Remove redundant information in `rustc_abi::Variants`#151742

moulins · 2026-01-27T14:26:28Z

Follow-up to #151040; partially addresses #113988.

Replaces the nested LayoutData in Variants::Multiple by a new, smaller VariantLayout struct, and adjust LayoutData::for_variantand the layout algorithm in consequence.
This PR is best reviewed commit-by-commit.

rustbot · 2026-01-27T14:26:32Z

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a, @makai410

rustbot · 2026-01-27T14:26:35Z

r? @wesleywiser

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moulins · 2026-01-27T14:28:30Z

compiler/rustc_abi/src/lib.rs

+#[derive(PartialEq, Eq, Hash, Clone, Debug)]
+#[cfg_attr(feature = "nightly", derive(HashStable_Generic))]
+pub struct VariantLayout<FieldIdx: Idx> {
+    pub size: Size,


Can't be removed, as it is used by the variant_size_differences lint.

moulins · 2026-01-27T14:31:15Z

compiler/rustc_abi/src/lib.rs

+#[cfg_attr(feature = "nightly", derive(HashStable_Generic))]
+pub struct VariantLayout<FieldIdx: Idx> {
+    pub size: Size,
+    pub backend_repr: BackendRepr,


I don't know enough about codegen to confidently say if it requires accurate reprs for enum variants, so I've left this field for now.

moulins · 2026-01-27T14:33:53Z

compiler/rustc_abi/src/lib.rs

+    pub size: Size,
+    pub backend_repr: BackendRepr,
+    pub field_offsets: IndexVec<FieldIdx, Size>,
+    fields_in_memory_order: IndexVec<u32, FieldIdx>,


Note: this field could be removed and recomputed from the field offsets in Layout::from_variant; it's unclear whether this is worth it.

moulins · 2026-01-27T14:35:01Z

compiler/rustc_codegen_llvm/src/debuginfo/metadata/enums/mod.rs

+                // Remove discriminant values of the other variants from the largest niche. This assumes
+                // that the largest niche, when it exists, always corresponds to the enum discriminant.


This assumption was already implicitly made by the original code.

moulins · 2026-01-27T14:38:24Z

compiler/rustc_codegen_llvm/src/debuginfo/metadata/enums/mod.rs

                let min = valid_range.start.min(valid_range.end);
                let min = tag.size(cx).truncate(min);

                let max = valid_range.start.max(valid_range.end);
                let max = tag.size(cx).truncate(max);


I think this logic is broken for valid ranges wrapping around uN::MAX? In any case, fixing this is out-of-scope of the PR, so I've left it as-is.

Kobzol · 2026-01-28T08:03:09Z

@bors try @rust-timer queue

Remove redundant information in `rustc_abi::Variants`

rust-bors · 2026-01-28T10:17:01Z

☀️ Try build successful (CI)
Build commit: 968e542 (968e5428d360caed1ceef0036386d8f8959a91bf, parent: e96bb7e44fbcc23c1e6009e8d0ee8ab208668fb4)

rust-timer · 2026-01-28T10:58:24Z

Finished benchmarking commit (968e542): comparison URL.

Overall result: ✅ improvements - no action needed

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.

@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
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.1%	[-0.3%, -0.0%]	6
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 1.4%, secondary -2.8%)

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

	mean	range	count
Regressions ❌ (primary)	3.8%	[1.6%, 5.7%]	4
Regressions ❌ (secondary)	1.9%	[0.5%, 3.4%]	3
Improvements ✅ (primary)	-1.7%	[-2.0%, -1.4%]	3
Improvements ✅ (secondary)	-4.7%	[-5.6%, -2.4%]	7
All ❌✅ (primary)	1.4%	[-2.0%, 5.7%]	7

Cycles

Results (secondary 3.7%)

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)	4.9%	[3.9%, 5.5%]	5
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.2%	[-2.2%, -2.2%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.0%)

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

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.0%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.0%]	1

Bootstrap: 470.891s -> 473.88s (0.63%)
Artifact size: 383.44 MiB -> 383.70 MiB (0.07%)

apiraino · 2026-03-26T12:34:01Z

r? compiler

cc @makai410 since you approved #151040?

makai410 · 2026-03-26T13:36:06Z

cc @workingjubilee IIRC you were involved in the related discussion on discord?

chenyukang · 2026-03-26T13:47:37Z

@rustbot reroll

jieyouxu · 2026-03-27T10:24:08Z

I'm finding another compiler reviewer for this PR.

