Rust GCC back end: Why and how
17 comments
·December 16, 2025mastax
saghm
I could be wrong, but my surface level understanding is that it's more of a library version of the external API of GCC than one that gives access to the internals.
keyle
If the author reads this...
I'd be very interested if the author could provide a post with a more in depth view of the passes, as suggested!
petcat
> Little side-note: If enough people are interested by this topic, I can write a (much) longer explanation of these passes.
Yes, please!
bfjjejskdjd
[flagged]
MangoToupe
I find it shocking that 20 years after LLVM was created, gcc still hasn't moved towards modularization of codegen.
pjmlp
LLVM wasn't the first modularization of codegen, see Amsterdam Compiler Kit for prior art, among others.
GCC approach is on purpose, plus even if they wanted to change, who would take the effort to make existing C, C++, Objective-C, Objective-C++, Fortran, Ada, D, and Go frontends adopt the new architecture?
Even clang with all the LLVM modularization is going to take a couple of years to move from plain LLVM IR into MLIR dialect for C based languages, https://github.com/llvm/clangir
ayende
Isn't that very much intentional on the part of GCC?
colejohnson66
Somewhat. Stallman claims to have tried to make it modular,[0] but also that he wants to avoid "misuse of [the] front ends".[1]
The idea is that you should link the front and back ends, to prevent out-of-process GPL runarounds. But because of that, the mingling of the front and back ends ended up winning out over attempts to stay modular.
[0]: https://lists.gnu.org/archive/html/emacs-devel/2015-02/msg00...
[1]: https://lists.gnu.org/archive/html/emacs-devel/2015-01/msg00...
phkahler
>> The idea is that you should link the front and back ends, to prevent out-of-process GPL runarounds.
Valid points, but also the reason people wanting to create a more modular compiler created LLVM under a different license - the ultimate GPL runaround. OTOH now we have two big and useful compilers!
Croak
When gcc was built most compilers were proprietary. Stallman wanted a free compiler and to keep it free. The GPL license is more restrictive, but it's philosophy is clear. At the end of the day the code's writer can choose if and how people are allowed to use it. You don't have to use it, you can use something else or build you own. And maybe, just maybe Linux is thriving while Windows is dying because in the Linux ecosystem everybody works together and shares, while in Windows everybody helps together paying for Satya Nadellas next yacht.
giancarlostoro
That sounds like Stallman wants proprietary OSS ;)
If you're going to make it hard for anyone anywhere to integrate with your open source tooling for fear of commercial projects abusing them and not ever sharing their changes, why even use the GPL license?
colechristensen
Good lord Stallman is such a zealot and hypocrite. It's not open vs. closed it's mine vs. yours and he's openly declaring that he's nerfing software in order to prevent people from using it in a way he doesn't like. And refusing to talk about it in public because normal people hate that shit "misunderstanding" him.
--- From the post:
I let this drop back in March -- please forgive me.
> Maybe that's the issue for GCC, but for Emacs the issue is to get detailed
> info out of GCC, which is a different problem. My understanding is that
> you're opposed to GCC providing this useful info because that info would
> need to be complete enough to be usable as input to a proprietary
> compiler backend.
I don't want to discuss the details on the list, because I think that would mean 50 messages of misunderstanding and tangents for each message that makes progress. Instead, is there anyone here who would like to work on this in detail?
wahern
Not anymore. Modularization is somewhat tangential, but for awhile Stallman did actively oppose rearchitecting GCC to better support non-free plugins and front-ends. But Stallman lost that battle years ago. AFAIU, the current state of GCC is the result of intentional technical choices (certain kinds of decoupling not as beneficial as people might think--Rust has often been stymied by lack of features in LLVM, i.e. defacto (semantic?) coupling), works in progress (decoupling ongoing), or lack of time or wherewithal to commit to certain major changes (decoupling too onerous).
torginus
Personally, I think when you are making bad technical decisions in service of legal goals (making it harder to circumvent the GPL), that's a sure sign that you made a wrong turn somewhere.
demurgos
It is intentional to avoid non-free projects from building on top of gcc components.
I am not familiar enough with gcc to know how it impacts out-of-tree free projects or internal development.
The decision was taken a long time ago, it may be worth revisiting it.
> On that note: GCC doesn't provide a nice library to give access to its internals (unlike LLVM). So we have to use libgccjit which, unlike the "jit" ("just in time", meaning compiling sub-parts of the code on the fly, only when needed for performance reasons and often used in script languages like Javascript) part in its name implies, can be used as "aot" ("ahead of time", meaning you compile everything at once, allowing you to spend more time on optimization).
Is libgccjit not “a nice library to give access to its internals?”