Remap def indices for deterministic metadata encoding by aerooneqq · Pull Request #162809 · rust-lang/rust

aerooneqq · 2026-09-15T12:17:30Z

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Now at the very early stage (however fixes corresponding issues), draft primarily for CI.

cc #t-compiler/const-eval > DefId reproducibility with the parallel frontend

r? @petrochenkov

susitsm · 2026-09-15T13:50:42Z

Are the changes in decoder.rs and def_path_hash_map.rs needed? It seems unrelated to me.

aerooneqq · 2026-09-15T14:01:33Z

Are the changes in decoder.rs and def_path_hash_map.rs needed? It seems unrelated to me.

We need to update map from hashes to def indices while encoding DefPathHashMap, now I inserted simple FxHashMap to test the approach, if I don't remove functions from decoder.rs there will be warnings, because they are not used anywhere else.

petrochenkov · 2026-09-16T12:21:55Z

@bors try @rust-timer queue

Remap def indices for deterministic metadata encoding

rust-bors · 2026-09-16T13:29:18Z

☀️ Try build successful (CI)
Build commit: ceb9839 (ceb98398abf30a57ca7f186c152d2adda7b079f9)
Base parent: 28e8a8c (28e8a8c81bf3b37909edac6c2a76e56f30cd492f)

rust-timer · 2026-09-16T14:13:41Z

Finished benchmarking commit (ceb9839): comparison URL.

Overall result: ❌ regressions - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@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)	4.4%	[0.1%, 164.6%]	276
Regressions ❌ (secondary)	11.8%	[0.2%, 130.1%]	350
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	4.4%	[0.1%, 164.6%]	276

Max RSS (memory usage)

Results (primary 3.0%, secondary 4.5%)

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

	mean	range	count
Regressions ❌ (primary)	3.0%	[0.8%, 9.4%]	203
Regressions ❌ (secondary)	4.6%	[0.9%, 14.8%]	327
Improvements ✅ (primary)	-2.3%	[-2.3%, -2.3%]	1
Improvements ✅ (secondary)	-3.6%	[-4.0%, -2.9%]	3
All ❌✅ (primary)	3.0%	[-2.3%, 9.4%]	204

Cycles

Results (primary 10.2%, secondary 14.6%)

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

	mean	range	count
Regressions ❌ (primary)	10.2%	[1.7%, 73.0%]	55
Regressions ❌ (secondary)	14.6%	[1.4%, 66.2%]	179
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	10.2%	[1.7%, 73.0%]	55

Binary size

Results (primary -0.4%, 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)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.4%	[-0.4%, -0.3%]	8
Improvements ✅ (secondary)	-0.4%	[-0.4%, -0.4%]	4
All ❌✅ (primary)	-0.4%	[-0.4%, -0.3%]	8

Bootstrap: 496.137s -> 496.27s (0.03%)
Artifact size: 406.79 MiB -> 406.77 MiB (-0.01%)

petrochenkov · 2026-09-16T14:16:05Z

@bors try @rust-timer queue

Remap def indices for deterministic metadata encoding

susitsm · 2026-09-16T14:34:22Z

+
+    #[inline]
+    fn map_index(&self, index: DefIndex) -> DefIndex {
+        self.def_indexes_remapping.get(&index).copied().unwrap_or(index)


This could be donewith a vec for better performance. Since we know that these indices are dense in a range. Either a full IndexVec mapping every LocalDefId, or a shorter Vec, that only contains the remapped ones, indexed by remapped_index - min_remapped_index

View changes since the review

It is very strong assumption that def ids that are needed to be remapped will be in the end, now it should be true, because resolution and AST -> HIR lowering is executed without any parallelism, however AST -> HIR lowering became more incremental recently by introducing lower_to_hir query which is very easy to parallelize in the beginning of the run_required_analysis function (now it is executed sequentially as we prefetch hir_crate_items which is a sequential HIR visit). If it is parallelized then from each lower_to_hir execution we may invoke arbitrary code that can allocate def ids that do not need to be remapped.

But I like the idea, I was thinking about something similar in a bit different direction: create separate IndexVecs in Definitions struct for different stages of compilation, thus we can maybe pre-allocate and remap ids only for selected parts.

I think that it is reasonable to track the last def id that is allocated during resolution stage, because it is not likely to get parallelized in the nearest time (I hope so) and then to remap all def ids that are allocated after it, I will try it tomorrow.

susitsm · 2026-09-16T14:45:54Z

+    let mut to_remap = vec![];
+    let mut def_ids = vec![];
+    for idx in 0..defs.num_definitions() {
+        let def_id = LocalDefId { local_def_index: idx.into() };


I think this could iterate backwards, stopping when it hits an index not needing remapping. Remapped indices should be at the end

View changes since the review

rust-bors · 2026-09-16T15:16:50Z

☀️ Try build successful (CI)
Build commit: 8bcd37a (8bcd37a7bc50acb44dc7181f601e93127ebf19af)
Base parent: 28e8a8c (28e8a8c81bf3b37909edac6c2a76e56f30cd492f)

rust-timer · 2026-09-16T16:35:40Z

Finished benchmarking commit (8bcd37a): comparison URL.

