Pre-RFC: Gradual Transition of NixOS x86_64 Baseline to x86-64-v3 with an Intermediate Step to x86-64-v2
Summary

This pre-RFC proposes a phased update of NixOS's x86_64 baseline architecture, transitioning to x86-64-v2 in 2024 and subsequently to x86-64-v3 in 2027. This strategy is designed to gradually optimize NixOS for modern CPU architectures, balancing performance improvements with compatibility and transition challenges.
Background

NixOS currently supports a broad range of x86_64 processors. However, this extensive compatibility limits the utilization of advancements in newer CPU architectures. A gradual transition would allow NixOS to progressively adopt modern CPU features while maintaining broader hardware support.
Phase 1: Transition to x86-64-v2 (2024)
Implementation

Compiler Updates: Shift compiler flags to -march=x86-64-v2, targeting CPUs supporting up to SSE4.2 and SSSE3​.
Evaluation: Assess compatibility and performance impacts on the existing package ecosystem.

Rationale

Intermediate Compatibility: x86-64-v2 offers improved performance over the current baseline while maintaining broader compatibility, including with some older CPUs.
Building Foundations for Phase 3: This phase sets the stage for the subsequent transition to x86-64-v3, allowing time for testing and community adaptation.

Phase 2: Transition to x86-64-v3 (2027)
Implementation

Compiler Adjustments: Update to -march=x86-64-v3, optimizing for CPUs with support for AVX2 and additional bit-manipulation instructions​​.
Legacy Support Strategy: Develop and maintain strategies for supporting systems incompatible with x86-64-v3.

Rationale

Maximizing Performance: The shift to x86-64-v3 aims to fully leverage modern CPU capabilities, significantly boosting performance for various applications​.
Alignment with Industry Trends: The transition aligns NixOS with industry trends towards utilizing more advanced CPU features.

Drawbacks and Challenges

Compatibility

Reduced Compatibility: Each phase might render NixOS incompatible with some older systems, potentially impacting a subset of the user base.
System Types vs. Features: There's an ongoing discussion about whether these architectural levels should be system types or features within NixOS, with implications for package compatibility and system configuration​.

Migration Complexity

Implementation Challenges: The phased approach requires meticulous planning, testing, and execution.
Toolchain and Build System Adjustments: Accommodating new compiler flags and architecture levels in the NixOS toolchain and build systems.

Maintenance Workload

Increased Burden: Switching architectures adds to the maintenance workload.

Contingency Plan

Staged Rollouts with Community Feedback: Implement changes in stages, with active monitoring and adjustments based on feedback.
Enhanced Testing and Validation: Prioritize extensive testing and validation before each phase.
Community-Led Support for Legacy Systems: Develop initiatives for supporting users with older, incompatible hardware.
Transparent Communication: Continuous communication with the community on transition status and issues.
Adaptation Rather Than Reversion: Focus on adapting the approach based on encountered issues.

Alternatives

Single-Step Transition to x86-64-v3: Potential for greater compatibility issues.
Extended Support Periods: Prolonging support for each architecture level for smoother community adaptation.

Unresolved Questions

Community Engagement: How to effectively communicate and implement the transition plan within the NixOS community?
Hardware Compatibility Checks: Mechanisms for users to easily determine the compatibility of their hardware with each new baseline.
Support Mechanisms: Strategies to support users and maintainers during each transition phase, especially those with older hardware.