Track x86-64 microarchitecture level upgrades per platform support windows

[thomasqueirozb]

Overview

As Vector's supported platforms age out, we can raise the minimum x86-64 microarchitecture level in our builds, unlocking better codegen (SIMD, BMI, FMA, etc.) without breaking users.

Related to #21721 (platform support tiers).

Background

The x86-64 microarchitecture levels define stable feature tiers:

Level	Key features	Oldest hardware
v1	SSE2 (baseline)	Any x86_64
v2	SSE4.2, POPCNT, SSSE3	~2008–2009 (Intel Nehalem / AMD Bulldozer 2011)
v3	AVX, AVX2, BMI1, BMI2, FMA	Intel Haswell 2013 / AMD EPYC 2017
v4	AVX-512	Too fragmented, skip

Upgrade milestones

x86-64-v2

• RHEL: RHEL 9 officially documents x86-64-v2 as its minimum CPU requirement. Once RHEL 8 reaches EOL (31 May 2029), all supported RHEL versions mandate v2 — clean policy justification.
• Other platforms: v2 hardware (SSE4.2) has been universal since ~2011. Pre-Bulldozer AMD is the only realistic gap, and that hardware is well beyond any reasonable support window today.

Action: Enable -C target-feature / target-cpu=x86-64-v2 for Linux x86_64 builds after 31 May 2029.

x86-64-v3

• RHEL: RHEL 10 (released 2025) reportedly raises the minimum to x86-64-v3. Once RHEL 9 reaches EOL (~2032), all supported RHEL versions would mandate v3.
• Ubuntu: Ubuntu does not publish explicit microarchitecture minimums, so EOL dates don't provide the same clean justification. Hardware running Ubuntu 24.04+ is practically v3 capable, but this is a heuristic rather than a policy guarantee.

Action: Enable target-cpu=x86-64-v3 for Linux x86_64 builds after RHEL 9 EOL (~2032), once confirmed.

Notes

• aarch64: NEON is the baseline and remains so. LSE is already handled via portable-atomic. No equivalent level-upgrade path applies.
• AVX-512 (v4): AMD only added this with Zen 4 (2022) — too fragmented to target in the near term.
• Ubuntu does not specify required x86-64 microarchitecture levels, but we should probably stick with what RHEL supports as a decider.

Checklist

• Confirm RHEL 10 officially mandates x86-64-v3 (track when Red Hat publishes final hardware requirements)
• Enable x86-64-v2 in Linux x86_64 build configs after RHEL 8 EOL (31 May 2029)
• Enable x86-64-v3 in Linux x86_64 build configs after RHEL 9 EOL (~2032)
• Update platform support documentation (Introduce platform support and related documentation #21721)