Mixed; 2 of them in rollups 66 artifact comparisons made in total

Regressions

Adapt table sizes to the contents, accommodating u64 rmeta offsets #113542 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	0.5%	[0.2%, 1.2%]	11
Regressions (secondary)	0.8%	[0.2%, 1.3%]	26
Improvements (primary)	-	-	0
Improvements (secondary)	-	-	0
All (primary)	0.5%	[0.2%, 1.2%]	11

already marked as triaged.

Use conditional synchronization for Lock #111713 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	2.3%	[2.3%, 2.3%]	1
Regressions (secondary)	-	-	0
Improvements (primary)	-	-	0
Improvements (secondary)	-	-	0
All (primary)	2.3%	[2.3%, 2.3%]	1

the measured regression appears spurious
marked as triaged

update Miri #115496 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	0.3%	[0.2%, 0.4%]	4
Regressions (secondary)	0.5%	[0.3%, 0.6%]	5
Improvements (primary)	-	-	0
Improvements (secondary)	-	-	0
All (primary)	0.3%	[0.2%, 0.4%]	4

believe to be spurious, marking as triaged

Rollup of 5 pull requests #115553 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	0.7%	[0.7%, 0.8%]	6
Regressions (secondary)	-	-	0
Improvements (primary)	-	-	0
Improvements (secondary)	-	-	0
All (primary)	0.7%	[0.7%, 0.8%]	6

believe to be spurious, marking as triaged

Improvements

Don’t record spans for predicates in coherence #115392 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	-	-	0
Regressions (secondary)	-	-	0
Improvements (primary)	-1.1%	[-2.5%, -0.3%]	18
Improvements (secondary)	-0.8%	[-1.0%, -0.2%]	8
All (primary)	-1.1%	[-2.5%, -0.3%]	18

undoes regression introduced by PR 115392

Use OnceLock for SingleCache #115422 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	-	-	0
Regressions (secondary)	-	-	0
Improvements (primary)	-0.4%	[-0.4%, -0.4%]	2
Improvements (secondary)	-0.5%	[-0.6%, -0.3%]	8
All (primary)	-0.4%	[-0.4%, -0.4%]	2

Fall through when resolving elided assoc const lifetimes #115429 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	-	-	0
Regressions (secondary)	-	-	0
Improvements (primary)	-1.0%	[-1.5%, -0.6%]	7
Improvements (secondary)	-	-	0
All (primary)	-1.0%	[-1.5%, -0.6%]	7

Don’t manually compute param indices when adding implicit Sized and ConstParamHasTy #115361 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	-	-	0
Regressions (secondary)	-	-	0
Improvements (primary)	-0.3%	[-0.5%, -0.2%]	5
Improvements (secondary)	-0.5%	[-0.6%, -0.4%]	2
All (primary)	-0.3%	[-0.5%, -0.2%]	5

Encode DepKind as u16 #115391 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	-	-	0
Regressions (secondary)	-	-	0
Improvements (primary)	-0.3%	[-0.3%, -0.3%]	5
Improvements (secondary)	-0.3%	[-0.3%, -0.3%]	3
All (primary)	-0.3%	[-0.3%, -0.3%]	5

Move RawOsError defination to sys #115493 (Comparison Link)

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

(I suspect this is spurious)

read_via_copy: don’t prematurely optimize away the read #115531 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	-	-	0
Regressions (secondary)	-	-	0
Improvements (primary)	-0.7%	[-0.7%, -0.7%]	5
Improvements (secondary)	-	-	0
All (primary)	-0.7%	[-0.7%, -0.7%]	5

(I suspect this is spurious)

Mixed

interpret: fix projecting into an unsized field of a local #114483 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	-	-	0
Regressions (secondary)	2.0%	[1.9%, 2.1%]	6
Improvements (primary)	-2.2%	[-2.2%, -2.2%]	1
Improvements (secondary)	-0.4%	[-0.4%, -0.4%]	3
All (primary)	-2.2%	[-2.2%, -2.2%]	1

Ralf notes: “Regressions only affect our ctfe stress test, so it’s likely some exaggerated consequence of a tiny change.”
marked as triaged

Lower Or pattern without allocating place #111752 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	0.4%	[0.2%, 1.0%]	8
Regressions (secondary)	0.9%	[0.5%, 1.6%]	21
Improvements (primary)	-1.0%	[-3.7%, -0.3%]	10
Improvements (secondary)	-0.4%	[-0.5%, -0.3%]	3
All (primary)	-0.3%	[-3.7%, 1.0%]	18

nnethercote notes: “Performance-wise, icounts/cycles/wall-times have a mix of wins and losses that more or less balance out. But binary size has lots of wins, which is nice.”
marked as triaged

