Runtime SIMD dispatch can select an FMA kernel tier on a CPU without FMA

[tobocop2]

Summary

The runtime SIMD dispatch in lance-core picks the SimdSupport::Avx2 tier when only AVX2 is detected:

// rust/lance-core/src/utils/cpu.rs (main)
} else if is_x86_feature_detected!("avx2") {
    SimdSupport::Avx2

but every kernel that tier dispatches to in lance-linalg::distance::{cosine, dot, l2, norm_l2} and simd::dist_table is annotated #[target_feature(enable = "avx,fma")]. AVX2 does not imply FMA in the x86 ISA, so a host that reports AVX2 without FMA takes this tier and executes an FMA instruction (vfmadd…) it cannot run — an illegal-instruction crash.

Impact

Latent, not live. Every AVX2-capable part Intel and AMD have shipped also has FMA, so no real CPU triggers this today. It is a hole in the feature-detection contract, not a crash anyone is currently hitting.

Fix

Found and fixed while working on #6630. The tier now requires both features:

} else if is_x86_feature_detected!("avx2") && is_x86_feature_detected!("fma") {
    SimdSupport::Avx2

The tier's contract is documented on the enum variant, and a test asserts that neither the Avx2 nor AvxFma tier can be selected on a host without FMA.

• Commit: bb8af46
• PR: feat(lance-linalg): runtime SIMD dispatch for pre-Haswell x86_64 from-source builds #6630