layout: Store inverse memory index in `FieldsShape::Arbitrary` #150116

moulins · 2025-12-17T23:37:18Z

All usages of memory_index start by calling invert_bijective_mapping, so storing the inverted mapping directly saves some work and simplifies the code.

rustbot · 2025-12-17T23:37:22Z

The Miri subtree was changed

cc @rust-lang/miri

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

rustbot · 2025-12-17T23:37:25Z

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workingjubilee · 2025-12-18T00:40:33Z

huh. I wonder what it's inverted relative to?

workingjubilee · 2025-12-18T01:06:25Z

@moulins I have no objection to the substance of the change but "inverse" suggests a relativeness this doesn't capture... IMO this is still just a "memory index" until you do something like specify the other half of the pair, whether it's the key or value, at which point they might adopt a logical ordering.

mem_src_field_index maybe? idk.

workingjubilee · 2025-12-18T01:08:58Z

kinda curious if this has any effect on perf or if it's just logical cleanup

@bors try @rust-timer queue

layout: Store inverse memory index in `FieldsShape::Arbitrary`

moulins · 2025-12-18T01:31:06Z

I have no objection to the substance of the change but "inverse" suggests a relativeness this doesn't capture... IMO this is still just a "memory index" until you do something like specify the other half of the pair, whether it's the key or value, at which point they might adopt a logical ordering.

mem_src_field_index maybe? idk

Hm, good point... what about in_memory_order?

workingjubilee · 2025-12-18T01:58:13Z

something like that sounds good to me!

rust-bors · 2025-12-18T03:24:22Z

☀️ Try build successful (CI)
Build commit: ed52dc8 (ed52dc8f27781eef22c18d699984ef94c758cc92, parent: f794a08738958bbef01c3dcab34ec21d3da1e51a)

rust-timer · 2025-12-18T04:19:17Z

Finished benchmarking commit (ed52dc8): 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
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.1%	[-0.2%, -0.0%]	2
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 1.8%)

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

	mean	range	count
Regressions ❌ (primary)	1.8%	[1.8%, 1.8%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.8%	[1.8%, 1.8%]	1

Cycles

Results (primary 2.3%)

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

	mean	range	count
Regressions ❌ (primary)	2.3%	[2.3%, 2.3%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.3%	[2.3%, 2.3%]	1

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 481.694s -> 485.432s (0.78%)
Artifact size: 390.59 MiB -> 390.53 MiB (-0.01%)

RalfJung · 2025-12-18T06:57:18Z

compiler/rustc_const_eval/src/interpret/visitor.rs

-            FieldsShape::Arbitrary { memory_index, .. } => {
-                for idx in Self::aggregate_field_iter(memory_index) {
+            FieldsShape::Arbitrary { inverse_memory_index, .. } => {
+                for idx in Self::aggregate_field_iter(inverse_memory_index) {


If iterating in memory order is now cheap, maybe we should just always do that... in fact that could unlock a bunch of follow-up cleanup here. Probably best to make all that a follow-up PR, but a good indication that this PR moves us in the right direction. :)

So you'd propose removing aggregate_field_iter entirely and and simply iterate the fields directly?

Yes. But I would suggest doing it in a separate PR so we can separately benchmark that change.

moulins · 2025-12-18T14:12:36Z

Renamed inverse_memory_index to in_memory_order, and updated some UI tests that I missed previously.

EDIT: I just noticed that rust-analyzer has its own version of FieldsShape, should I perform the same change here too?

workingjubilee · 2025-12-18T18:07:26Z

@moulins I don't think this type is related in a meaningful way? https://github.com/rust-lang/rust/blob/main/src/tools/rust-analyzer/crates/hir-def/src/item_tree.rs#L519

Do you mean something else?

workingjubilee · 2025-12-18T20:31:49Z

To be clear, I am 100% certain that rust-analyzer uses rustc_abi::FieldsShape also but have no idea what uses need to be changed if any.

moulins · 2025-12-18T21:04:38Z

You're totally right; from what I can see, the uses are here in Layout::{tail_padding, largest_padding}, but for some reason my ./x check still succeeds and trying to rename these uses causes an error.

All usages of `memory_index` start by calling `invert_bijective_mapping`, so storing the inverted mapping directly saves some work and simplifies the code.

rustbot · 2025-12-18T21:26:59Z

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

rustbot · 2025-12-18T21:27:01Z

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.

workingjubilee · 2025-12-19T15:34:53Z

Cool!

r? workingjubilee
@bors r+ rollup

( I don't believe this is truly performance sensitive by itself, I only asked the bot because it was truly just idle curiosity and I'm trying to get a sense of where the hotter spots in rustc_abi actually are (since they're effectively only realized at usage sites). )

bors · 2025-12-19T15:34:56Z

📌 Commit b31ee3a has been approved by workingjubilee

It is now in the queue for this repository.

…=workingjubilee layout: Store inverse memory index in `FieldsShape::Arbitrary` All usages of `memory_index` start by calling `invert_bijective_mapping`, so storing the inverted mapping directly saves some work and simplifies the code.

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Dec 17, 2025

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moulins force-pushed the layout-inv-memory-index branch from c3ca7dc to 8db435e Compare December 18, 2025 00:12

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rust-bors bot added a commit that referenced this pull request Dec 18, 2025

Auto merge of #150116 - moulins:layout-inv-memory-index, r=<try>

ed52dc8

layout: Store inverse memory index in `FieldsShape::Arbitrary`

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Dec 18, 2025

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layout: Store inverse memory index in FieldsShape::Arbitrary

b31ee3a

All usages of `memory_index` start by calling `invert_bijective_mapping`, so storing the inverted mapping directly saves some work and simplifies the code.

moulins force-pushed the layout-inv-memory-index branch from e0bc269 to b31ee3a Compare December 18, 2025 21:26

rustbot assigned workingjubilee and unassigned nnethercote Dec 19, 2025

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Dec 19, 2025

matthiaskrgr mentioned this pull request Dec 19, 2025

Rollup of 6 pull requests #150167

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matthiaskrgr mentioned this pull request Dec 19, 2025

Rollup of 7 pull requests #150168

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rust-timer commented Dec 18, 2025

Overall result: ✅ improvements - no action needed

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layout: Store inverse memory index in `FieldsShape::Arbitrary` #150116

layout: Store inverse memory index in `FieldsShape::Arbitrary` #150116

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