Skip rendering metadata strings from include_str!/include_bytes! #115286 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	0.3%	[0.3%, 0.3%]	1
Regressions (secondary)	0.4%	[0.3%, 0.7%]	6
Improvements (primary)	-2.9%	[-10.3%, -0.3%]	28
Improvements (secondary)	-2.6%	[-3.7%, -1.7%]	9
All (primary)	-2.8%	[-10.3%, 0.3%]	29

saethlin notes: “Regressions look like recovery from a spurious improvement in the previous run, and are way less significant than the improvements.”
already marked as triaged

bootstrap: use git merge-base for LLVM CI download logic #115409 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	1.0%	[0.6%, 1.5%]	7
Regressions (secondary)	-	-	0
Improvements (primary)	-0.4%	[-0.4%, -0.4%]	1
Improvements (secondary)	-0.4%	[-0.5%, -0.3%]	2
All (primary)	0.8%	[-0.4%, 1.5%]	8

this is a collection of spurious regressions to bitmaps-3.1.0
- see e.g. this graph and note the blip.
marked as triaged

Add note on non-exhaustiveness when matching on str and nested non-exhaustive enums #115270 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	1.1%	[0.8%, 1.4%]	6
Regressions (secondary)	-	-	0
Improvements (primary)	-	-	0
Improvements (secondary)	-0.5%	[-0.5%, -0.5%]	1
All (primary)	1.1%	[0.8%, 1.4%]	6

seems like continued spurious noise on bitmaps-3.1.0
marked as triaged

Rollup of 5 pull requests #115518 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	7.1%	[0.7%, 13.6%]	2
Regressions (secondary)	-	-	0
Improvements (primary)	-1.1%	[-1.5%, -0.7%]	7
Improvements (secondary)	-	-	0
All (primary)	0.8%	[-1.5%, 13.6%]	9

13.6% regression to cargo-0.60.0 seems spurious (!), since it immediately returned to normal in #115491
marked as triaged

Outline panicking code for RefCell::borrow and RefCell::borrow_mut #115491 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	1.0%	[0.6%, 1.5%]	8
Regressions (secondary)	-	-	0
Improvements (primary)	-6.3%	[-12.0%, -0.6%]	2
Improvements (secondary)	-0.5%	[-0.6%, -0.5%]	4
All (primary)	-0.4%	[-12.0%, 1.5%]	10

as lqd notes: “bitmaps and cargo opt incr-unchanged kinda look noisy atm”
marked as triaged

Do not require associated types with Self: Sized to uphold bounds when confirming object candidate #115467 (Comparison Link)

(instructions:u)	mean	range	count
Regressions (primary)	0.6%	[0.6%, 0.6%]	2
Regressions (secondary)	-	-	0
Improvements (primary)	-1.0%	[-1.2%, -0.8%]	5
Improvements (secondary)	-	-	0
All (primary)	-0.6%	[-1.2%, 0.6%]	7

all these changes (for better or for worse) look like yet more spurious noise.
marked as triaged

Nominated Issues

T-compiler

“SIGILL Illegal Instruction on PPC64” rust#110278
- from last week (notes)
- the xvor instruction (an AltiVec optimisation) in cargo and rustup-init binaries prevents anything to compile under PPC64 on IBM hardware (9406-520) and similar POWER5 machines (comment), that model being discontinued in 2019
- Platform is “Tier 2 with Host tools” (powerpc64-unknown-linux-gnu)
- nomination by @Josh Stone (cuviper): do we want to reduce the default CPU target to something that supports that very old model?

RFC

No I-compiler-nominated RFCs this time.

Oldest PRs waiting for review

T-compiler

“Rebased: Mark drop calls in landing pads cold instead of noinline” rust#102099
- probably needs a second look from someone with more MSVC expertise (comment)
“Fix ICE when trying to convert ConstKind::Error to usize” rust#113712(last review activity: about 46 days ago)
- cc: @uwu
“Pretty-print argument-position impl trait to name it.” rust#113955(last review activity: about 31 days ago)
- cc @Waffle Lapkin

Next week’s WG checkins

Impl Trait initiative by @oli
@_WG-llvm by @nagisa and @Nikita Popov

Next meetings’ agenda draft: https://hackmd.io/FuBR9iC1T4ukhQ8QPFod3Q