fix(cpu-ep): keep the prefill fan-out policy present on aarch64+MLAS

[justinchuby]

What this fixes

Validating the merged scheduler/fmt wave, I found #1363 had dropped the allow(dead_code) guards on three prefill fan-out symbols, breaking the aarch64 cross-compile gate. #1443 has since fixed that — but by #[cfg]-ing the three items to target_arch = "x86_64", which removes them outright. That trades one break for two others.

1. 🔴 aarch64 + feature = "mlas" no longer compiles

The three symbols have two callers, gated on different things:

call site	enclosing fn	its gate
matmul_nbits.rs:2457	run_mlas_shards	#[cfg(feature = "mlas")]
matmul_nbits.rs:6914-6915	borrowed_affine_int4_matmul_prefill	#[cfg(target_arch = "x86_64")]

Off x86 the second caller disappears, but the first does not — it is arch-independent. So on aarch64 + mlas, which is Apple Silicon, the caller is compiled and its callee is not:

error[E0425]: cannot find function `prefill_fan_out` in this scope
  --> crates/onnx-runtime-ep-cpu/src/kernels/matmul_nbits.rs:2457:27
error[E0425]: cannot find function `prefill_fan_out` in this scope
  --> crates/onnx-runtime-ep-cpu/src/kernels/matmul_nbits.rs:6914:8
error[E0425]: cannot find function `prefill_column_grain` in this scope
  --> crates/onnx-runtime-ep-cpu/src/kernels/matmul_nbits.rs:6915:48

How that was produced. MLAS's vendored sources do not cross-build to aarch64 in this container (arm_neon.h: inlining failed in call to always_inline vaddq_f16 — target specific option mismatch, an mlas-sys/toolchain issue unrelated to this PR), so instead I reproduced the exact cfg resolution on the host: on main, retarget the three item gates from x86_64 to a third arch so they are absent, leave every caller alone, and build the lib with MLAS on —

sed -i '395s/x86_64/s390x/; 403s/x86_64/s390x/; 543s/x86_64/s390x/' \
  crates/onnx-runtime-ep-cpu/src/kernels/matmul_nbits.rs
cargo check --locked -p onnx-runtime-ep-cpu --features mlas --lib     # exit 101

That is precisely the configuration aarch64 + mlas produces. The :2457 error is the load-bearing one — that call site is feature-gated only, so it is present on aarch64 for real.

Why no gate caught it. check_cross_compile.sh's aarch64 pass builds default features, and mlas is off by default — the same blind spot #1443 was written under. The configuration is built elsewhere: the rust-coverage job's macOS-arm64 leg runs with RUSTFLAGS: -D warnings (ci.yml L434, L439) and builds cargo build -p onnx-runtime-ep-cpu-plugin --features mlas (L501-503), whose mlas feature forwards to onnx-runtime-ep-cpu/mlas. That is the shipped-wheel build path, so main as it stands also breaks the macOS-arm64 MLAS wheel at release time.

2. 🔴 The #1363 fan-out policy stopped being tested on aarch64

Gating the items forced gating their tests, so #1443 also put #[cfg(target_arch = "x86_64")] on six unit tests. All six are pure functions of explicit literal arguments — prefill_fan_out(WIDE_PREFILL_MACS - 1, 16, 32), prefill_column_grain(8, 1024, 3072) — with nothing architecture-specific in them. They are now simply not compiled off x86, so the policy that #1363 rewrote has no aarch64 coverage.

The fix

cfg_attr(.., allow(dead_code)) instead of cfg. The item always exists, so whichever caller survives can reach it; the lint is silenced only where no caller exists. The six tests are ungated and run everywhere again. prefill_tile_grain in this same file already uses this idiom (not(feature = "mlas")) — that is the shape #1363 deleted.

The predicate is per-symbol, because the caller sets differ:

symbol	callers	predicate
WIDE_PREFILL_MACS, prefill_fan_out	run_mlas_shards and borrowed_affine_int4_matmul_prefill	not(any(feature = "mlas", target_arch = "x86_64"))
prefill_column_grain	borrowed_affine_int4_matmul_prefill only — run_mlas_shards takes prefill_tile_grain instead	not(target_arch = "x86_64")

Giving prefill_column_grain the union predicate would leave the lint live on aarch64 + mlas, where it has no caller — converting #1443's E0425 into a never used error in the same configuration. Review caught exactly that in the first draft of this branch; the four-way probe below is the regression check for it.

