Re: [rust-dev] compiling Rust to C?
The biggest problem would be probably be handling stack unwinding (IIRC the LLVM C backend never tried to handle this either). The only option when targeting C is to use setjmp / longjmp, but that is going to be pretty inefficient. Alternatively you could just abort instead of unwinding. Cameron On Jul 18, 2014, at 12:29 AM, Josh Haberman jhaber...@gmail.com wrote: Is there any prospect of compiling Rust to C anytime in the mid to near future? This would be a really attractive option for anyone who wants to write in Rust, but wants the extreme portability of C. Actually maybe I should first ask if this is actually a tractable problem. Are there technical reasons that would prevent compiling Rust into portable C? LLVM's C Backend seems to have fallen out of maintenance -- would this provide the solution I am looking for, if it were maintained? Thanks, Josh ___ Rust-dev mailing list Rust-dev@mozilla.org https://mail.mozilla.org/listinfo/rust-dev ___ Rust-dev mailing list Rust-dev@mozilla.org https://mail.mozilla.org/listinfo/rust-dev
Re: [rust-dev] compiling Rust to C?
On Fri, Jul 18, 2014 at 12:35 AM, Cameron Zwarich zwar...@mozilla.com wrote: The biggest problem would be probably be handling stack unwinding (IIRC the LLVM C backend never tried to handle this either). Interesting, I can see what that would be a challenge. The only option when targeting C is to use setjmp / longjmp, but that is going to be pretty inefficient. Why do you think of setjmp/longjmp as inefficient? If you use the _setjmp/_longjmp variants that don't fiddle with the signal mask, they seem pretty efficient to me. The bigger problem with setjmp/longjmp to me is that they don't let you clean up variables sitting on the stack, as Rust language semantics do I believe? I can think of a way to do this portably, but it costs two extra pointers every time you declare any stack variables. Basically you could, every time you declare local variables, make them part of a struct that has a pointer to the enclosing local var struct, and a pointer to an unwind function. Then when a task fails, traverse this list of frames and run the unwind function for each one before calling longjmp(). It's wasteful of stack space (since this is basically duplicating unwind info that exists at the system level, like in .eh_frame), but it is portable. In any case, it sounds like no one is working on this, so it's probably unlikely to happen unless someone takes it up. Thanks for the info! Josh ___ Rust-dev mailing list Rust-dev@mozilla.org https://mail.mozilla.org/listinfo/rust-dev
Re: [rust-dev] compiling Rust to C?
On Jul 18, 2014, at 9:52 AM, Josh Haberman jhaber...@gmail.com wrote: The only option when targeting C is to use setjmp / longjmp, but that is going to be pretty inefficient. Why do you think of setjmp/longjmp as inefficient? If you use the _setjmp/_longjmp variants that don't fiddle with the signal mask, they seem pretty efficient to me. The bigger problem with setjmp/longjmp to me is that they don't let you clean up variables sitting on the stack, as Rust language semantics do I believe? I can think of a way to do this portably, but it costs two extra pointers every time you declare any stack variables. Basically you could, every time you declare local variables, make them part of a struct that has a pointer to the enclosing local var struct, and a pointer to an unwind function. Then when a task fails, traverse this list of frames and run the unwind function for each one before calling longjmp(). It's wasteful of stack space (since this is basically duplicating unwind info that exists at the system level, like in .eh_frame), but it is portable. This is more along the lines of what I meant. The 32-bit ARM Darwin ABI actually uses this form of exception handling, and LLVM has some support for it. Unlike DWARF unwinding you incur a cost for every cleanup that you register, so it can be quite expensive. Having had to debug issues with it in the past in LLVM, I'm not sure I would really trust the codegen to be correct. Cameron ___ Rust-dev mailing list Rust-dev@mozilla.org https://mail.mozilla.org/listinfo/rust-dev
Re: [rust-dev] compiling Rust to C?
On Fri, Jul 18, 2014 at 12:29 AM, Josh Haberman jhaber...@gmail.com wrote: Is there any prospect of compiling Rust to C anytime in the mid to near future? This would be a really attractive option for anyone who wants to write in Rust, but wants the extreme portability of C. Actually maybe I should first ask if this is actually a tractable problem. Are there technical reasons that would prevent compiling Rust into portable C? LLVM's C Backend seems to have fallen out of maintenance -- would this provide the solution I am looking for, if it were maintained? FWIW I removed the C Backend a few years ago because it was largely unmaintained. It would likely need to be rewritten from scratch to do this. If you're curious about it, let me know. -eric ___ Rust-dev mailing list Rust-dev@mozilla.org https://mail.mozilla.org/listinfo/rust-dev
