Add a `layout_of_val` intrinsic for when you want both `size_of_val`+`align_of_val` by scottmcm · Pull Request #152867 · rust-lang/rust

scottmcm · 2026-02-19T21:01:34Z

(This is a replacement for #152786 where github broke)

Today we're emitting a bunch more MIR and LLVM-IR than we need for stuff like this.

Closes #152641 because this solves the motivating example from that PR of avoiding the alignment transmutes when getting a layout from a DST, just in a better way.

While I was in the area I also put a more specific alignment !range metadata on loading alignment from a vtable and some extra no-wrap flags when calculating the size of an ADT with an unsized tail.

Same reviewer as was rolled before,
r? @JonathanBrouwer

rustbot · 2026-02-19T21:01:38Z

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

⚠️ #[miri::intrinsic_fallback_is_spec] must only be used if the function actively checks for all UB cases,
and explores the possible non-determinism of the intrinsic.

cc @rust-lang/miri

scottmcm · 2026-02-19T21:03:42Z

This has another library change that the previous perf run didn't, so checking things are still good
@bors try @rust-timer queue

Add a `layout_of_val` intrinsic for when you want both `size_of_val`+`align_of_val`

rust-bors · 2026-02-19T23:13:51Z

☀️ Try build successful (CI)
Build commit: bb0e06e (bb0e06e2d9fe1b22d66a34677fe80747042cb320, parent: b6bed6f2d9f27e15d425958bbd288138ecdff61c)

rust-timer · 2026-02-19T23:53:36Z

Finished benchmarking commit (bb0e06e): comparison URL.

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

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.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@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)	1.7%	[1.7%, 1.7%]	1
Regressions ❌ (secondary)	0.8%	[0.8%, 0.8%]	1
Improvements ✅ (primary)	-0.6%	[-1.0%, -0.3%]	3
Improvements ✅ (secondary)	-1.3%	[-3.2%, -0.4%]	9
All ❌✅ (primary)	-0.0%	[-1.0%, 1.7%]	4

Max RSS (memory usage)

Results (primary -0.3%, secondary -3.8%)

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

	mean	range	count
Regressions ❌ (primary)	4.8%	[4.8%, 4.8%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.8%	[-3.4%, -2.2%]	2
Improvements ✅ (secondary)	-3.8%	[-4.4%, -3.2%]	2
All ❌✅ (primary)	-0.3%	[-3.4%, 4.8%]	3

Cycles

Results (secondary 1.7%)

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)	9.0%	[4.7%, 13.2%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.2%	[-4.2%, -2.4%]	3
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.3%, secondary -1.3%)

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%]	3
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	1
Improvements ✅ (primary)	-0.4%	[-0.8%, -0.0%]	19
Improvements ✅ (secondary)	-1.4%	[-3.8%, -0.0%]	15
All ❌✅ (primary)	-0.3%	[-0.8%, 0.0%]	22

Bootstrap: 481.703s -> 481.307s (-0.08%)
Artifact size: 397.81 MiB -> 395.82 MiB (-0.50%)

rustbot · 2026-02-20T02:59:32Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

scottmcm · 2026-02-20T03:58:56Z

@bors try @rust-timer queue

Add a `layout_of_val` intrinsic for when you want both `size_of_val`+`align_of_val`

rust-bors · 2026-02-20T06:10:54Z

☀️ Try build successful (CI)
Build commit: 42ba6e2 (42ba6e2cd86001f68af90745a6e3a0e2bdc16b6f, parent: ef70767064ab87b0a41400f69e1dc0b55c8d5284)

rust-timer · 2026-02-20T06:50:36Z

Finished benchmarking commit (42ba6e2): comparison URL.

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

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.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@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)	1.7%	[1.7%, 1.7%]	1
Regressions ❌ (secondary)	0.7%	[0.2%, 1.2%]	2
Improvements ✅ (primary)	-0.6%	[-1.0%, -0.3%]	3
Improvements ✅ (secondary)	-1.1%	[-3.0%, -0.4%]	10
All ❌✅ (primary)	0.0%	[-1.0%, 1.7%]	4

Max RSS (memory usage)

Results (primary 0.6%, 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)	2.2%	[2.2%, 2.3%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.7%	[-2.7%, -2.7%]	1
Improvements ✅ (secondary)	-2.8%	[-2.8%, -2.7%]	2
All ❌✅ (primary)	0.6%	[-2.7%, 2.3%]	3