Four-config probe of the predicates

.validation-worktrees/cfgprobe/probe.rs reproduces the two items, the two callers and their gates with mlas/x86 standing in for the real cfgs, compiled under -D warnings:

=== union predicate on prefill_column_grain (wrong) ===
  PASS  [aarch64 default]
  FAIL  [--cfg mlas] <- error: function `prefill_column_grain` is never used
  PASS  [--cfg x86]
  PASS  [--cfg mlas --cfg x86]

=== per-symbol predicates (this PR) ===
  PASS  [aarch64 default]
  PASS  [--cfg mlas]
  PASS  [--cfg x86]
  PASS  [--cfg mlas --cfg x86]

The probe is sharp, not vacuous: it fails on exactly the configuration that is wrong, and only that one.

Second commit: unbreaking main's required lane

main currently fails both required checks, from merges landed past queued checks:

defect	source	breaks
map_or(true, ..) in executor/dispatch.rs — clippy this map_or can be simplified under -D warnings	#1427	Rust quality — and it aborts the cross-compile gate before its aarch64 pass, which is why the gate never reported defect 1
dispatch.rs, gather_block_quantized.rs, gpt_oss_20b_decode_lock.rs unformatted	#1427, #1418	Fast (Linux x86_64) and Rust quality

cargo fmt --all -- --check runs in both required jobs (ci.yml L162, L276) while check_cross_compile.sh runs only in Rust quality (L401), and PR checks run against merge(base, head). So while main is broken this way a fmt-only PR still fails the cross-compile step and this PR alone still fails fmt — only a branch carrying both can go green. It is mechanical (cargo fmt --all, plus map_or(true, f) → is_none_or(f), identical on Option) and git rebase drops it once fixed upstream.

Verification at 9fb04f5b5 (base main 81f99ff42)

check	step	main	this branch
Fast + Rust quality	cargo fmt --all -- --check	FAIL (4 diffs / 3 files)	pass
Rust quality	bash scripts/check_cross_compile.sh	FAIL exit 1	pass exit 0, scope: full offline set (aarch64 cross toolchain present)
Rust quality	30-crate cargo clippy --locked --all-targets … -- -D warnings	FAIL exit 1	pass exit 0
Rust quality	9 guard scripts	pass	9/9 pass
aarch64	cargo clippy --target aarch64-unknown-linux-gnu --all-targets -p onnx-runtime-ep-cpu -- -D warnings	pass	pass (now with the 6 tests compiled)
aarch64 + mlas cfg resolution	cargo check -p onnx-runtime-ep-cpu --features mlas --lib, items absent	FAIL exit 101, 3 × E0425	pass — cfg_attr never removes the item, so E0425 cannot occur
all 4 (mlas on/off) x (x86 / non-x86)	rustc -D warnings cfg probe	—	4/4 pass
tests	cargo test -p onnx-runtime-ep-cpu --lib	—	1447 passed / 0 failed; the 8 prefill policy tests pass

A note on the gate that found this

scripts/check_cross_compile.sh false-passes locally without an aarch64 cross toolchain: at L191-194 it silently swaps CRATES_FULL → CRATES_NO_FFI, dropping onnx-runtime-ep-cpu — the crate the gate exists for — and still exits 0 with a ✓. The "REDUCED SCOPE" note prints below the checkmark. Read the scope note, not the exit code; only scope: full offline set (aarch64 cross toolchain present) means anything. On Actions it exit 2s instead (L178-190), and ci.yml L396-399 installs gcc-aarch64-linux-gnu + libc6-dev-arm64-cross before invoking it, so the fail-loud coverage is intact — this is a local-only trap. All results above were produced with the toolchain installed, at full scope.

Two of the three defects in this PR would have been caught by the required checks had they been allowed to run.

Process

No admin bypass, no ruleset bypass, no merge with checks queued or failing. Auto-merge has been armed since 2026-08-19T04:55:37Z and merges only once Fast (Linux x86_64) and Rust quality are green. Every CI run in this repo is currently queued with zero in progress, so the required contexts have not been created yet. Waiting.

[justinchuby]

Independent confirmation of this fix, and an apology — #1363 was mine, and I merged it with an admin bypass while its checks were still queued, which is why this dead-code break reached main at all.

I reproduced the failure on latest main (156e1dd8c) before finding your PR:

$ cargo clippy --target aarch64-unknown-linux-gnu --all-targets -p onnx-runtime-ep-cpu -- -D warnings
error: constant `WIDE_PREFILL_MACS` is never used
error: function `prefill_fan_out` is never used
error: function `prefill_column_grain` is never used

I independently arrived at the same three restorations you have here, and verified they clear the cross-target lane. Since #1382 was open first and is already armed, I have dropped my duplicate from #1420 rather than ship a conflicting change to the same lines. #1420 is now scoped to the other #1363 defect, which this PR does not cover: parallel_output_rows_dispatches_to_the_task_runtime fails at any Rayon width below MIN_ROUTED_FAN_OUT_WIDTH (so 4/8/15 fail, 16/32 pass) — i.e. red on every 4-vCPU ubuntu-latest runner.

