[bug#54239] [PATCH] gnu: Add cross-clang.

[Maxime Devos <maximedevos@telenet.be>]

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Julien Lepiller schreef op do 03-03-2022 om 17:02 [+0100]:
> Hi Guix!
> 
> This small patch series adds cross-clang, a cross-compiler version of
> clang. Clang doesn't really make a distinction between a native and a
> cross-build, it is already a cross-compiler, but this ensures that:
> 
> 1. it actually works
> 2. it targets (%current-target-architecture) by default

Do you mean (%current-target-system)?

Also, WDYT of making 'cross-clang' a memoising procedure, such
that there's only one package object for a cross-clang of a fixed
target system (and version)?

>      (native-inputs
> -     (list clang llvm))
> +     (list (if (%current-target-system)
> +               (cross-clang (%current-target-system) #:clang clang-9)
> +               clang-9)))

Probably a few other packages built with clang need such a thing as well.
How about making doing the right thing a bit easier?  Suggestion: introduce
a 'clang-for-target' procedure, automatically returning the right clang:

(define (clang-for-target #:optional (clang clang))
  (if (%current-target-system)
      (cross-clang [...])
      clang)) ; not cross-compiling

then packages just need to do

  (native-inputs (list (clang-for-target) libfoo libbar ...))


The rest of the series ensures that libcxx and libcxxabi can be
cross-compiled with it.

Customarily, cross-compilers are named $TARGET-foo.  WDYT of
renaming the clang binary to '$TARGET-clang', such that a package
can have both a native clang and a cross-clang in native-inputs if desired?
Also, Autoconf looks for $TARGET-compiler, where compiler is at least gcc,
but possibly also clang

And perhaps the package name can be changed '$TARGET-clang' like done for gcc?

> + ;; Support the same variables as clang, even in cross-compilation
> + ;; context.
> + ;; Clang does not make a difference between native and
> + ;; cross-compilation.

Upstream clang doesn't, but this is in a 'cross-clang' procedure,
so I think it would make sense for Guix' cross-clang to ignore
LIBRARY_PATH and only use CROSS_LIBRARY_PATH.  Mixing up
C_INCLUDE_PATH and CROSS_C_INCLUDE_PATH (& friends) is unlikely
to lead anything good, e.g. include/bits/setjmp.h is architecture-dependent.

> +           (files '("lib" "lib64"))))

I don't think Guix does a "lib" / "lib64" split, "lib" might
be sufficient.  At least, there are a few comments like

  ;; Force powerpc libdir to be /lib and not /lib64

in Guix (though the gcc packages still includes "lib64" but
maybe that's only due to historical reasons).

How does this patch series interact with 'with-c-toolchain'?
Would "guix build hello --target=aarch64-linux-gnu --with-c-toolchain=..."
succesfully compile 'hello' with clang?

Greetings,
Maxime.

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