cpu_guard: build-time warning when a target feature has no guard row

[AdaWorldAPI]

What

build.rs compares CARGO_CFG_TARGET_FEATURE with the feature names in cpu_guard's x86_64 FEATURES table. If the build enables a feature that has no row, it emits a cargo:warning. The x86_64 baseline (sse, sse2, x87) is excluded.

Why

The guard checks each row with cfg!(target_feature = ...). A feature that is compiled in but has no row is never compared with the running CPU, so the guard lets the binary through and it can still die with SIGILL. This happens when a toolchain bump lets LLVM enable a feature the table predates. The existing the_table_covers_every_feature_a_cpu_model_can_enable test checks a hand-copied list. This check reads the features the actual build enabled.

Shape

• src/cpu_guard_drift.rs holds the pure parser and filter. build.rs and a test module in cpu_guard.rs both include it with include!.
• build.rs is added to the Cargo.toml include list so it is packaged.

Evidence

• Tests:
  • the parser reads exactly the 54 FEATURES rows, in order;
  • an input containing avx10.1 reports only avx10.1;
  • the table plus the baseline reports nothing.
• Disable runs (each red, then restored):
  • dropping the table filter fails both filter tests;
  • dropping the => check fails the parser test.
• End to end:
  • On stable at -Ctarget-cpu=sapphirerapids it stays silent.
  • On nightly at -Ctarget-cpu=sapphirerapids it warns: cpu_guard has no row for enabled target features: amx-bf16, amx-int8, amx-tile, clflushopt, ermsb, lahfsahf, movdir64b, movdiri, prfchw.
• cargo clippy --lib --tests -- -D warnings and cargo fmt --check are clean.

Limits

• On stable rustc, CARGO_CFG_TARGET_FEATURE omits unstable features (avx10.x, amx-*). The guard's cfg! misses them the same way, so on stable the guard and this check are blind together.
• Cargo hides build-script warnings from git and registry dependencies. The warning therefore shows when building ndarray itself, not inside a consumer.

Open

The nightly output lists the features the guard cannot see. Adding rows for them (AMX needs XCR0 tile-state gating) is a separate change.

🤖 Generated with Claude Code

https://claude.ai/code/session_011BMJnDBAfzcuQh5HeT39Rh

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Generated by Claude Code

Summary by CodeRabbit

• Build & Compatibility
  • Builds now warn when an enabled x86_64 CPU feature is not covered by the compatibility checks. If the relevant compatibility information cannot be read, the check is skipped with a warning. Other architectures are unaffected.
• Tests
  • Added checks for detecting uncovered CPU features and confirming that the listed features and baseline are recognized.

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📝 Walkthrough

The package now includes build.rs in its published file list. On x86_64, the build script checks enabled target features against the CPU guard table and warns about unchecked features.

Changes: CPU Guard Drift Check

Layer / File(s)	Summary
Build-time CPU guard drift check Cargo.toml, build.rs, src/cpu_guard.rs, src/cpu_guard_drift.rs	The build script checks enabled x86_64 features against names parsed from the guard table and warns about unchecked features. Tests verify table parsing, feature detection, and consistency with all 54 guard rows. The package include list adds build.rs.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant Cargo
  participant BuildScript as build.rs
  participant GuardTable as src/cpu_guard.rs
  participant DriftHelpers as cpu_guard_drift helpers
  Cargo->>BuildScript: Run build script
  BuildScript->>BuildScript: Read target features
  BuildScript->>GuardTable: Read guard table
  BuildScript->>DriftHelpers: Find enabled features missing from table
  BuildScript-->>Cargo: Emit warning for unchecked features

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Suggested reviewers: claude

Merge Risk: ⚪ Minimal · up to d8f83

No actionable issue remains from the supplied evidence; the change is mergeable after normal checks.

Pre-merge checks · 5

Passed checks (5 passed)

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: adding a build-time warning when an enabled target feature lacks a cpu_guard row.
Docstring Coverage		Docstring coverage is 83.33% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 6 functions across 3 files. (1 skipped: 1 u…
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[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-13T06:12:05.745636Z	d8f8305	Draft marked ready

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[chatgpt-codex-connector]

Exclude crt-static from the CPU feature warning

On targets such as x86_64-unknown-linux-musl, CARGO_CFG_TARGET_FEATURE includes crt-static by default, even though it describes the C runtime linkage rather than a CPU capability. Because it is neither in the guard table nor this exclusion list, every such build warns that the CPU guard cannot detect a CPU lacking crt-static and asks for an impossible guard row. Add it to the non-checkable exclusions so static x86_64 targets do not receive this false warning.

Useful? React with 👍 / 👎.

[AdaWorldAPI]

Agreed. rustc --print cfg --target x86_64-unknown-linux-musl lists crt-static, and of the default x86_64 sets (gnu, musl, msvc, darwin) it was the only feature that does not describe the CPU. Fixed in 9869e02 (not yet pushed): it is excluded alongside the baseline. The new test crt_static_is_not_a_cpu_feature was red before the exclusion (6ace106) and checks that a real missing row next to crt-static is still reported.

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