perf(codegen): Restore noundef On PassMode::Cast Args In Rust ABI

[TKanX]

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Summary:

Problem:

Small aggregate arguments passed via PassMode::Cast in the Rust ABI (e.g. [u32; 2] cast to i64) are missing noundef in the emitted LLVM IR, even when the type contains no uninit bytes:

#[no_mangle] pub fn f(v: [u32; 2]) -> u32 { v[0] }

; expected: define i32 @f(i64 noundef %0) ; actual: define i32 @f(i64 %0) ← noundef missing

This blocks LLVM from applying optimizations that require value-defined semantics on function arguments.

Root Cause:

adjust_for_rust_abi calls arg.cast_to(Reg::Integer), which internally creates a CastTarget with ArgAttributes::new() — always empty. Any validity attribute that was present before the cast is silently dropped.

This affects all PassMode::Cast arguments and return values in the Rust ABI: plain arrays, newtype wrappers, and any BackendRepr::Memory type small enough to fit in a register.

A prior attempt (#127210) used Ty/repr attributes to detect padding.

Solution:

After adjust_for_rust_abi, iterate all PassMode::Cast args and the return value. For each, call layout_is_noundef on the original layout; if it returns true, set NoUndef on the CastTarget's attrs.

layout_is_noundef uses only the computed layout — BackendRepr, FieldsShape, Variants, Scalar::is_uninit_valid() — and never touches Ty or repr attributes. Anything it cannot prove returns false.

Covered cases:

• Scalar / ScalarPair (both halves initialized, fields contiguous)
• FieldsShape::Array (element type recursively uninit-free)
• FieldsShape::Arbitrary with Variants::Single (fields cover 0..size with no gaps, each recursively uninit-free) — handles newtype wrappers, multi-field structs, single-variant enums, repr(transparent), repr(C) wrappers

Conservatively excluded with FIXMEs:

• Multi-variant enums (per-variant padding analysis needed)
• Foreign-ABI casts (cast target may exceed layout size, needs a size guard)

Changes:

• compiler/rustc_ty_utils/src/abi.rs: add restoration loop after adjust_for_rust_abi; add layout_is_noundef and fields_cover_layout.
• tests/codegen-llvm/abi-noundef-cast.rs: new FileCheck test covering arrays, newtype wrappers (repr(Rust), repr(transparent), repr(C)), multi-field structs, single-variant enums, return values, and negative cases (MaybeUninit, struct with trailing padding).
• tests/codegen-llvm/debuginfo-dse.rs: update one CHECK pattern — Aggregate_4xi8 (struct { i8, i8, i8, i8 }) now correctly gets noundef.

Fixes #123183.

r? @RalfJung

[rustbot]

RalfJung is not on the review rotation at the moment.
They may take a while to respond.

[RalfJung]

I'll take a look at parts of this but I can't do the full review.
@rustbot reroll

[TKanX]

@rustbot label +A-LLVM +A-codegen +A-ABI +C-optimization +T-compiler

[TKanX]

@bjorn3 Ready for review. Thanks for the advice!

[RalfJung]

@bors try
@rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 336510d (336510dc97e872320307f66d5356820a14af413e, parent: d3e8bd94cc042c65e30a6c60146a8a00531d1292)

[rust-timer]

Finished benchmarking commit (336510d): comparison URL.

Overall result: no relevant changes - 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

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

Max RSS (memory usage)

Results (primary 2.0%, secondary 3.1%)

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

	mean	range	count
Regressions ❌ (primary)	2.0%	[1.8%, 2.2%]	2
Regressions ❌ (secondary)	3.1%	[2.1%, 4.5%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.0%	[1.8%, 2.2%]	2

Cycles

Results (primary 1.6%, secondary -2.2%)

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

	mean	range	count
Regressions ❌ (primary)	1.6%	[1.6%, 1.6%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.2%	[-2.2%, -2.2%]	1
All ❌✅ (primary)	1.6%	[1.6%, 1.6%]	1