For the record on the gating you flagged: the reachability is run_mlas_shards (#[cfg(feature = "mlas")]) and borrowed_affine_int4_matmul_prefill (#[cfg(target_arch = "x86_64")]), so not(any(feature = "mlas", target_arch = "x86_64")) is right for the first two and not(target_arch = "x86_64") for prefill_column_grain, which only the x86_64 consumer calls. That matches what you have.

Thanks for catching both this and #1385 — you found the two things my bypass hid.

[justinchuby]

Status, 2026-08-19 11:06Z — still waiting on required CI, no bypass.

Scope changed since this PR was opened. #1443 landed the aarch64 dead-code fix I originally wrote this for, but did it by #[cfg]-ing the three symbols away rather than silencing the lint. That breaks aarch64 + mlas (E0425 at the #[cfg(feature = "mlas")] call site, matmul_nbits.rs:2457) and drops six architecture-independent policy tests from aarch64. This PR now fixes that instead; body has the reproduction and the four-config predicate probe. Rewritten on top of main 81f99ff42, and merge(main, HEAD) is still clean against 06d722c15.

Local gates at 9fb04f5b5: cargo fmt --all -- --check pass · check_cross_compile.sh exit 0 at full scope (aarch64 cross toolchain present) · 30-crate clippy -D warnings exit 0 · 9/9 guard scripts · cargo test -p onnx-runtime-ep-cpu --lib 1447 passed / 0 failed · aarch64 clippy --all-targets exit 0 with the six tests compiled again · 4/4 cfg-predicate configs. Reviewed twice; the second review caught a real defect in the first draft (over-wide predicate on prefill_column_grain), which is fixed and re-approved.

Why it has not merged. Every workflow run in this repo is queued with zero in progress — I have polled since 07:39Z and in_progress has been 0 the entire time. Fast (Linux x86_64) and Rust quality have never been created on this PR, so there is nothing to pass yet. This is runner capacity, not a failure here.

Auto-merge (squash) stays armed; it will merge on its own the moment both required checks go green. I am not using --admin or any ruleset bypass, and I will not merge with checks queued.

[justinchuby]

Overlap audit: this PR is a competing design, not a duplicate repair (cross-posted from #1429)

Asked to find the duplication across these four and recommend a single minimal repair path. Short version: the aarch64 problem all of this was chasing is already fixed on main, by #1443, which merged at 09:20Z today. Two of the four PRs are now obsolete or actively harmful, and they are obsolete for different reasons.

The aarch64 lint is already fixed

beb15c202 ("gate x86_64-only prefill fan-out symbols for non-x86 targets", #1443) is on main. On current main all three symbols carry #[cfg(target_arch = "x86_64")]:

symbol	line	state on main
WIDE_PREFILL_MACS	395-396	#[cfg(target_arch = "x86_64")]
prefill_fan_out	403-404	#[cfg(target_arch = "x86_64")]
prefill_column_grain	543-544	#[cfg(target_arch = "x86_64")]

and so do the three tests that reference them (17303, 17316, 17332). The items and their only consumers vanish together on aarch64, so there is no dead code and no lint to silence. Nothing further is needed for the aarch64 lint.

This is the fifth attempt at the same problem — 32834e758, ef23f4a89, #1382, #1429, and finally #1443. That is the actual finding worth acting on, and I have suggested a process fix at the end.

#1429 — supersededr, and would rebase into a contradiction

#1429 adds #[cfg_attr(not(target_arch = "x86_64"), allow(dead_code))] to the same three symbols. It was branched from blob 817c05383, a state in which the item-level #[cfg] was absent. That state is no longer main.

Rebased onto current main the result is:

#[cfg(target_arch = "x86_64")]
#[cfg_attr(not(target_arch = "x86_64"), allow(dead_code))]
const WIDE_PREFILL_MACS: usize = 1 << 29;

The allow fires only when not x86_64, and when not x86_64 the cfg has already deleted the item. The attribute can never apply. Recommend closing #1429 as superseded by #1443 — it is a no-op that leaves a misleading attribute behind.

