RFC 0020: `@[TargetFeature]` annotation

straight-shoota · March 10, 2026, 10:14am

This RFC is about ready to be accepted: RFC 0020: `@[TargetFeature]` annotation by straight-shoota · Pull Request #20 · crystal-lang/rfcs · GitHub

Here is the full proposal. It’s time for a final glance that everything is in order:

github.com/crystal-lang/rfcs

target-annotation/text/0020-target-annotation.md?rgh-link-date=2026-02-17T22%3A30%3A59Z

rfc

---
Feature Name: target_annotation
Start Date: 2026-02-17
RFC PR: "https://github.com/crystal-lang/rfcs/pull/0020"
Issue: "https://github.com/crystal-lang/crystal/issues/16570"
Implementation PR: "https://github.com/crystal-lang/crystal/pull/16571"
---

## Summary

Introduces a new `@[TargetFeature]` annotation that allows setting codegen options for individual methods.
This enables programs to include code paths that use advanced CPU instructions (such as SIMD extensions) and select compatible implementations at runtime.


## Motivation

For performance-critical routines, it is important to be able to use advanced CPU features.
Yet it's not always feasible to build dedicated executables for each specific feature combination.
Instead, programs should be able to include multiple code paths and select the appropriate one at runtime based on CPU feature detection.

This file has been truncated. show original

A draft implementation is already available at Add `@[TargetFeature]` annotation by stakach · Pull Request #16717 · crystal-lang/crystal · GitHub

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RFC 0020: `@[TargetFeature]` annotation

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