Add a fast path for lowering trivial consts #148040

saethlin · 2025-10-23T16:28:27Z

No description provided.

saethlin · 2025-10-23T20:34:39Z

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-log-analyzer · 2025-10-25T03:49:08Z

The job aarch64-gnu-llvm-20-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

failures:

---- [mir-opt] tests/mir-opt/unusual_item_types.rs stdout ----

thread '[mir-opt] tests/mir-opt/unusual_item_types.rs' panicked at src/tools/compiletest/src/runtest/mir_opt.rs:63:21:
Output file `/checkout/obj/build/aarch64-unknown-linux-gnu/test/mir-opt/unusual_item_types/unusual_item_types.{impl#0}-ASSOCIATED_CONSTANT.built.after.mir` from test does not exist, available files are in `/checkout/obj/build/aarch64-unknown-linux-gnu/test/mir-opt/unusual_item_types`
stack backtrace:
   5: __rustc::rust_begin_unwind
             at /rustc/bb624dcb4c8ab987e10c0808d92d76f3b84dd117/library/std/src/panicking.rs:698:5
   6: core::panicking::panic_fmt
             at /rustc/bb624dcb4c8ab987e10c0808d92d76f3b84dd117/library/core/src/panicking.rs:75:14

rust-bors · 2025-10-25T05:32:22Z

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

rust-timer · 2025-10-25T06:42:10Z

Finished benchmarking commit (8907237): 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%, 0.4%]	16
Regressions ❌ (secondary)	0.6%	[0.0%, 1.6%]	21
Improvements ✅ (primary)	-3.7%	[-15.8%, -0.1%]	71
Improvements ✅ (secondary)	-4.3%	[-8.1%, -0.2%]	20
All ❌✅ (primary)	-3.0%	[-15.8%, 0.4%]	87

Max RSS (memory usage)

Results (primary -3.7%, secondary 2.0%)

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

	mean	range	count
Regressions ❌ (primary)	1.3%	[1.0%, 1.5%]	3
Regressions ❌ (secondary)	5.4%	[1.6%, 8.2%]	6
Improvements ✅ (primary)	-4.5%	[-8.6%, -0.6%]	20
Improvements ✅ (secondary)	-1.5%	[-1.8%, -1.0%]	6
All ❌✅ (primary)	-3.7%	[-8.6%, 1.5%]	23

Cycles

Results (primary -7.5%, secondary -3.4%)

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

	mean	range	count
Regressions ❌ (primary)	1.5%	[1.5%, 1.5%]	1
Regressions ❌ (secondary)	4.3%	[3.6%, 5.8%]	4
Improvements ✅ (primary)	-7.8%	[-15.2%, -2.5%]	27
Improvements ✅ (secondary)	-6.5%	[-8.7%, -5.1%]	10
All ❌✅ (primary)	-7.5%	[-15.2%, 1.5%]	28

Binary size

Results (primary -1.7%, secondary -3.5%)

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)	0.1%	[0.0%, 0.3%]	20
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: 474.979s -> 473.593s (-0.29%)
Artifact size: 390.46 MiB -> 390.54 MiB (0.02%)

oli-obk · 2025-10-25T08:00:02Z

compiler/rustc_mir_transform/src/lib.rs

+            | DefKind::AnonConst
+    ) && trivial_const(&body).is_some()
+    {
+        return tcx.alloc_steal_mir(body);


Why are we building the Mir for trivial consts?

My initial assessment indicated that most of the savings were from what we do after building MIR, and detecting trivial consts on THIR looked hard.

I think that pushing this detection earlier in compilation would make sense as a later extension.

Absolutely. Pls drop that as a comment right here

oli-obk · 2025-10-25T18:43:19Z

Even the instruction count regressions (non-incremental) seem to be an improvement in time measured. I don't think this PR needs more performance tuning, as it's somewhat expected that incremental may do more work now due to the extra query, but often still is an improvement

saethlin · 2025-10-25T19:03:30Z

I agree that the performance is pretty good. Also, all of my attempts are reducing the perf overhead have been completely ineffectual. The latest report looks better than the one before it because I let the optimization apply to more DefKinds.

I'm cleaning up the code so that it integrates a bit better in ctfe, which will probably make Ralf happier and also make this easier to extend.

Add a fast path for lowering trivial consts

314bd45

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Oct 23, 2025

Ban getting mir_for_ctfe of trivial consts

dd23814