[Tinycc-devel] Questions regarding ARM Thumb code generation

[Erlend Sveen]

Hi all!

I have been trying to get a compiler running on a Cortex-M7. I've had success

with some of the GNU binutils (linker, assembler, objdump, ...), but getting

GCC to run seemed to be a bit too ambitious. I went looking around and found

TCC. Sadly it does not support thumb instructions, but I did find that someone

has attempted to get going with it:

  https://github.com/gfwilliams/tinycc

While it barely compiles an empty function, I decided to spend a couple of

weekends trying to understand it and figure out if I could get it working.

Those weekends have now turned into month(s). Using the tests in the

tests/tests2 directory, I'm having a success rate of about 88% (I'm ignoring

those that are intended to not compile). The ones that fail are 4 due to

floating point support, 1 because of array copying, 1 due to C library issues

and 1 due to an immediate being out of range (still working on these). Note that

I'm running the tests manually since I can't run the test system on my MCU

(yet), so take the numbers with a grain of salt. I've also been using an older

version of TCC (from the link above), so I'll have to migrate my changes and

run all the newer tests too.

While I'm still hacking away on the original arm-gen.c it is starting to look

rather serious. So I figured I should ask a few questions on the mailing list

to ensure that I'm on the right track and that my contributions are welcome.

 * Is anyone else working on something like this? I've searched the mailing

   list but could not find any.

 * The repository I found mentions a licensing issue. I've searched the

   mailing list and found that the author of the original ARM codegen does not

   want to re-license it. Personally, I don't mind leaving the license as is.

   There is no way I will be able to write the thumb code gen from scratch,

   so my code will be based on the ARM one and subject to the same issue. Do I

   need to worry about this? It would really suck if my code gets dropped due

   to relicensing in the future.

 * Generally speaking, information to new contributors seems to be a bit

   scarce. I'm guessing that I'll have to make a new target to the build system

   at the very least, and new files for the thumb code gen. Is there anything I

   should read up on before I attemt to do this?

 * Code style: It is of course best to follow the existing code, but if I

   contribute a new file, am I allowed to use my own style for it? I've read

   the CodingStyle text file but it does not say much on this.

 * If I do substantial modifications to a function, may I reformat it? I see

   that some functions have a mix of spaces and tab indentation, I'd love to

   make this consistent.

 * C99: I could not find anything about mixed code and declarations. Am I

   allowed to use this? I personally find this to be a really important

   feature, since it improves readability by a lot. It also helps avoiding

   uninitialized variables (RAII and all).

I will probably have more questions for later, but these are the most important

ones. If there are no major roadblocks I may even have a diff ready some time

in the next few months! Also if there is something special I should know

regarding adding a new codegen I'd love to know.

Regards,

Erlend Sveen