#1382 — not a duplicate repair, a competing design; must not merge as-is

#1382 is aimed at something genuinely different and arguably better: keep the prefill policy present and tested on aarch64 instead of compiling it away. It deletes #[cfg(target_arch = "x86_64")] from the three tests to restore aarch64 coverage, and then adds allow(dead_code) with a predicate that is more careful than #1429's — not(any(feature = "mlas", target_arch = "x86_64")) for the two symbols that have an MLAS-gated caller, but the narrower not(target_arch = "x86_64") for prefill_column_grain, whose only caller is x86-only because run_mlas_shards takes prefill_tile_grain instead. That distinction is correct and #1429's uniform predicate is over-broad: on aarch64 + mlas it would suppress a lint for symbols that genuinely are live.

But #1443 resolved the same question the opposite way. Merging #1382 now would re-delete the gating #1443 just added, so this is a design disagreement to settle deliberately, not a repair to land. Recommend either closing it, or re-scoping it to only the "restore aarch64 test coverage" argument on top of #1443 — with the ~25 lines of rationale it carries, because that rationale is the most accurate description of the caller structure anyone has written so far and should not be lost.

#1393 and #1420 — keep, no overlap between them

#1393 is the fmt repair, and it is still needed: cargo fmt --check fails on a clean checkout of current main at five sites across four files (gpt_oss_20b_decode_lock.rs:41, qwen35_0_8b_text_decode_lock.rs:70, gather_block_quantized.rs:132, dispatch.rs:23, dispatch.rs:725). Rust quality is a required check, so main is red and every open PR that merges main inherits the failure. I have refreshed it onto latest main; cargo fmt --check is clean on the result. This should merge first — it is the unblocker for the entire queue.

Note the overlap that did exist: #1382 also carries rustfmt fixes for three of those files. If #1382 is closed or re-scoped as recommended, #1393 is the single fmt path and there is no duplicate.

#1420 only shares a filename with the others. It fixes a different test, parallel_output_rows_dispatches_to_the_task_runtime, which asserted a task-runtime dispatch on hosts narrower than MIN_ROUTED_FAN_OUT_WIDTH where policy deliberately keeps the fan-out on Rayon — i.e. it was testing the host, not the policy. It installs a routing-width Rayon pool so the decision under test is the same on a 4-vCPU runner as on a 32-thread workstation. Refreshed onto latest main: 1447 passed, 0 failed. No interaction with the aarch64 gating.

Recommended path

1. Merge ci: unbreak the Rust quality lane on main, a fourth time #1393 first (unblocks the required check for everything else).
2. Close fix(cpu): keep the int4 prefill fan-out policy off the aarch64 dead-code lint #1429 as superseded by fix(cpu-ep): gate x86_64-only prefill fan-out symbols for non-x86 targets #1443.
3. Close or re-scope fix(cpu-ep): keep the prefill fan-out policy present on aarch64+MLAS #1382; do not merge it as a repair.
4. Merge fix(cpu): make the flat fan-out dispatch test host-independent #1420 on its own merits; it is unrelated to the aarch64 cluster.

The process point

Five PRs from three people attacked one lint, and the reason is visible in the history: the fmt/clippy gates only run on PR branches, so main can go red from an interaction between two independently-green PRs, and whoever notices opens a fix. That is why #1393 is titled "a fourth time" and is now on its fifth site. A push-triggered cargo fmt --check and cross-clippy job on main would attribute the breakage to the commit that caused it instead of to whoever merges next, and would have made four of these five PRs unnecessary.

[justinchuby]

Author recusal + merge-order finding (Pris, Tester)

I am the author of this PR, so I am not approving it — it needs an independent reviewer (Gaff, Luv, or Chew). What follows is evidence for whoever picks it up, plus one finding that affects sequencing across all the open candidates.

Main is red four independent ways

Audited against origin/main @ 6bd006b7e. All four are merged, and every one of them is catchable by a gate this repo already has:

#	breakage	lane it breaks	fixed by
1	cargo fmt --all -- --check — 5 diffs	both required jobs (ci.yml:162, :276)	this PR (+ one hunk added below)
2	clippy -D warnings: map_or in dispatch.rs:23	Rust quality; also aborts check_cross_compile.sh	this PR
3	aarch64 + mlas → E0425 prefill_fan_out	Rust (Windows ARM64), macOS arm64	this PR
4	parallel_output_rows_dispatches_to_the_task_runtime fails at <16 rayon threads	required Fast (Linux x86_64) on ubuntu-latest (4 vCPU)	#1420

No single PR makes main green. This PR and #1420 are each necessary and only jointly sufficient. They should land back-to-back; anything merged between them still sees a red main.

