Replace OffsetOf by an actual sum of calls to intrinsic. #148151

cjgillot · 2025-10-26T20:55:55Z

This PR changes the way we compute the value of the offset_of! macro in MIR. The current implementation uses a dedicated MIR rvalue.

This PR proposes to replace it by an inline constant which sums calls to a new intrinsic offset_of(variant index, field index). The desugaring is done at THIR building time, easier that doing it on MIR.

The new intrinsic is only meant to be used by const-eval. LLVM codegen will refuse to generate code for it.

We replace:

a = offset_of!(T, Variant1.Field1.Variant2.Field2);

By:

a = const {constant#n};

{constant#n}: usize = {
    _1 = offset_of::<T>(index of Variant1, index of Field1);
    _2 = offset_of::<U>(index of Variant2, index of Field2); // Where T::Variant1::Field1 has type U
    _0 = _1 + _2
}

The second commit modifies intrinsic const checking to take allow_internal_unstable into account. The new intrinsic should only be called from stable offset_of! macro. The intrinsic itself is unstable, const-unstable, but rustc_intrinsic_const_stable_indirect.

Fixes #123959
Fixes #125680
Fixes #129425
Fixes #136175

r? @ghost

library/core/src/intrinsics/mod.rs

cjgillot · 2025-10-27T00:53:27Z

@bors try @rust-timer queue

Replace OffsetOf by an actual sum of calls to intrinsic.

rust-bors · 2025-10-27T03:07:33Z

☀️ Try build successful (CI)
Build commit: 7721497 (772149783c4c312fd88b7dfaa68a045b14db3280, parent: f37aa9955f03bb1bc6fe08670cb1ecae534b5815)

rust-timer · 2025-10-27T05:51:54Z

Finished benchmarking commit (7721497): comparison URL.

Overall result: ❌✅ regressions and 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)	0.1%	[0.0%, 0.1%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	3
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (secondary -1.9%)

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)	2.6%	[2.1%, 3.2%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.7%	[-4.9%, -1.5%]	5
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary -0.8%)

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)	2.7%	[1.6%, 5.0%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-6.1%	[-7.9%, -4.2%]	2
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 473.876s -> 473.729s (-0.03%)
Artifact size: 390.50 MiB -> 390.46 MiB (-0.01%)

bors · 2025-10-31T16:23:55Z

☔ The latest upstream changes (presumably #148324) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot · 2025-11-01T23:47:25Z

Some changes occurred in match checking

cc @Nadrieril

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

⚠️ #[rustc_intrinsic_const_stable_indirect] controls whether intrinsics can be exposed to stable const
code; adding it needs t-lang approval.

cc @rust-lang/wg-const-eval

Some changes occurred to constck

cc @fee1-dead

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

This PR changes rustc_public

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

Some changes occurred in rustc_ty_utils::consts.rs

cc @BoxyUwU

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @vakaras

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

This PR changes a file inside tests/crashes. If a crash was fixed, please move into the corresponding ui subdir and add 'Fixes #' to the PR description to autoclose the issue upon merge.

BoxyUwU · 2025-11-02T20:43:45Z

What's the justification/benefit for doing this, and what user facing impact does it have (i.e. why does it need a lang nomination)?

cjgillot · 2025-11-02T20:58:55Z

My justification is simplifying MIR. This feature was implemented as a specific MIR statement, but does not need to, an intrinsic is sufficient.

There should be no user-facing change because of the offset_of manipulation. Maybe some diagnostics, but not more.