Overall result: ❌ regressions - BENCHMARK(S) FAILED

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

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

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

Job failure
cranelift-codegen-0.119.0

❗ ❗ ❗ ❗ ❗

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%, 0.6%]	60
Regressions ❌ (secondary)	0.2%	[0.1%, 0.3%]	13
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.3%	[0.1%, 0.6%]	60

Max RSS (memory usage)

Results (secondary -4.1%)

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)	-4.1%	[-5.1%, -3.3%]	3
All ❌✅ (primary)	-	-	0

Cycles

Results (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)	-	-	0
Regressions ❌ (secondary)	4.6%	[4.6%, 4.6%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.5%	[-2.7%, -2.3%]	2
All ❌✅ (primary)	-	-	0

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 496.137s -> 495.635s (-0.10%)
Artifact size: 406.79 MiB -> 406.74 MiB (-0.01%)

petrochenkov · 2026-09-17T11:59:39Z

@bors try @rust-timer queue

Remap def indices for deterministic metadata encoding

rust-bors · 2026-09-17T12:58:07Z

☀️ Try build successful (CI)
Build commit: 501ad34 (501ad34e355f975a1f1d71121c6738cf001435d7)
Base parent: c999cef (c999cef531ea9059e189e82fe0e82c5daf249bc9)

petrochenkov · 2026-09-17T13:56:21Z

r? @oli-obk for example (so I'm not the only one who reviews @aerooneqq's PRs)

rust-timer · 2026-09-17T14:22:41Z

Finished benchmarking commit (501ad34): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

@bors rollup=never rustc-perf
@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
Regressions ❌ (secondary)	0.2%	[0.2%, 0.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.3%	[-0.4%, -0.1%]	8
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -2.3%, 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)	-	-	0
Regressions ❌ (secondary)	3.4%	[3.1%, 3.7%]	2
Improvements ✅ (primary)	-2.3%	[-2.3%, -2.3%]	1
Improvements ✅ (secondary)	-2.0%	[-3.3%, -0.4%]	3
All ❌✅ (primary)	-2.3%	[-2.3%, -2.3%]	1

Cycles

Results (primary 4.4%, secondary 4.3%)

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

	mean	range	count
Regressions ❌ (primary)	4.4%	[2.4%, 6.4%]	2
Regressions ❌ (secondary)	4.3%	[4.3%, 4.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	4.4%	[2.4%, 6.4%]	2

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 500.132s -> 498.753s (-0.28%)
Artifact size: 406.81 MiB -> 408.93 MiB (0.52%)

rust-bors · 2026-09-18T04:48:54Z

☔ The latest upstream changes (presumably #162920) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

rustbot assigned petrochenkov Sep 15, 2026

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 Sep 15, 2026

aerooneqq changed the title ~~Remap def indices for deterministic metadata encoing~~ Remap def indices for deterministic metadata encoding Sep 15, 2026

aerooneqq added 7 commits September 16, 2026 12:25

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rust-bors Bot pushed a commit that referenced this pull request Sep 16, 2026

Auto merge of #162809 - aerooneqq:def-index-remapping, r=<try>

ceb9839

Remap def indices for deterministic metadata encoding

aerooneqq added 2 commits September 16, 2026 15:35

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aerooneqq force-pushed the def-index-remapping branch from cc58a69 to 9736cd8 Compare September 16, 2026 14:03

rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Sep 16, 2026

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rust-bors Bot pushed a commit that referenced this pull request Sep 16, 2026

Auto merge of #162809 - aerooneqq:def-index-remapping, r=<try>

8bcd37a

Remap def indices for deterministic metadata encoding

susitsm reviewed Sep 16, 2026

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petrochenkov mentioned this pull request Sep 16, 2026

Parallel frontend: reproducible def ids for RPITITs #162555

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Sep 16, 2026

aerooneqq added 2 commits September 17, 2026 13:51

Fix wrong remapping of def index

517be51

Remap indices only after last deterministic index

e01cdf1

aerooneqq force-pushed the def-index-remapping branch from 4f2feb6 to e01cdf1 Compare September 17, 2026 11:54

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rust-bors Bot pushed a commit that referenced this pull request Sep 17, 2026

Auto merge of #162809 - aerooneqq:def-index-remapping, r=<try>

501ad34

Remap def indices for deterministic metadata encoding

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aerooneqq mentioned this pull request Sep 17, 2026

Some refactorings around metadata encoding #162900

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rustbot assigned oli-obk and unassigned petrochenkov Sep 17, 2026

rustbot removed S-waiting-on-perf Status: Waiting on a perf run to be completed. perf-regression Performance regression. labels Sep 17, 2026

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Overall result: ❌ regressions - please read:

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Overall result: ❌ regressions - BENCHMARK(S) FAILED

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Overall result: ✅ improvements - no action needed

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