Verification of the pair

Scratch worktree, origin/main + this branch + #1420, conflicts=0:

cargo fmt --all -- --check                                   0 diffs
cargo clippy -p onnx-runtime-session --all-targets -D warnings  EXIT=0
RAYON_NUM_THREADS=4 … parallel_output_rows_dispatches_…       ok
RAYON_NUM_THREADS=4 cargo test -p onnx-runtime-ep-cpu --lib   1447 passed; 0 failed
bash scripts/check_cross_compile.sh                           EXIT=0, both legs full scope
simulated aarch64+mlas                                        0 errors

Evidence for defect 3 (the part reviewers should check hardest)

run_mlas_shards (matmul_nbits.rs:2396) is #[cfg(feature = "mlas")] — arch-independent — and calls prefill_fan_out at :2457. #1443 hard-gated that symbol to #[cfg(target_arch = "x86_64")] at :403. On aarch64 + mlas the caller survives and the callee does not.

MLAS will not cross-build to aarch64 in this container (arm_neon.h … vaddq_f16: target specific option mismatch), so I reproduced the cfg resolution on an x86_64 host by relabelling target_arch = "x86_64" → "s390x" throughout that one file and building --features mlas:

negative control (unmodified, host x86_64 + mlas)  → EXIT=0, no errors
origin/main, simulated non-x86 + mlas              → error[E0425]: cannot find function `prefill_fan_out`
this PR,     simulated non-x86 + mlas              → clean

The negative control matters: without it the probe could have been failing for an unrelated reason.

This is why the per-symbol predicates differ, and it is worth not "simplifying":

• WIDE_PREFILL_MACS, prefill_fan_out → cfg_attr(not(any(feature = "mlas", target_arch = "x86_64")), allow(dead_code)) — two callers, differently gated.
• prefill_column_grain → cfg_attr(not(target_arch = "x86_64"), allow(dead_code)) — only the x86 caller (run_mlas_shards uses prefill_tile_grain). Widening this one to the union relocates the failure to never used on aarch64+mlas. An Opus review caught exactly that in my first draft.

New commit: 481f47247

#1456 landed two trailing blank lines in qwen35_0_8b_text_decode_lock.rs after this branch was written, which left the merge result at 1 fmt diff — enough to keep both required jobs red. Pushed a 2-line deletion so this branch now fully clears fmt (0 diffs). No other file touched.

Process note

Auto-merge (squash) has been armed on this PR since 2026-08-19T04:55:37Z. I have not used --admin or any ruleset bypass on it or anything else. The queue remains fully stalled — 75 queued, 0 in progress repo-wide — and the required checks have not been created yet, so mergeStateStatus is UNKNOWN.

Why the existing gate never reported defect 3

check_cross_compile.sh runs clippy as its first pass, so defect 2 aborted it before it ever reached onnx-runtime-ep-cpu. Separately, the script false-passes locally when no aarch64 cross toolchain is installed: :191-194 swaps CRATES_FULL→CRATES_NO_FFI, dropping onnx-runtime-ep-cpu, and exits 0 with the "REDUCED SCOPE" note printed below the ✓. On Actions it correctly exit 2s (:178-190), so this is a local-validation trap specifically. Read the scope note, not the exit code — only scope: full offline set (aarch64 cross toolchain present) is evidence. Every cross-compile result I have reported is at full scope.

[justinchuby]

Closing: fully superseded on one half, reverting on the other

Consolidating the aarch64/fmt repair PRs (#1382 / #1393 / #1420 / #1429 / #1434)
down to one path. This one is now the odd one out, in two separate ways.

1. The fmt half is already on main. The gpt_oss_20b_decode_lock.rs token
regrouping and the qwen35_0_8b_text_decode_lock.rs trailing-newline removal
landed in #1393 (4b1cabb82). origin/main today:

$ git show origin/main:crates/onnx-genai-engine/tests/gpt_oss_20b_decode_lock.rs | sed -n '43,46p'
const EXPECTED_TOKENS: &[u32] = &[
    13719, 1511, 290, 19458, 11, 326, 290, 68347, 382, 290, 32840, 13983, 306, 290, 19458, 13, 623,
    68347, 382, 290, 32840, 13983, 306, 290,
];

Byte-identical to what this PR proposes. Those hunks are now no-ops.

