Add a fast path for lowering trivial consts #148040

saethlin · 2025-10-23T16:28:27Z

The objective of this PR is to improve compilation performance for crates that define a lot of trivial consts. This is a flamegraph of a build of a library crate that is just 100,000 trivial consts, taken from a nightly compiler:

My objective is to target all of the cycles in eval_to_const_value_raw that are not part of mir_built, because if you look at the mir_built for a trivial const, we already have the value available.

In this PR, the definition of a trivial const is this:

const A: usize = 0;

Specifically, we look for if the mir_built body is a single basic block containing one assign statement and a return terminator, where the assign statement assigns an Operand::Constant(Const::Val). The MIR dumps for these look like:

const A: usize = {
    let mut _0: usize;

    bb0: {
        _0 = const 0_usize;
        return;
    }
}

The implementation is built around a new query, trivial_const(LocalDefId) -> Option<(ConstValue, Ty)> which returns the contents of the Const::Val in the mir_built if the LocalDefId is a trivial const.

Then I added debug assertions to the beginning of mir_for_ctfe and mir_promoted to prevent trying to get the body of a trivial const, because that would defeat the optimization here. But these are deliberately debug assertions because the consequence of failing the assertion is that compilation is slow, not corrupt. If we made these hard assertions, I'm sure there are obscure scenarios people will run into where the compiler would ICE instead of continuing on compilation, just a bit slower. I'd like to know about those, but I do not think serving up an ICE is worth it.

With the assertions in place, I just added logic around all the places they were hit, to skip over trying to analyze the bodies of trivial consts.

In the future, I'd like to see this work extended by:

Pushing detection of trivial consts before MIR building
Including DefKind::Static and DefKind::InlineConst
Including consts like _1 = const 0_usize; _0 = &_1, which would make a lot of promoteds into trivial consts
Handling less-trivial consts like const A: usize = B, which have Operand::Constant(Const::Unevaluated)

saethlin · 2025-10-23T20:34:39Z

@bors try @rust-timer queue

Add a fast path for lowering trivial consts

rust-bors · 2025-10-23T22:48:20Z

☀️ Try build successful (CI)
Build commit: 4c15d20 (4c15d2003befc82fb2064960f5520c8643947469, parent: 6501e64fcb02d22b49d6e59d10a7692ec8095619)

rust-timer · 2025-10-24T00:16:31Z

Finished benchmarking commit (4c15d20): comparison URL.

Overall result: ❌✅ regressions and improvements - BENCHMARK(S) FAILED

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

❗ ❗ ❗ ❗ ❗
Warning ⚠️: The following benchmark(s) failed to build:

serde-1.0.219-threads4

❗ ❗ ❗ ❗ ❗

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%	[0.0%, 0.9%]	77
Regressions ❌ (secondary)	0.5%	[0.0%, 1.6%]	29
Improvements ✅ (primary)	-2.6%	[-5.6%, -1.8%]	13
Improvements ✅ (secondary)	-2.5%	[-2.8%, -2.3%]	3
All ❌✅ (primary)	-0.1%	[-5.6%, 0.9%]	90

Max RSS (memory usage)

Results (primary -0.2%, secondary 2.5%)

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

	mean	range	count
Regressions ❌ (primary)	0.7%	[0.4%, 1.3%]	7
Regressions ❌ (secondary)	7.1%	[4.6%, 8.2%]	4
Improvements ✅ (primary)	-1.7%	[-2.0%, -1.5%]	4
Improvements ✅ (secondary)	-1.3%	[-2.2%, -0.8%]	5
All ❌✅ (primary)	-0.2%	[-2.0%, 1.3%]	11

Cycles

Results (primary -1.5%, secondary -0.4%)

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

	mean	range	count
Regressions ❌ (primary)	3.5%	[3.5%, 3.5%]	1
Regressions ❌ (secondary)	3.0%	[2.8%, 3.3%]	3
Improvements ✅ (primary)	-2.5%	[-3.2%, -2.0%]	5
Improvements ✅ (secondary)	-3.0%	[-6.0%, -1.7%]	4
All ❌✅ (primary)	-1.5%	[-3.2%, 3.5%]	6

Binary size

Results (primary -0.6%, secondary 0.0%)

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

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.0%, 0.8%]	72
Regressions ❌ (secondary)	0.0%	[0.0%, 0.2%]	14
Improvements ✅ (primary)	-8.9%	[-8.9%, -8.8%]	8
Improvements ✅ (secondary)	-0.1%	[-0.2%, -0.0%]	2
All ❌✅ (primary)	-0.6%	[-8.9%, 0.8%]	80