However, there are 2 user-facing changes in this PR that t-lang may want to know:

const-stability of intrinsics now takes allow_internal_unstable into account;
we get an additional intrinsic with rustc_intrinsic_const_stable_indirect.

rustbot · 2025-11-18T00:33:17Z

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.

oli-obk · 2025-11-18T09:16:00Z

@bors r=scottmcm

bors · 2025-11-18T09:16:03Z

📌 Commit e799160 has been approved by scottmcm

It is now in the queue for this repository.

bors · 2025-11-18T10:56:04Z

⌛ Testing commit e799160 with merge f9e7961...

bors · 2025-11-18T14:06:38Z

☀️ Test successful - checks-actions
Approved by: scottmcm
Pushing f9e7961 to main...

github-actions · 2025-11-18T14:09:21Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing c199562 (parent) -> f9e7961 (this PR)

Test differences

Show 82 test diffs

Stage 1

[crashes] tests/crashes/123959.rs: pass -> [missing] (J3)
[crashes] tests/crashes/125680.rs: pass -> [missing] (J3)
[crashes] tests/crashes/129425.rs: pass -> [missing] (J3)
[crashes] tests/crashes/136175-2.rs: pass -> [missing] (J3)
[mir-opt] tests/mir-opt/building/offset_of.rs: [missing] -> pass (J3)
[mir-opt] tests/mir-opt/dataflow-const-prop/offset_of.rs: pass -> [missing] (J3)
[ui] tests/ui/offset-of/inside-array-length.rs: [missing] -> pass (J3)
[ui] tests/ui/thir-print/offset_of.rs: [missing] -> pass (J3)

Stage 2

[crashes] tests/crashes/123959.rs: pass -> [missing] (J0)
[crashes] tests/crashes/125680.rs: pass -> [missing] (J0)
[crashes] tests/crashes/129425.rs: pass -> [missing] (J0)
[crashes] tests/crashes/136175-2.rs: pass -> [missing] (J0)
[ui] tests/ui/offset-of/inside-array-length.rs: [missing] -> pass (J1)
[ui] tests/ui/thir-print/offset_of.rs: [missing] -> pass (J1)
[mir-opt] tests/mir-opt/building/offset_of.rs: [missing] -> pass (J2)
[mir-opt] tests/mir-opt/dataflow-const-prop/offset_of.rs: pass -> [missing] (J2)

Additionally, 66 doctest diffs were found. These are ignored, as they are noisy.

Job group index

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard f9e7961506a97b318ad4815b8ce94bb045562f89 --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

x86_64-gnu-gcc: 2788.1s -> 3512.0s (+26.0%)
aarch64-apple: 7671.6s -> 9098.6s (+18.6%)
pr-check-1: 1633.7s -> 1918.4s (+17.4%)
aarch64-gnu-llvm-20-1: 3230.0s -> 3765.5s (+16.6%)
x86_64-rust-for-linux: 2666.8s -> 3098.8s (+16.2%)
x86_64-gnu-llvm-20: 2257.8s -> 2593.6s (+14.9%)
dist-x86_64-solaris: 5282.4s -> 5894.9s (+11.6%)
dist-aarch64-msvc: 6224.0s -> 5534.3s (-11.1%)
aarch64-gnu-debug: 3940.7s -> 4360.6s (+10.7%)
x86_64-gnu-llvm-20-1: 3166.7s -> 3489.0s (+10.2%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

rust-timer · 2025-11-18T15:25:50Z

Finished benchmarking commit (f9e7961): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

If the regression was expected or you think it can be justified,
please write a comment with sufficient written justification, and add
@rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
If you think that you know of a way to resolve the regression, try to create
a new PR with a fix for the regression.
If you do not understand the regression or you think that it is just noise,
you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
were already notified of this PR).