2. The cfg half would revert #1443. #1443 (beb15c202) fixed the aarch64
lint by gating WIDE_PREFILL_MACS and prefill_fan_out — and their tests —
with #[cfg(target_arch = "x86_64")]. That is what main carries now:

$ git show origin/main:crates/onnx-runtime-ep-cpu/src/kernels/matmul_nbits.rs | grep -B1 'const WIDE_PREFILL_MACS\|fn prefill_fan_out'
#[cfg(target_arch = "x86_64")]
const WIDE_PREFILL_MACS: usize = 1 << 29;
--
#[cfg(target_arch = "x86_64")]
fn prefill_fan_out(macs: usize, lanes: usize, wide: usize) -> PrefillFanOut {

This PR replaces exactly those two attributes with
#[cfg_attr(not(any(feature = "mlas", target_arch = "x86_64")), allow(dead_code))].
That is a deliberate and defensible competing design — keep the policy
compiled and unit-tested on aarch64 rather than delete it there — but it is not
a rebase of #1443, it is an undo of it. Landing it now would silently reverse a
merged decision.

I am closing rather than rebasing because rebasing produces a contradiction:
once #1443's #[cfg] has removed the item on aarch64, an allow(dead_code)
attached to it can only fire where the item no longer exists. That is the same
reason #1429 was closed.

Auto-merge was armed here, which made this a live hazard — if the queue had
ever gone green it would have reverted #1443 unattended. Disarmed before
closing.

If the "keep the policy tested on aarch64" design is still wanted, it should
come back as a fresh PR on top of #1443 that re-introduces the items and their
tests deliberately, with a stated reason why aarch64 coverage of an x86-only
fan-out policy is worth the dead code — not as a diff that happens to undo the
attribute. Happy to review that.

Merged consolidation outcome: #1393 4b1cabb82, #1407 10486a7c9,
#1420 1557a355d, #1434 6501bc6b4. #1429 and #1382 closed as superseded.

[justinchuby]

Validation before direct squash merge (Sebastian, Performance Engineer)

Reopened per Justin's local-validation authorization and Pris's audit verdict (#1382 APPROVED). Latest origin/main (7de4bb1dc) merged into this head; the merge was clean and the fmt/clippy hunks this PR carried have since landed on main via #1393, so the net diff against main is now exactly one file:

crates/onnx-runtime-ep-cpu/src/kernels/matmul_nbits.rs | 29 +++++++++++-------

3 item gates #[cfg(target_arch = "x86_64")] → #[cfg_attr(…, allow(dead_code))], and 6 unit tests un-gated.

Headline: this PR repairs a lane that is currently RED on main

I ran the exact Rust (Windows ARM64) build step, using aarch64-unknown-linux-gnu as the arch proxy (the same substitution scripts/check_cross_compile.sh makes deliberately for the arch dimension), with the job's RUSTFLAGS: -D warnings:

cargo build --target aarch64-* -p onnx-runtime-ep-cpu-plugin --features mlas

arm	result
main 7de4bb1dc	❌ error[E0425]: cannot find function prefill_fan_out in this scope — exit 101
this PR	✅ compiles and links — exit 0

Mechanism: #1443 gated prefill_fan_out on target_arch = "x86_64", but run_mlas_shards calls it under feature = "mlas" on any arch. So aarch64 + mlas has the caller without the callee. ci.yml:585 builds exactly that configuration, so main's Windows ARM64 lane cannot compile. This PR's union predicate not(any(feature = "mlas", target_arch = "x86_64")) is the correct one, and the narrower not(target_arch = "x86_64") on prefill_column_grain is also right — its only non-test caller is x86-only, so widening it would leave the lint live on aarch64 + mlas.

Un-gating the 6 tests is the other half of the value: it restores aarch64 coverage of the prefill fan-out policy that #1443 dropped. All 6 run and pass.

Gates run on the merged head

gate	result
cargo fmt --all --check	PASS (0 diffs)
cargo clippy --locked --all-targets offline-linux (86 pkg args) -D warnings	PASS (exit 0)
cargo test --locked offline-linux	3989 passed, 0 failed, 95 ignored
scripts/check_cross_compile.sh	PASS — x86_64 and aarch64, full offline set
aarch64 + --features mlas, -D warnings (exact CI step)	PASS (main: FAIL)
6 un-gated prefill tests execute	all ... ok
guard scripts (8)	PASS

Toolchain note: the aarch64 MLAS C++ needed -march=armv8.2-a+fp16+dotprod+i8mm+bf16 and CARGO_TARGET_..._LINKER=aarch64-linux-gnu-gcc on this host. GCC requires the explicit fp16 arch that MSVC/Apple clang enable by default, so this is a host-toolchain accommodation, not a source issue — and both arms got identical treatment, so the A/B is valid.

Known, out of scope, pre-existing

• --all-targets --features mlas on aarch64 reports 2 dead_code errors for half_yielded_to_widened_calls / reset_half_yielded_to_widened_calls in matmul.rs (a file this PR does not touch). They are lib-test-target only, so the CI step (lib only) is unaffected, and on main they are masked behind the harder E0425. Not a regression from this PR; flagged for separate follow-up.
• verify_cuda_test_honesty.py fails identically on main and here (CUDA inventory for matmul_nbits_marlin_numerics on a no-CUDA host).