For now, this is a 1-to-1 copy of `LayoutData`, but this will change.

It was always set to `Variants::Single`.

Enum variants always have `Arbitrary` layout, so the enum isn't needed.

This field is only used during layout calculations, so re-synthetized `LayoutData`s for enum variants can use a dummy value instead.

Reusing the alignment of the enclosing enum in `LayoutData::for_variant` doesn't appear to cause any issues.

The only place inspecting the niches of a variant layout is `compiler/rustc_codegen_llvm/src/debuginfo/metadata/enums/mod.rs`, which can be rewritten to use the niches of the enclosing enum instead.

rustbot · 2026-04-05T16:34:29Z

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.

rustbot added the A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. label Jan 27, 2026

rustbot assigned wesleywiser Jan 27, 2026

moulins commented Jan 27, 2026

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jan 28, 2026

rust-bors bot pushed a commit that referenced this pull request Jan 28, 2026

Auto merge of #151742 - moulins:variant-layout, r=<try>

968e542

Remove redundant information in `rustc_abi::Variants`

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jan 28, 2026

rustbot assigned chenyukang and unassigned wesleywiser Mar 26, 2026

rustbot assigned jieyouxu and unassigned chenyukang Mar 26, 2026

apiraino removed T-rustdoc-frontend Relevant to the rustdoc-frontend team, which will review and decide on the web UI/UX output. T-rustdoc Relevant to the rustdoc team, which will review and decide on the PR/issue. labels Apr 2, 2026

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jieyouxu added the S-no-work-capacity-tracking Status: Exempted from triagebot work capacity tracking. label Apr 5, 2026

moulins added 6 commits April 5, 2026 18:33

Introduce VariantLayout struct

d1d2efd

For now, this is a 1-to-1 copy of `LayoutData`, but this will change.

Remove redundant VariantLayout.variants field

dcc9f81

It was always set to `Variants::Single`.

Store FieldsShape::Arbitrary fields directly in VariantLayout

82a3a13

Enum variants always have `Arbitrary` layout, so the enum isn't needed.

Remove unused VariantLayout.randomization_seed field

8f0df41

This field is only used during layout calculations, so re-synthetized `LayoutData`s for enum variants can use a dummy value instead.

Remove per-variant alignment in VariantLayout

db81f4e

Reusing the alignment of the enclosing enum in `LayoutData::for_variant` doesn't appear to cause any issues.

Remove VariantLayout::largest_niche field

7c77389

The only place inspecting the niches of a variant layout is `compiler/rustc_codegen_llvm/src/debuginfo/metadata/enums/mod.rs`, which can be rewritten to use the niches of the enclosing enum instead.

moulins force-pushed the variant-layout branch from 8d1fb7e to 7c77389 Compare April 5, 2026 16:34

rustbot added the T-rustdoc-frontend Relevant to the rustdoc-frontend team, which will review and decide on the web UI/UX output. label Apr 5, 2026

apiraino removed the T-rustdoc-frontend Relevant to the rustdoc-frontend team, which will review and decide on the web UI/UX output. label Apr 9, 2026

		// Remove discriminant values of the other variants from the largest niche. This assumes
		// that the largest niche, when it exists, always corresponds to the enum discriminant.

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rust-timer commented Jan 28, 2026

Overall result: ✅ improvements - no action needed

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moulins commented Jan 27, 2026 •

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