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

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)	3.1%	[3.1%, 3.1%]	1
Regressions ❌ (secondary)	0.3%	[0.1%, 0.6%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.4%	[-0.9%, -0.0%]	7
All ❌✅ (primary)	3.1%	[3.1%, 3.1%]	1

Max RSS (memory usage)

Results (primary 2.1%, secondary -0.6%)

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

	mean	range	count
Regressions ❌ (primary)	2.1%	[2.0%, 2.2%]	2
Regressions ❌ (secondary)	3.4%	[2.4%, 4.4%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.6%	[-5.5%, -0.7%]	6
All ❌✅ (primary)	2.1%	[2.0%, 2.2%]	2

Cycles

Results (secondary -0.3%)

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)	4.0%	[2.3%, 6.6%]	5
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.5%	[-7.6%, -1.3%]	5
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 1.1%)

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

	mean	range	count
Regressions ❌ (primary)	1.1%	[1.1%, 1.1%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.1%	[1.1%, 1.1%]	1

Bootstrap: 474.876s -> 471.624s (-0.68%)
Artifact size: 388.76 MiB -> 388.73 MiB (-0.01%)

lqd · 2025-11-18T15:29:00Z

clap_derive is noise, include-blob looks like noise.

Relevant upstream PR: - rust-lang/rust#148151 (Replace OffsetOf by an actual sum of calls to intrinsic.) Resolves: model-checking#4481

panstromek · 2025-11-19T13:24:32Z

perf triage:

As indicated by the comment above, clap_derive is noise, include-blob is also noise (at least the opt variants, other ones didn't return to previous state, but they are tiny in absolute numbers).

@rustbot label: +perf-regression-triaged

Relevant upstream PR: - rust-lang/rust#148151 (Replace OffsetOf by an actual sum of calls to intrinsic.) Resolves: #4481 By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

cjgillot force-pushed the no-offset-of branch from 0dc894f to e46804b Compare October 26, 2025 20:56

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rust-bors bot added a commit that referenced this pull request Oct 27, 2025

Auto merge of #148151 - cjgillot:no-offset-of, r=<try>

7721497

Replace OffsetOf by an actual sum of calls to intrinsic.

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Oct 27, 2025

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cjgillot force-pushed the no-offset-of branch from 875c6f6 to 9fd16d6 Compare November 1, 2025 23:45

cjgillot marked this pull request as ready for review November 1, 2025 23:47

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Nov 1, 2025

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cjgillot force-pushed the no-offset-of branch from fc6c34f to 5b48fd9 Compare November 16, 2025 22:00

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cjgillot force-pushed the no-offset-of branch from 5b48fd9 to 85eca56 Compare November 16, 2025 22:03

cjgillot added 5 commits November 18, 2025 00:10

Replace OffsetOf by an actual sum.

7244437

Honor allow_internal_unstable for const intrinsics.

12e91cf

Simplify implementation.

63a7712

Complete doc.

aeb9332

Add THIR building test.

e799160

cjgillot force-pushed the no-offset-of branch from 85eca56 to e799160 Compare November 18, 2025 00:33

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 Nov 18, 2025

bors added the merged-by-bors This PR was explicitly merged by bors. label Nov 18, 2025

bors merged commit f9e7961 into rust-lang:main Nov 18, 2025
12 checks passed

rustbot added this to the 1.93.0 milestone Nov 18, 2025

bors mentioned this pull request Nov 18, 2025

Offload intrinsic #147936

Open

rustbot added the perf-regression Performance regression. label Nov 18, 2025

cjgillot deleted the no-offset-of branch November 18, 2025 21:30

tautschnig added a commit to tautschnig/kani that referenced this pull request Nov 19, 2025

Upgrade Rust toolchain to 2025-11-19

e738898

Relevant upstream PR: - rust-lang/rust#148151 (Replace OffsetOf by an actual sum of calls to intrinsic.) Resolves: model-checking#4481

tautschnig mentioned this pull request Nov 19, 2025

Upgrade Rust toolchain to 2025-11-19 model-checking/kani#4482

Merged

rustbot added the perf-regression-triaged The performance regression has been triaged. label Nov 19, 2025

Replace OffsetOf by an actual sum of calls to intrinsic. #148151

Replace OffsetOf by an actual sum of calls to intrinsic. #148151

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Overall result: ❌✅ regressions and improvements - no action needed

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Test differences

Stage 1

Stage 2

Job duration changes

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Overall result: ❌✅ regressions and improvements - please read the text below

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