Remove `const FLAGS`. by nnethercote · Pull Request #152791 · rust-lang/rust

nnethercote · 2026-02-18T09:38:22Z

The performance wins provided by these types are meagre, and I don't think they justify the code complexity they introduce.

r? @Zalathar

nnethercote · 2026-02-18T09:38:39Z

@bors try @rust-timer queue

Remove `const FLAGS`.

Zalathar · 2026-02-18T09:55:10Z

See also [EXPERIMENT] Remove static booleans from rustc_query_impl::DynamicConfig #151633 where I experimented with putting the flags in the vtable.

I wonder if this PR's approach will preserve some of the benefits of static flags via inlining. It's certainly nicer than having FLAGS everywhere.

rust-bors · 2026-02-18T11:50:45Z

☀️ Try build successful (CI)
Build commit: 744269b (744269beec5ae0238f5da62161b8168c2e7f8956, parent: 8387095803f21a256a9a772ac1f9b41ed4d5aa0a)

rust-timer · 2026-02-18T12:30:26Z

Finished benchmarking commit (744269b): comparison URL.

Overall result: ❌ regressions - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

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

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.1%, 0.9%]	42
Regressions ❌ (secondary)	0.4%	[0.1%, 1.0%]	78
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.3%	[0.1%, 0.9%]	42

Max RSS (memory usage)

Results (primary 2.4%, secondary -0.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.4%	[1.8%, 3.6%]	3
Regressions ❌ (secondary)	3.3%	[1.8%, 6.0%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.6%	[-4.0%, -3.2%]	3
All ❌✅ (primary)	2.4%	[1.8%, 3.6%]	3

Cycles

Results (primary 3.0%, secondary 3.3%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	3.0%	[2.4%, 4.1%]	6
Regressions ❌ (secondary)	3.3%	[2.1%, 4.5%]	29
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.0%	[2.4%, 4.1%]	6

Binary size

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

Bootstrap: 480.396s -> 481.316s (0.19%)
Artifact size: 397.88 MiB -> 397.75 MiB (-0.03%)

nnethercote · 2026-02-19T00:43:52Z

Just enough of a regression that it's hard to justify, alas. I see now that SemiDynamicQueryDispatcher and QueryVTable could be merged if FLAGS went away.

Relatedly, I found the following functions take a FLAGS generic param but don't actually use it:

mk_cycle
handle_cycle_error
cycle_error
wait_for_query
try_load_from_disk_and_cache_in_memory
query_key_hash_verify
try_load_from_on_disk_cache_inner

They can be changed to take a QueryVTable instead of a SemiDynamicQueryDispatcher, though then some methods on the latter need to be moved to the former in such a way that's it's not a uniform facade type.

nnethercote · 2026-02-19T05:25:15Z

@bors try @rust-timer queue

Remove `const FLAGS`.

rust-bors · 2026-02-19T07:40:51Z

☀️ Try build successful (CI)
Build commit: 06d3be4 (06d3be4e20551ccb3f23a19b0d08dabb3542e3fa, parent: e0cb264b814526acb82def4b5810e394a2ed294f)

rust-timer · 2026-02-19T08:21:06Z

Finished benchmarking commit (06d3be4): comparison URL.

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

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

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

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.2%, 0.5%]	14
Regressions ❌ (secondary)	0.3%	[0.1%, 0.6%]	33
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.4%	[-0.5%, -0.1%]	8
All ❌✅ (primary)	0.3%	[0.2%, 0.5%]	14

Max RSS (memory usage)

Results (secondary -5.5%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-5.5%	[-5.5%, -5.5%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results (primary -2.8%, secondary -3.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.8%	[-2.8%, -2.8%]	1
Improvements ✅ (secondary)	-3.2%	[-3.2%, -3.2%]	1
All ❌✅ (primary)	-2.8%	[-2.8%, -2.8%]	1

Binary size

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

Bootstrap: 487.064s -> 481.364s (-1.17%)
Artifact size: 397.85 MiB -> 395.69 MiB (-0.54%)

rustbot · 2026-02-20T02:58:10Z

Zalathar is not on the review rotation at the moment.
They may take a while to respond.

rustbot · 2026-02-22T07:16:13Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

nnethercote · 2026-02-22T07:16:22Z

I rebased.

compiler/rustc_query_impl/src/lib.rs

compiler/rustc_middle/src/query/plumbing.rs

Zalathar · 2026-02-22T10:33:37Z

r=me after considering nits, though I assume you'll want to wait for Clarify aspects of query macros #152779.

`SemiDynamicQueryDispatcher` is just a `QueryVTable` wrapper with an additional `const FLAGS: QueryFlags` generic parameter that contains three booleans. This arrangement exists as a performance optimization. But the performance effects are very small and it adds quite a bit of complexity to an already overly-complex part of the codebase. If it didn't exist and somebody proposed adding it and asked me to review, I almost certainly wouldn't approve it. This commit removes it. The three booleans in `QueryFlags` are moved into `QueryVTable` The non-trivial methods of `SemiDynamicQueryDispatcher` become methods of `QueryVTable`.

It's now `query_vtable` because its return type changed. And thanks to the previous commit it can be manually inlined in several places. (The only remaining calls to it are in `make_dep_kind_vtable_for_query`, which are more challenging to remove.)

nnethercote · 2026-02-22T23:50:57Z

@bors r=Zalathar

rust-bors · 2026-02-22T23:50:59Z

📌 Commit 5aebfd6 has been approved by Zalathar

It is now in the queue for this repository.

