I586 cross compiling, ambiguous "i686" use

goertzenator · November 20, 2018, 9:39pm

I ambitiously set out today to start adding i586 support and immediately ran into the issue of the loose use of “i686”. Sometimes it is interpreted as “32 bit x86” and other times specifically as the i686 cpu (ie c build flag -march=i686). Has anybody given thought about how to address this?

I am working with Vortex86 CPUs which are a few instructions short of i686.

matthewbauer · November 21, 2018, 4:15am

I have a branch that should let you cross compile to i586:

goertzenator · November 21, 2018, 2:45pm

Thanks. Is the systems/parse and binutils commits the crux of it, or is there other stuff driving i586 support?
Is this likely to be merged to master?

Just trying to get the lay of the land here. I’m happy to work on i586 support and vet it on actual i586 hardware. My inclination is to run my own branch with the intention of eventually merging it to master.

vcunat · November 21, 2018, 2:47pm

The “ambiguity” is due to originally not counting on supporting any x86 not compatible with i686.

These changes probably won’t be controversial (I just looked quickly), but by itself it won’t help you to get binaries from cache.nixos.org anyway.

goertzenator · November 21, 2018, 5:52pm

I’ve sketched an RFC for this. Initial thoughts?

github.com

goertzenator/rfcs/blob/8679399a5e0bd2dab2acac515806981caba00222/0000-general-x86_32-support.md

---
feature: (fill me in with a unique ident, my_awesome_feature)
start-date: (fill me in with today's date, YYYY-MM-DD)
author: (name of the main author)
co-authors: (find a buddy later to help our with the RFC)
related-issues: (will contain links to implementation PRs)
---

# Summary
[summary]: #summary

Expand x86 32 bit support beyond i686 to include i586, i486, and i386.  This biggest part of this change is to disambiguate usages of `i686` which is currently overloaded to mean both "32 bit x86" and "exactly the i686 architecture".  The term `x86_32` is proposed here to replace many current usages of `i686`.


# Motivation
[motivation]: #motivation

- To support embedded x86 SoCs such as the i586 [Vortex86](https://en.wikipedia.org/wiki/Vortex86).  I personally would like to use nix on such a CPU.
- To provide the framework for other contributers to expand support for these architectures.

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oxij · November 21, 2018, 9:58pm

I’ve read it. It LGTM.

See also some discussion in treewide: support i386-pc-elf target, make errors prettier by oxij · Pull Request #50562 · NixOS/nixpkgs · GitHub. I think most, if not all, of us interested in cross-compilation here would agree with the change RFC proposes.

I’m more worried about the vendor and kernel parts of the target string.

Ericson2314 · November 28, 2018, 5:35pm

LGTM! Thanks for doing this—and providing the real-world use-case.

goertzenator · December 21, 2018, 5:14pm

Formal RFC PR is [RFC 0037] add x86_32 by goertzenator · Pull Request #37 · NixOS/rfcs · GitHub
Implementation PR is add generic x86_32 support by goertzenator · Pull Request #52634 · NixOS/nixpkgs · GitHub

I tested this by building an i586 kernel and running it on actual i586 hardware. It worked!