ARC MWDT toolchain support is broken

[tagunil]

Every attempt to build Zephyr v4.1.0-rc1 with ARC MWDT toolchain ends up with failure while building one or another assembly file, like:

In file included from /home/tagunil/tests2/zephyr/arch/arc/core/fast_irq.S:18:
In file included from /home/tagunil/tests2/zephyr/include/zephyr/toolchain.h:44:
In file included from /home/tagunil/tests2/zephyr/include/zephyr/toolchain/mwdt.h:18:
/home/tagunil/tests2/zephyr/include/zephyr/toolchain/common.h:101:6: error: Architecture unsupported
  101 |     #error Architecture unsupported

I've bisected it down to 65471a6. I'm not sure why our toolchain is the only one affected, need to look into it further.

[LoveKarlsson]

Probably because we separated C/C++/ASM COMPILE_LANGUAGE options, since IAR don't use the same flags for compiler and assembler like GCC, but I'm not sure.

But checking the error line it seems CONFIG_ARC is not set, which seems weird

[bjorniuppsala]

Autoconf.h is probably not being included due to the separation between compilation languages.
Look at how it is done for gcc (cmake/compiler/gcc/compiler_flags.cmake:235)

Adding something like this in cmake/compiler/arcmwdt/compiler_flags.cmake should fix the missing include:

# GCC compiler flags for imacros. The specific header must be appended by user.
set_property(TARGET asm PROPERTY imacros "-imacros")

[fabiobaltieri]

cc @tejlmand

[kartben]

@RobinKastberg

[bjorniuppsala]

I took the liberty to suggest the fix in PR #85871. But, I need help testing it since I dont have the arc MWDT toolchain installed.

[RobinKastberg]

I profusely apologize, I was under the mistaken impression that all toolchains apart from IAR inherited from gcc/compiler_flags.cmake and should have been more careful.

It would make me feel better if we could detect this condition somehow since all the files getting out of sync feels like it will become a more and more common problem.

[bjorniuppsala]

I profusely apologize, I was under the mistaken impression that all toolchains apart from IAR inherited from gcc/compiler_flags.cmake and should have been more careful.

It would make me feel better if we could detect this condition somehow since all the files getting out of sync feels like it will become a more and more common problem.

For these properties that are not optional to have, I think we could at least add a obviously broken default that screams in the build-log about not being set. That or have a explicit check in cmake after the toolchain files are included that the toolchain has set up the required things