Bootstrap: 476.496s -> 475.064s (-0.30%)
Artifact size: 390.49 MiB -> 390.55 MiB (0.02%)

saethlin · 2025-10-24T21:28:19Z

@bors try @rust-timer queue

Add a fast path for lowering trivial consts

rust-bors · 2025-10-24T23:46:27Z

☀️ Try build successful (CI)
Build commit: 4931b5e (4931b5e2c7edb857fab6e19e39dd4e4e22a37a91, parent: ab925646fae038b02bd462cd328ae9eef1639236)

rust-timer · 2025-10-25T00:56:20Z

Finished benchmarking commit (4931b5e): 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%	[0.1%, 0.9%]	67
Regressions ❌ (secondary)	0.5%	[0.0%, 1.6%]	32
Improvements ✅ (primary)	-9.7%	[-15.6%, -5.5%]	13
Improvements ✅ (secondary)	-2.5%	[-2.8%, -2.4%]	3
All ❌✅ (primary)	-1.3%	[-15.6%, 0.9%]	80

Max RSS (memory usage)

Results (primary -0.8%, secondary -1.1%)

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

	mean	range	count
Regressions ❌ (primary)	1.0%	[0.5%, 3.5%]	11
Regressions ❌ (secondary)	4.9%	[0.8%, 8.0%]	6
Improvements ✅ (primary)	-2.8%	[-4.0%, -1.3%]	10
Improvements ✅ (secondary)	-4.1%	[-5.4%, -2.4%]	12
All ❌✅ (primary)	-0.8%	[-4.0%, 3.5%]	21

Cycles

Results (primary -9.2%, secondary -1.6%)

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)	4.1%	[0.9%, 10.7%]	6
Improvements ✅ (primary)	-9.2%	[-14.1%, -4.2%]	13
Improvements ✅ (secondary)	-4.2%	[-8.6%, -2.1%]	13
All ❌✅ (primary)	-9.2%	[-14.1%, -4.2%]	13

Binary size

Results (primary -0.7%, secondary 0.0%)

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

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.0%, 0.8%]	72
Regressions ❌ (secondary)	0.1%	[0.0%, 0.2%]	22
Improvements ✅ (primary)	-9.0%	[-9.1%, -9.0%]	8
Improvements ✅ (secondary)	-0.1%	[-0.2%, -0.0%]	2
All ❌✅ (primary)	-0.7%	[-9.1%, 0.8%]	80

Bootstrap: 474.337s -> 474.199s (-0.03%)
Artifact size: 390.48 MiB -> 390.80 MiB (0.08%)

saethlin · 2025-10-25T03:20:35Z

@bors try @rust-timer queue

Add a fast path for lowering trivial consts

rust-bors · 2025-10-25T05:32:22Z

☀️ Try build successful (CI)
Build commit: 8907237 (8907237f16e88f2a9ee9a48ba052804abac7e239, parent: f435972085b697a1ece8ee6a1ac76efff8d1df7b)

rustbot · 2025-10-25T21:02:23Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

RalfJung · 2025-10-26T10:55:05Z

compiler/rustc_mir_transform/src/lib.rs

+    matches!(tcx.def_kind(def), DefKind::AssocConst | DefKind::Const | DefKind::AnonConst)
+}
+
+fn trivial_const_provider<'tcx>(


Seems worth moving this query impl into a separate file, to avoid growing this already-big file even bigger.

Also please add more comments, in particular explaining the overall contract this query must abide by.

I added a big doc comment to the main function that has all the logic that explains the contract.

RalfJung · 2025-10-26T10:56:14Z

compiler/rustc_mir_transform/src/lib.rs

        tcx.ensure_done().coroutine_by_move_body_def_id(def);
    }

+    // the `trivial_const` query uses mir_built, so make sure it is run.


mir_built also uses trivial_const, so I am confused... this sounds cyclic?

tcx.mir_built uses the free function, not the query. So it's not cyclic, but if the first call is tcx.trivial_const, we call tcx.mir_built to get the body, which checks if the Body is trivial in order to skip its passes internally, then returns that to trivial_const_provider which analyzes the Body again.

It's a bit goofy, but that's all.

That code flow feels worth putting in a comment somewhere.

Also, doesn't that mean we do the work of checking triviality twice?

Just noting that I did add this to a comment in which I noted that we are checking for triviality multiple times which is undesirable.

compiler/rustc_mir_transform/src/lib.rs

oli-obk · 2025-10-27T07:06:49Z

@bors r+

bors · 2025-10-27T07:06:51Z

📌 Commit a63035f has been approved by oli-obk

It is now in the queue for this repository.

bors · 2025-10-27T11:02:45Z

⌛ Testing commit a63035f with merge 4b53279...

bors · 2025-10-27T14:11:51Z

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing 4b53279 to master...

github-actions · 2025-10-27T14:15:20Z

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 34a8c73 (parent) -> 4b53279 (this PR)

Test differences

Show 10 test diffs

10 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 4b53279854fcc60b063398181f5dc13ddc319cb8 --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

aarch64-apple: 7133.3s -> 10864.5s (52.3%)
i686-gnu-1: 7316.0s -> 8265.3s (13.0%)
aarch64-gnu-llvm-20-2: 2294.2s -> 2571.1s (12.1%)
x86_64-gnu-llvm-20: 2528.1s -> 2826.0s (11.8%)
aarch64-gnu-debug: 3989.6s -> 4445.5s (11.4%)
x86_64-gnu-gcc: 3053.5s -> 3394.6s (11.2%)
x86_64-gnu-miri: 4386.6s -> 4870.1s (11.0%)
i686-gnu-2: 5594.6s -> 6168.3s (10.3%)
arm-android: 5817.8s -> 6386.1s (9.8%)
tidy: 173.1s -> 189.9s (9.7%)

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 · 2025-10-27T16:24:44Z

Finished benchmarking commit (4b53279): comparison URL.

