What is rustc_codegen_gcc?

rustc_codegen_gcc is a GCC ahead-of-time codegen for rustc, meaning that it can be loaded by the existing rustc frontend, but benefits from GCC by having more architectures supported and having access to GCC’s optimizations. It is not to be confused with gccrs, which is a GCC frontend for Rust.

State of rustc_codegen_gcc

Here’s what has been done during the past months:

Remove passing tests from failing-ui-tests.txt

Fix RAM usage by splitting deeply nested expressions

Implement reg vector type

Regenerate intrinsics (thanks to GuillaumeGomez!)

Sync from rust 2025/11/01

Replace allow attributes with expect and remove unused attributes (thanks to GuillaumeGomez!)

Sync from rust 2025/11/13

Clarify -Cllvm-args forwarding to GCC in Readme (thanks to harin-ramesh!)

Update to nightly-2025-11-24

Dlopen libgccjit.so in order to support multiple targets more easily

Update gccjit dependency (thanks to GuillaumeGomez!)

Move libgccjit.so in the sysroot

Better document some combinations of flags (thanks to harin-ramesh!)

Use generic builtin for checked binary operation

Use generic builtin for saturating add/sub

Use fallback sysroot directory if we cannot find libgccjit.so in the explicit directory

Update gccjit_sys

Regenerate intrinsics (thanks to GuillaumeGomez!)

Remove [no-mentions] handler in our triagebot config (thanks to Urgau!)

Some of these PRs were made to make it easier to cross-compile rustc to new platforms, which is currently my focus. This needed a few PRs on the Rust side:

Move the libgccjit.so file in a target directory

Bootstrap config: libgccjit libs dir (thanks to Kobzol for the help!)

With those, it is possible to build a native rustc for m68k with very few workarounds! Currently, it requires to patch a few crates used by rustc since the support for m68k isn’t merged yet. (A PR adding m68k support to rustix will also need to be done since both of these crates are dependencies of tempfile which is a rustc dependency.) The resulting binary cannot yet correctly compile a simple program, but I’m going to work on improving this in the coming months.

In the next months, I plan to debug all this so that we can compile Rust programs on m68k and in doing so, I will make more tests pass on m68k.

Here’s a rough summary of what has been implemented:

Feature	Last month completion	Completion	Delta
Run tests in the Rust CI	60%	60%
Rustup distribution.	40%	40%
Unwinding.	80%	80%
More function and variable attributes.	22%	22%
Target features (to detect what is supported in an architecture, like SIMD).	82%	82%
Debug info.	20%	20%
Thin LTO.	5%	5%
Support for new architectures in libraries (libc, object, …) and rustc.	0%	2%	+2%
SIMD for other architectures than x86-64.	0%
Refactor to rustc_codegen_ssa to make it easier for the GCC codegen	0%
LTO	Done
Endianness support for non-native 128-bit integers.	Done
SIMD (x86-64).	Done
Basic and aggregate types.	Done
Operations, local and global variables, constants, functions, basic blocks.	Done
Atomics.	Done
Thread-local storage.	Done
Inline assembly.	Done
Many intrinsics.	Done
Metadata.	Done
Setting optimization level.	Done
Packed structures.	Done
Alignment, symbol visibility, attributes.	Done
128-bit integers.	Done

UI tests progress

Here are the results of running the UI tests in the CI:

https://github.com/rust-lang/rustc_codegen_gcc/actions/runs/20039113246/job/57468316723#step:18:4162

https://github.com/rust-lang/rustc_codegen_gcc/actions/runs/20039113246/job/57468316741#step:18:4176

https://github.com/rust-lang/rustc_codegen_gcc/actions/runs/20038468084#summary-57466180045 (failures)

Category	Last Month	This Month	Delta
Passed	6676	6774	+98
Failed	47	57	+10

How to contribute

rustc_codegen_gcc

If you want to help on the project itself, please do the following:

Run the tests locally. 1.

Choose a test that fails. 1.

Investigate why it fails. 1.

Fix the problem.

Even if you can’t fix the problem, your investigation could help, so if you enjoy staring at assembly code, have fun!

Crates and rustc

If you would like to contribute on adding support for Rust on currently unsupported platforms, you can help by adding the support for those platforms in some crates like libc and object and also in the rust compiler itself.

Test this project

Otherwise, you can test this project on new platforms and also compare the assembly with LLVM to see if some optimization is missing.

Good first issue

Finally, another good way to help is to look at good first issues. Those are issues that should be easier to start with.

Thanks for your support!

I wanted to personally thank all the people that sponsor this project: your support is very much appreciated.

A special thanks to the following sponsors:

Futurewei

Shnatsel

Rust Foundation

A big thank you to bjorn3 for his help, contributions and reviews. And a big thank you to lqd and GuillaumeGomez for answering my questions about rustc’s internals and to Kobzol and GuillaumeGomez for their contributions. Another big thank you to Commeownist for his contributions.

Also, a big thank you to the rest of my sponsors:

kpp

0x7CFE

oleid

joshtriplett

djc

sdroege

pcn

alanfalloon

davidlattimore

colelawrence

zmanian

berkus

belzael

yvt

yerke

srijs

kkysen

riking

Lemmih

memoryruins

senden9

robjtede

Jonas Platte

Sam Harrington

Jonas

Eugene Bulkin

Joseph Garvin

MarcoFalke

athre0z

Sebastian Zivota

Oskar Nehlin

Nicolas Barbier

Daniel

Justin Ossevoort

kiyoshigawa

Daniel Sheehan

Marvin Löbel

nacaclanga

0x0177b11f

L.apz

JockeTF

davidcornu

stuhood

Mauve

icewind1991

nicholasbishop

David Vasak

Eric Driggers

Olaf Leidinger

UtherII

simonlindholm

lemmih

Eddddddd

rrbutani

Mateusz K

thk1

teh

KirilMihaylov

Vladislav Sukhmel

ximou

Kate Kiesel

jplatte

VasanthakumarV

thesamesam

sbstp

Laine Taffin Altman

LunNova

Robin Moussu

and a few others who preferred to stay anonymous.

Former sponsors/patreons:

igrr

embark-studios

saethlin

Traverse-Research

finfet

Alovchin91

wezm

mexus

raymanfx

ghost

gilescope

olanod

Denis Zaletaev

Chai T. Rex

Paul Ellenbogen

Dakota Brink

Botlabs

Cass

Oliver Marshall

pthariensflame

tedbyron

sstadick

Absolucy

rafaelcaricio

dandxy89

luizirber

regiontog

vincentdephily

zebp

Hofer-Julian

messense

fanquake

jam1garner

evanrichter

Nehliin

nevi-me

TimNN

steven-joruk

seanpianka

spike grobstein

Jeff Muizelaar

robinmoussu

Chris Butler

sierrafiveseven

icewind

Thomas Colliers

Tommy Thorn

Bálint Horváth

Matthew Conolly

Lapz

Myrik Lord

T

Emily A. Bellows

Chris

repi

opensrcsec

NobodyXu

alexkirsz

0xdeafbeef

l-const

CohenArthur

bes

acshi

Shoeboxam

teohhanhui

g4titanx