Merging by squash under the local-validation authorization.

[github-actions]

🔴 Benchmark Regression Detected

Comparison of criterion micro-benchmarks: PR head vs merge-base, measured on the same runner in the same job (base first → PR second).

ℹ️ Absolute times are informational only — they vary with runner load. The % change column is the reliable signal because both sides ran under identical conditions.

Status	Scenario	Base	PR	Change
🔴	sampling_latency/top_k_per_token	51.61 µs	87.43 µs	+69.4%
🔴	qwen3_sampling_processors/top_k_full_sort_baseline	2.15 ms	3.10 ms	+43.7%
⚠️	qwen3_sampling_processors/top_k_top_p_fast	658.09 µs	807.62 µs	+22.7%
⚠️	sampling_latency/top_p_per_token	394.34 µs	482.69 µs	+22.4%
⚠️	qwen3_sampling_processors/top_p_full_sort_after_top_k_baseline	3.84 ms	4.69 ms	+22.3%
⚠️	add/large_f32_threads=1-internal/4194304	559.06 µs	678.65 µs	+21.4%
⚠️	kv_cache/alloc_dealloc_pages	40.97 µs	48.39 µs	+18.1%
⚠️	sampling_latency/min_p_per_token	221.81 µs	260.38 µs	+17.4%
⚠️	add/medium_bf16_threads=1-internal/262144	94.45 µs	109.65 µs	+16.1%
⚠️	qwen3_sampling_processors/top_p_fast_after_top_k	540.15 µs	622.93 µs	+15.3%
✅	add/small_f16_threads=1-internal/1024	413.1 ns	469.2 ns	+13.6%
✅	gather/large_bf16_threads=1-internal/131072	17.49 µs	19.44 µs	+11.1%
✅	add/medium_f16_threads=1-internal/262144	98.31 µs	108.80 µs	+10.7%
✅	grammar_masking/llguidance_compute_mask/32	79.62 µs	87.82 µs	+10.3%
✅	reduce_mean/small_f32_threads=1-internal/4096	15.02 µs	16.49 µs	+9.8%
✅	qwen3_sampling_processors/top_k_top_p_full_sort_baseline	5.96 ms	6.45 ms	+8.3%
✅	sampling_latency/greedy_per_token	3.27 µs	3.48 µs	+6.6%
✅	logit_processing/seven_processor_chain_per_step	374.33 µs	397.17 µs	+6.1%
✅	reduce_mean/medium_f32_threads=1-internal/65536	225.65 µs	239.16 µs	+6.0%
✅	reduce_mean/large_f32_threads=1-internal/262144	908.99 µs	959.32 µs	+5.5%
✅	add/small_bf16_threads=1-internal/1024	428.0 ns	443.9 ns	+3.7%
✅	matmul/small_generic_f32_threads=1/1x256x256	47.97 µs	49.18 µs	+2.5%
✅	qwen3_sampling_processors/top_k_partial_selection	195.55 µs	198.35 µs	+1.4%
✅	add/small_f32_threads=1-internal/1024	195.9 ns	197.9 ns	+1.0%
✅	gather/small_f16_threads=1-internal/4096	520.7 ns	514.5 ns	-1.2%
✅	tokenization/encode_tokens_per_second	422.41 µs	413.52 µs	-2.1%
✅	matmul/medium_generic_f32_threads=1/32x512x512	2.61 ms	2.55 ms	-2.4%
✅	gather/small_bf16_threads=1-internal/4096	506.7 ns	483.6 ns	-4.5%
✅	add/medium_f32_threads=1-internal/262144	23.86 µs	22.72 µs	-4.8%
✅	add/large_f16_threads=1-internal/4194304	1.83 ms	1.71 ms	-6.2%
✅	gather/small_f32_threads=1-internal/4096	767.5 ns	710.2 ns	-7.5%
✅	gather/medium_bf16_threads=1-internal/32768	2.51 µs	2.32 µs	-7.6%
✅	matmul/large_generic_f32_threads=1/32x1024x1024	10.43 ms	9.46 ms	-9.3%
✅	matmul/medium_generic_bf16_threads=1/32x512x512	621.87 µs	558.39 µs	-10.2%
✅	gather/large_f16_threads=1-internal/131072	19.12 µs	17.01 µs	-11.0%
✅	gather/medium_f32_threads=1-internal/32768	4.12 µs	3.64 µs	-11.6%
🟢	tokenization/decode_tokens_per_second	7.75 ms	6.42 ms	-17.2%
🟢	matmul/medium_generic_f16_threads=1/32x512x512	50.19 µs	41.49 µs	-17.3%
🟢	matmul/medium_generic_f16_threads=8/32x512x512	48.97 µs	40.46 µs	-17.4%
🟢	add/large_bf16_threads=1-internal/4194304	2.17 ms	1.68 ms	-22.4%
🟢	matmul/large_generic_bf16_threads=1/32x1024x1024	2.39 ms	1.85 ms	-22.4%
🟢	matmul/medium_generic_f32_threads=8/32x512x512	1.78 ms	1.38 ms	-22.5%
🟢	matmul/large_generic_f16_threads=1/32x1024x1024	98.28 µs	73.48 µs	-25.2%
🟢	gather/large_f32_threads=1-internal/131072	38.06 µs	28.33 µs	-25.6%
🟢	gather/medium_f16_threads=1-internal/32768	3.21 µs	2.38 µs	-25.8%
🟢	matmul/small_generic_bf16_threads=8/1x256x256	41.41 µs	30.12 µs	-27.3%
🟢	matmul/small_generic_f16_threads=1/1x256x256	40.29 µs	28.75 µs	-28.6%
🟢	matmul/small_generic_bf16_threads=1/1x256x256	41.69 µs	28.89 µs	-30.7%
🟢	matmul/small_generic_f32_threads=8/1x256x256	63.04 µs	40.78 µs	-35.3%
🟢	matmul/medium_generic_bf16_threads=8/32x512x512	673.84 µs	420.82 µs	-37.5%
🟢	matmul/small_generic_f16_threads=8/1x256x256	45.98 µs	28.58 µs	-37.8%
🟢	block_quantized_moe_cached_dense/mxfp4_uncached_expert_dequant_each_call/rows=1,H=256,I=256,E=4,top_k=1	585.17 µs	355.54 µs	-39.2%
🟢	matmul/large_generic_f32_threads=8/32x1024x1024	6.63 ms	3.98 ms	-39.9%
🟢	matmul/large_generic_f16_threads=8/32x1024x1024	140.50 µs	77.87 µs	-44.6%
🟢	matmul/large_generic_bf16_threads=8/32x1024x1024	2.36 ms	1.28 ms	-45.9%
🟢	block_quantized_matmul_cached_dense/mxfp4_uncached_dequant_each_call/1x1024x1024	1.17 ms	539.10 µs	-53.8%
🟢	block_quantized_matmul_cached_dense/mxfp4_cached_dense_repeated_call/1x1024x1024	97.91 µs	45.17 µs	-53.9%
🟢	block_quantized_moe_cached_dense/mxfp4_cached_dense_expert_repeated_call/rows=1,H=256,I=256,E=4,top_k=1	115.19 µs	52.79 µs	-54.2%
🟢	block_quantized_matmul_cached_dense/mxfp4_preexpanded_dense_oncelock_like_proxy/1x1024x1024	102.55 µs	43.69 µs	-57.4%