Binary size

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

Bootstrap: 480.206s -> 479.468s (-0.15%)
Artifact size: 397.91 MiB -> 397.92 MiB (0.00%)

[rustbot]

⚠️ Warning ⚠️

• There are issue links (such as #123) in the commit messages of the following commits.
  Please move them to the PR description, to avoid spamming the issues with references to the commit, and so this bot can automatically canonicalize them to avoid issues with subtree.
  • 97bd985

[TKanX]

@rustbot ready

[RalfJung]

@bors r+

[rust-bors]

📌 Commit 97bd985 has been approved by RalfJung

It is now in the queue for this repository.

[rust-bors]

💔 Test for a3f0079 failed: CI. Failed job:

• auto - aarch64-apple (web logs, enhanced plaintext logs)

[JonathanBrouwer]

@bors retry
Spurious...

[rust-bors]

☀️ Test successful - CI
Approved by: RalfJung
Duration: 3h 31m 19s
Pushing c2c6f74 to main...

[github-actions]

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 8215374 (parent) -> c2c6f74 (this PR)

Test differences

Show 8 test diffs

Stage 1

• [codegen] tests/codegen-llvm/abi-noundef-cast.rs: [missing] -> pass (J2)
• [codegen] tests/codegen-llvm/abi-noundef-cast.rs: [missing] -> ignore (only executed when the pointer width is 64bit) (J3)

Stage 2

• [codegen] tests/codegen-llvm/abi-noundef-cast.rs: [missing] -> pass (J0)
• [codegen] tests/codegen-llvm/abi-noundef-cast.rs: [missing] -> ignore (only executed when the pointer width is 64bit) (J1)

Additionally, 4 doctest diffs were found. These are ignored, as they are noisy.

Job group index

• J0: aarch64-apple, aarch64-gnu, aarch64-gnu-llvm-20-1, aarch64-msvc-1, test-various, x86_64-gnu, x86_64-gnu-llvm-20-2, x86_64-gnu-llvm-21-2, x86_64-gnu-nopt, x86_64-gnu-stable, x86_64-mingw-1, x86_64-msvc-1
• J1: arm-android, armhf-gnu, dist-i586-gnu-i586-i686-musl, i686-gnu-1, i686-gnu-nopt-1, i686-msvc-1
• J2: x86_64-gnu-llvm-20-3, x86_64-gnu-llvm-21-3
• J3: pr-check-2

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \ test-dashboard c2c6f74fd2216a63c7d180d5dda4c825d503c2fa --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

1. dist-aarch64-apple: 2h 20m -> 1h 52m (-19.7%)
2. dist-x86_64-apple: 2h 27m -> 2h 3m (-16.3%)
3. dist-aarch64-llvm-mingw: 1h 43m -> 1h 30m (-13.0%)
4. dist-apple-various: 1h 32m -> 1h 41m (+9.9%)
5. x86_64-gnu-llvm-21-3: 1h 48m -> 1h 58m (+9.2%)
6. dist-x86_64-solaris: 1h 37m -> 1h 28m (-9.0%)
7. i686-msvc-1: 2h 46m -> 3h (+8.9%)
8. aarch64-apple: 3h 43m -> 3h 24m (-8.8%)
9. x86_64-gnu-miri: 1h 28m -> 1h 20m (-8.8%)
10. x86_64-gnu-aux: 2h 8m -> 2h 19m (+8.4%)

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]

Finished benchmarking commit (c2c6f74): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -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)	-0.5%	[-0.5%, -0.5%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -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)	-	-	0
Improvements ✅ (primary)	-3.7%	[-3.7%, -3.7%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.7%	[-3.7%, -3.7%]	1

Cycles

Results (secondary -2.5%)

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)	-2.5%	[-2.5%, -2.5%]	1
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 479.99s -> 479.752s (-0.05%)
Artifact size: 397.51 MiB -> 397.58 MiB (0.02%)