Cycles

Results (primary 3.2%, secondary -3.8%)

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

	mean	range	count
Regressions ❌ (primary)	3.2%	[2.9%, 3.4%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.8%	[-3.8%, -3.8%]	1
All ❌✅ (primary)	3.2%	[2.9%, 3.4%]	2

Binary size

Results (primary -0.3%, secondary -1.4%)

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

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.4%]	4
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	1
Improvements ✅ (primary)	-0.4%	[-0.8%, -0.1%]	18
Improvements ✅ (secondary)	-1.5%	[-3.8%, -0.3%]	14
All ❌✅ (primary)	-0.3%	[-0.8%, 0.4%]	22

Bootstrap: 496.246s -> 480.362s (-3.20%)
Artifact size: 397.86 MiB -> 397.94 MiB (0.02%)

JonathanBrouwer · 2026-02-20T14:45:28Z

I think the perf is still not as nicely green as #152786 (comment)
Haven't taken a look at the code yet, it seems like you're still working on this, let me know when it's ready for review :)
@rustbot author

…r=mati865 Tighten the `!range` bounds on alignments in vtables Right now we're only telling LLVM that they're non-zero, but alignments must be powers of two so can't be more than `isize::MAX+1`. And we actually never emit anything beyond LLVM's limit of 2²⁹, so outside of 16-bit targets the limit is that. (Pulled out from rust-lang#152867 which is starting to have too much in it.)

Rollup merge of #152929 - scottmcm:better-alignment-ranges, r=mati865 Tighten the `!range` bounds on alignments in vtables Right now we're only telling LLVM that they're non-zero, but alignments must be powers of two so can't be more than `isize::MAX+1`. And we actually never emit anything beyond LLVM's limit of 2²⁹, so outside of 16-bit targets the limit is that. (Pulled out from #152867 which is starting to have too much in it.)

It was already computing both anyway with `size_of_val::size_and_align_of_dst`, so we just need to return them both instead of ignoring one.

Same as rust-lang/rust 152689 did for align/size.

rustbot · 2026-02-22T00:06:55Z

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.

scottmcm · 2026-02-22T00:07:55Z

@bors try @rust-timer queue

Add a `layout_of_val` intrinsic for when you want both `size_of_val`+`align_of_val`

rust-bors · 2026-02-22T02:19:01Z

☀️ Try build successful (CI)
Build commit: 6db21b4 (6db21b409cc72631f20d5c8e2509975486b890ca, parent: 5fb2ff8611e5a4af4dc85977cfdecfbf3ffa6ade)

rust-timer · 2026-02-22T02:59:36Z

Finished benchmarking commit (6db21b4): comparison URL.

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

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.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@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)	1.9%	[1.9%, 1.9%]	1
Regressions ❌ (secondary)	1.6%	[1.6%, 1.6%]	1
Improvements ✅ (primary)	-0.7%	[-1.0%, -0.3%]	2
Improvements ✅ (secondary)	-1.4%	[-3.1%, -0.4%]	7
All ❌✅ (primary)	0.2%	[-1.0%, 1.9%]	3

Max RSS (memory usage)

Results (primary -0.6%, 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)	4.5%	[4.3%, 4.7%]	2
Regressions ❌ (secondary)	2.5%	[2.5%, 2.5%]	1
Improvements ✅ (primary)	-3.1%	[-5.2%, -1.8%]	4
Improvements ✅ (secondary)	-3.1%	[-3.1%, -3.1%]	1
All ❌✅ (primary)	-0.6%	[-5.2%, 4.7%]	6

Cycles

Results (primary 2.3%, secondary -0.9%)

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)	2.8%	[2.3%, 3.3%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.3%	[-3.7%, -2.7%]	3
All ❌✅ (primary)	2.3%	[2.3%, 2.3%]	1

Binary size

Results (primary -0.4%, secondary -1.6%)

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%	[-0.8%, -0.1%]	21
Improvements ✅ (secondary)	-1.6%	[-3.9%, -0.3%]	14
All ❌✅ (primary)	-0.4%	[-0.8%, -0.1%]	21

Bootstrap: 480.585s -> 482.646s (0.43%)
Artifact size: 395.91 MiB -> 395.90 MiB (-0.00%)

rustbot assigned JonathanBrouwer Feb 19, 2026

scottmcm mentioned this pull request Feb 19, 2026

Add a layout_of_val intrinsic for when you want both size_of_val+align_of_val #152786

Closed