Subject: Re: [bug#78102] [PATCH v2 2/3] gnu: libcxx: Fix cross-compilation build.

From: Greg Hogan <[email-masked]>
Date: Thu, 1 May 2025 12:08:55 -0400

On Tue, Apr 29, 2025 at 11:33 AM Maxim Cournoyer
<[email-masked]> wrote:
>
> * gnu/packages/llvm.scm (libcxx) [arguments] <#:implicit-inputs?>: Set to #f.
> <#:phases>: Remove adjust-CPLUS_INCLUDE_PATH.
> [native-inputs]: Explicitly provide standard inputs, filtering out gcc.
>
> Change-Id: I8cbc73ef90dce0a57bf2f8198c3176932bf24185
> ---
>  gnu/packages/llvm.scm | 27 ++++++++++-----------------
>  1 file changed, 10 insertions(+), 17 deletions(-)
>
> diff --git a/gnu/packages/llvm.scm b/gnu/packages/llvm.scm
> index a0f47447bee..49b6e27bff9 100644
> --- a/gnu/packages/llvm.scm
> +++ b/gnu/packages/llvm.scm
> @@ -17,7 +17,7 @@
>  ;;; Copyright © 2020 Giacomo Leidi <[email-masked]>
>  ;;; Copyright © 2020 Jakub Kądziołka <[email-masked]>
>  ;;; Copyright © 2021, 2022 Maxime Devos <[email-masked]>
> -;;; Copyright © 2020, 2021, 2022, 2024 Maxim Cournoyer <[email-masked]>
> +;;; Copyright © 2020-2022, 2024-2025 Maxim Cournoyer <[email-masked]>
>  ;;; Copyright © 2021 Julien Lepiller <[email-masked]>
>  ;;; Copyright © 2021 Lars-Dominik Braun <[email-masked]>
>  ;;; Copyright © 2021, 2022 Guillaume Le Vaillant <[email-masked]>
> @@ -52,6 +52,7 @@ (define-module (gnu packages llvm)
>    #:use-module (guix memoization)
>    #:use-module (guix search-paths)
>    #:use-module (guix utils)
> +  #:use-module (guix build-system gnu)

Should we keep this build-system section sorted?

>    #:use-module (guix build-system cmake)
>    #:use-module (guix build-system emacs)
>    #:use-module (guix build-system python)
> @@ -1905,6 +1906,7 @@ (define-public libcxx
>      (build-system cmake-build-system)
>      (arguments
>       (list
> +      #:implicit-inputs? #f             ;to avoid conflicting GCC headers
>        #:tests? #f
>        #:configure-flags
>        #~(list "-DLLVM_ENABLE_RUNTIMES=libcxx;libcxxabi;libunwind"
> @@ -1920,23 +1922,14 @@ (define-public libcxx
>        #~(modify-phases %standard-phases
>            (add-after 'unpack 'enter-subdirectory
>              (lambda _
> -              (chdir "runtimes")))
> -          (add-after 'set-paths 'adjust-CPLUS_INCLUDE_PATH
> -            (lambda* (#:key inputs #:allow-other-keys)
> -              (let ((gcc (assoc-ref inputs  "gcc")))
> -                ;; Hide GCC's C++ headers so that they do not interfere with
> -                ;; the ones we are attempting to build.
> -                (setenv "CPLUS_INCLUDE_PATH"
> -                        (string-join (delete (string-append gcc "/include/c++")
> -                                             (string-split (getenv "CPLUS_INCLUDE_PATH")
> -                                                           #\:))
> -                                     ":"))
> -                (format #t
> -                        "environment variable `CPLUS_INCLUDE_PATH' changed to ~a~%"
> -                        (getenv "CPLUS_INCLUDE_PATH"))
> -                #t))))))
> +              (chdir "runtimes"))))))
>      (native-inputs
> -     (list clang-19 libunwind-headers llvm python))
> +     (append (list clang-19 libunwind-headers llvm python)

Does this work to combine clang-19 with llvm-13?

> +             ;; Remove GCC from the build environment, to avoid its C++
> +             ;; headers (include/c++), which would interfere and cause build
> +             ;; failures.
> +             (map second

With the map are we not also dropping the package output as the third
field? So we wind up with a second "libc" but no "libc:static". It
might not matter here but this kind of trick is often copied into
additional packages. Can we use modify-inputs on standard-packages,
delete "gcc", and append the list of additional packages? Or revert to
the old-style inputs.

> +                  (alist-delete  "gcc" (standard-packages)))))

There is an extra space after alist-delete.

>      (home-page "https://libcxx.llvm.org")
>      (synopsis "C++ standard library")
>      (description
> --
> 2.49.0