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

aerooneqq · 2026-09-15T12:17:30Z

Remaps local def ids that are allocated after Definitions::commit_end_of_determinism was called.
Fixes #162202, fixes #162203.

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

cc #t-compiler/gpgpu-backend > Deterministic encoding of offload manifest

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%)

aerooneqq · 2026-10-14T10:38:30Z

The problem was that we have impl FixedSizeEncoding for Option<RawDefId> which does not respect remapping. It worked earlier because I remapped RawDefId explicitly and when I changed it to impl Encodable<EncodeContext<'_, '_>> for RawDefId and removed manual remappings it stopped working in cases where we use record_some! macro, as set_some in Table uses FixedSizeEncoding::write_to_bytes to encode values. Need to take care of it too.

While I was debugging this yesterday I also learned about existence of offload manifest which writes ty::Instance, seems like we need a remapping here too, as those instances can be decoded and then used during collect_crate_mono_items.

aerooneqq · 2026-10-15T11:40:40Z

@rustbot ready
Asked about offload manifest in https://rust-lang.zulipchat.com/#narrow/channel/422870-t-compiler.2Fgpgpu-backend/topic/Deterministic.20encoding.20of.20offload.20manifest/with/631708893, but overall ready to review.

oli-obk · 2026-10-17T13:58:59Z

@bors squash msg="Remap local def ids for deterministic metadata encoding"

rust-bors · 2026-10-17T13:59:22Z

🔨 14 commits were squashed into d8980d3.

oli-obk · 2026-10-17T13:59:36Z

@bors r+

rust-bors · 2026-10-17T13:59:40Z

📌 Commit d8980d3 has been tentatively approved by oli-obk

It will be put into the queue for this repository once PR CI succeeds.

JonathanBrouwer · 2026-10-17T15:50:29Z

@bors rollup=iffy

rustbot assigned petrochenkov Sep 15, 2026

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

ceb9839

Remap def indices for deterministic metadata encoding

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

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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

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petrochenkov mentioned this pull request Sep 16, 2026

Parallel frontend: reproducible def ids for RPITITs #162555

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

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Comment thread compiler/rustc_metadata/src/rmeta/encoder.rs

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aerooneqq force-pushed the def-index-remapping branch from 2e0d0ff to e1a82ef Compare October 13, 2026 14:40

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aerooneqq force-pushed the def-index-remapping branch from e1a82ef to 79353ac Compare October 13, 2026 16:31

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aerooneqq force-pushed the def-index-remapping branch from 79353ac to 5bc2553 Compare October 14, 2026 10:05

aerooneqq force-pushed the def-index-remapping branch 2 times, most recently from 857fb2e to ab3da1b Compare October 14, 2026 14:54

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tiny-ac mentioned this pull request Oct 15, 2026

Crate metadata and crate hash depend on the host CPU #164379

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Remap local def ids for deterministic metadata encoding

d8980d3

rust-bors Bot force-pushed the def-index-remapping branch from 8c692ca to d8980d3 Compare October 17, 2026 13:59

JonathanBrouwer mentioned this pull request Oct 17, 2026

Rollup of 11 pull requests #164479

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susitsm mentioned this pull request Oct 18, 2026

Re-enable parallelism for coroutine_by_move_body_def_id #164505

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Overall result: ❌ regressions - please read:

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Overall result: ❌ regressions - BENCHMARK(S) FAILED

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