Overall result: ❌✅ regressions and improvements - 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)	0.3%	[0.2%, 0.4%]	14
Regressions ❌ (secondary)	0.5%	[0.0%, 2.1%]	36
Improvements ✅ (primary)	-3.7%	[-15.5%, -0.1%]	71
Improvements ✅ (secondary)	-4.0%	[-8.2%, -0.0%]	22
All ❌✅ (primary)	-3.0%	[-15.5%, 0.4%]	85

Max RSS (memory usage)

Results (primary -3.5%, secondary 2.5%)

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)	5.0%	[1.5%, 7.5%]	7
Improvements ✅ (primary)	-3.5%	[-9.5%, -0.5%]	29
Improvements ✅ (secondary)	-1.9%	[-2.6%, -0.5%]	4
All ❌✅ (primary)	-3.5%	[-9.5%, -0.5%]	29

Cycles

Results (primary -6.8%, secondary -4.6%)

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

	mean	range	count
Regressions ❌ (primary)	2.2%	[2.2%, 2.2%]	1
Regressions ❌ (secondary)	2.9%	[2.0%, 3.8%]	2
Improvements ✅ (primary)	-7.1%	[-13.6%, -2.5%]	29
Improvements ✅ (secondary)	-5.7%	[-8.2%, -2.2%]	13
All ❌✅ (primary)	-6.8%	[-13.6%, 2.2%]	30

Binary size

Results (primary -1.7%, secondary -3.8%)

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

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.0%, 0.7%]	64
Regressions ❌ (secondary)	0.2%	[0.0%, 0.3%]	17
Improvements ✅ (primary)	-4.9%	[-12.5%, -0.9%]	40
Improvements ✅ (secondary)	-9.5%	[-14.9%, -0.0%]	12
All ❌✅ (primary)	-1.7%	[-12.5%, 0.7%]	104

Bootstrap: 473.665s -> 474.938s (0.27%)
Artifact size: 390.50 MiB -> 390.59 MiB (0.02%)

saethlin · 2025-10-27T16:53:40Z

The regressions are caused by the small growth in the size of the query graph, and they show up on crates that don't benefit from this optimization (or perhaps just don't benefit yet, see my suggestions for how to expand this in the PR description).

therealprof · 2025-10-28T10:56:03Z

@saethlin I'm a bit surprised by the binary size regressions, even for optimised builds.

Kobzol · 2025-10-28T11:45:06Z

Note that we don't have many binaries in the perf. suite, those did not experience any binary size changes (https://perf.rust-lang.org/compare.html?start=34a8c7368c84fc699fc83a8851a02f93fd655931&end=4b53279854fcc60b063398181f5dc13ddc319cb8&stat=size%3Alinked_artifact&library=false&nonRelevant=true). The binary size changes reported are mostly in the crate metadata (https://perf.rust-lang.org/compare.html?start=34a8c7368c84fc699fc83a8851a02f93fd655931&end=4b53279854fcc60b063398181f5dc13ddc319cb8&stat=size%3Acrate_metadata&nonRelevant=true).

Accept trivial consts based on trivial consts This is an expansion of #148040. The previous implementation only accepted trivial consts that assign a literal. For example: ```rust const A: usize = 0; const B: usize = A; ``` Before this PR, only `A` was a trivial const. Now `B` is too.

Accept trivial consts based on trivial consts This is an expansion of rust-lang/rust#148040. The previous implementation only accepted trivial consts that assign a literal. For example: ```rust const A: usize = 0; const B: usize = A; ``` Before this PR, only `A` was a trivial const. Now `B` is too.

Kobzol · 2025-11-03T20:52:44Z

Improvements greatly outweigh the regressions.

@rustbot label: +perf-regression-triaged

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Auto merge of #148040 - saethlin:trivial-consts, r=<try>

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Add a fast path for lowering trivial consts

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Auto merge of #148040 - saethlin:trivial-consts, r=<try>

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Add a fast path for lowering trivial consts

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Auto merge of #148040 - saethlin:trivial-consts, r=<try>

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Add a fast path for lowering trivial consts

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RalfJung reviewed Oct 26, 2025

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compiler/rustc_mir_transform/src/lib.rs Show resolved Hide resolved

saethlin added 2 commits October 26, 2025 22:09

Move trivial_const to a separate module with a doc comment

6ba56d2

Explain that the current impl is only silly, not recursive

a63035f

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Hack together inline-always-overrides #141055

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saethlin deleted the trivial-consts branch October 27, 2025 16:48

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saethlin mentioned this pull request Oct 28, 2025

Accept trivial consts based on trivial consts #148182

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Add a fast path for lowering trivial consts #148040

Add a fast path for lowering trivial consts #148040

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Overall result: ❌✅ regressions and improvements - BENCHMARK(S) FAILED

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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 and improvements - please read the text below

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