Zalathar · 2026-02-23T08:59:29Z

Scheduling: Giving this a bump as it's on the critical path for any subsequent work involving query vtables.

@bors p=1

@Zalathar

Remove `const FLAGS`. *[View all comments](https://triagebot.infra.rust-lang.org/gh-comments/rust-lang/rust/pull/152791)* The performance wins provided by these types are meagre, and I don't think they justify the code complexity they introduce. r? @Zalathar

JonathanBrouwer · 2026-02-23T14:57:22Z

@bors cancel
x86_64-gnu-aux looks super stuck

rust-bors · 2026-02-23T14:57:25Z

Auto build cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/22301250669

The next pull request likely to be tested is #153013.

rust-bors · 2026-02-23T22:33:00Z

☀️ Test successful - CI
Approved by: Zalathar
Duration: 3h 20m 56s
Pushing b3869b9 to main...

github-actions · 2026-02-23T22:36:06Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing eeb94be (parent) -> b3869b9 (this PR)

Test differences

Show 2 test diffs

2 doctest diffs were found. These are ignored, as they are noisy.

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard b3869b94cd1ed4bfa2eb28f301535d5e9599c713 --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

dist-apple-various: 1h 41m -> 2h 18m (+36.0%)
dist-aarch64-llvm-mingw: 1h 25m -> 1h 45m (+23.7%)
dist-aarch64-apple: 2h 4m -> 2h 25m (+16.9%)
aarch64-gnu-debug: 1h 16m -> 1h 6m (-13.6%)
i686-gnu-2: 1h 45m -> 1h 31m (-13.6%)
x86_64-gnu-stable: 2h 26m -> 2h 7m (-12.6%)
x86_64-msvc-ext2: 1h 48m -> 1h 36m (-11.6%)
x86_64-gnu-gcc: 1h 8m -> 1h (-10.6%)
aarch64-gnu-llvm-20-1: 1h 2m -> 55m 45s (-10.5%)
armhf-gnu: 1h 30m -> 1h 21m (-10.5%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

rust-timer · 2026-02-23T23:14:39Z

Finished benchmarking commit (b3869b9): comparison URL.

Overall result: ❌ regressions - please read the text below

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

If the regression was expected or you think it can be justified,
please write a comment with sufficient written justification, and add
@rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
If you think that you know of a way to resolve the regression, try to create
a new PR with a fix for the regression.
If you do not understand the regression or you think that it is just noise,
you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
were already notified of this PR).

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.1%, 0.5%]	17
Regressions ❌ (secondary)	0.3%	[0.1%, 0.6%]	37
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[0.1%, 0.5%]	17

Max RSS (memory usage)

Results (primary -1.4%, secondary -5.3%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.4%	[-1.4%, -1.4%]	1
Improvements ✅ (secondary)	-5.3%	[-5.3%, -5.3%]	1
All ❌✅ (primary)	-1.4%	[-1.4%, -1.4%]	1

Cycles

Results (secondary 8.6%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	8.6%	[8.6%, 8.6%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 481.498s -> 479.41s (-0.43%)
Artifact size: 395.88 MiB -> 395.68 MiB (-0.05%)

nnethercote · 2026-02-23T23:28:29Z

Perf regressions were minor and deemed worthwhile above for the simplicity improvements.

@rustbot label: +perf-regression-triaged

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rust-bors bot pushed a commit that referenced this pull request Feb 18, 2026

Auto merge of #152791 - nnethercote:rm-const-FLAGS, r=<try>

744269b

Remove `const FLAGS`.

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Feb 18, 2026

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Feb 18, 2026

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nnethercote force-pushed the rm-const-FLAGS branch from cd94110 to 940ba86 Compare February 19, 2026 05:25

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rust-bors bot pushed a commit that referenced this pull request Feb 19, 2026

Auto merge of #152791 - nnethercote:rm-const-FLAGS, r=<try>

06d3be4

Remove `const FLAGS`.

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Feb 19, 2026

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Feb 19, 2026

nnethercote force-pushed the rm-const-FLAGS branch from 940ba86 to e7fb201 Compare February 20, 2026 02:56

rustbot assigned Zalathar Feb 20, 2026

nnethercote mentioned this pull request Feb 22, 2026

Streamline QueryVTableUnerased into GetQueryVTable #152841

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Zalathar reviewed Feb 22, 2026

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nnethercote added 2 commits February 23, 2026 07:20

nnethercote force-pushed the rm-const-FLAGS branch from f69a4e1 to 5aebfd6 Compare February 22, 2026 20:58

rust-bors bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Feb 22, 2026

Zalathar mentioned this pull request Feb 23, 2026

Improve how QueryCache/QueryState/QueryEngine are stored #152835

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rust-bors bot added merged-by-bors This PR was explicitly merged by bors. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Feb 23, 2026

rust-bors bot merged commit b3869b9 into rust-lang:main Feb 23, 2026
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rustbot added this to the 1.95.0 milestone Feb 23, 2026

nnethercote deleted the rm-const-FLAGS branch February 23, 2026 23:28

rustbot added the perf-regression-triaged The performance regression has been triaged. label Feb 23, 2026

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Overall result: ❌ regressions - please read the text below

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

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Test differences

Job duration changes

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Overall result: ❌ regressions - please read the text below

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