Visual flags: ⚠️ ≥ 15% slower, 🔴 ≥ 30% slower — calibrated against measured runner noise (~27% worst-case on multi-threaded matmul)

Host info

CPU: Apple M1 (Virtual)
Cores: 3
OS: Darwin 25.5.0 arm64
Rust: rustc 1.97.1 (8bab26f4f 2026-07-14)
Load avg: { 6.55 4.40 5.97 }

What this cannot catch

• Regressions in code paths not covered by these benchmarks (e.g., end-to-end decode with a real model)
• Sub-threshold regressions that compound over multiple PRs
• Performance changes that only manifest under GPU execution
• Latency changes in the ORT integration path (these benchmarks exercise the native Rust kernels)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.13%. Comparing base (4a9f4ec) to head (2aa6997).
⚠️ Report is 72 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1382       +/-   ##
===========================================
- Coverage   82.10%   80.13%    -1.97%     
===========================================
  Files          12      376      +364     
  Lines        5471   164287   +158816     
  Branches     5471   164287   +158816     
===========================================
+ Hits         4492   131659   +127167     
- Misses        780    27801    +27021     
- Partials      199     4827     +4628     

Flag	Coverage Δ
cli-ort-linux	82.60% <ø> (?)
cli-ort-windows	82.19% <ø> (+0.09%)	⬆️
offline	80.05% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...es/onnx-runtime-ep-cpu/src/kernels/matmul_nbits.rs	77.03% <ø> (ø)

... and 364 files with indirect coverage changes

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