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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Tue, 21 Jul 2020 10:55:50 -0700
Message-ID: <CAKwvOdkXs+Z28XmNA5i35Bun4kZM1fn5j9msPo8ajSi7JjMyCw@mail.gmail.com>
Subject: Re: [PATCH v3] Makefile: Fix GCC_TOOLCHAIN_DIR prefix for Clang cross compilation
To:     Fangrui Song <maskray@google.com>
Cc:     Masahiro Yamada <masahiroy@kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Jian Cai <jiancai@google.com>,
        Bill Wendling <morbo@google.com>,
        Manoj Gupta <manojgupta@google.com>,
        "# 3.4.x" <stable@vger.kernel.org>,
        Nathan Chancellor <natechancellor@gmail.com>
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On Tue, Jul 21, 2020 at 10:31 AM Fangrui Song <maskray@google.com> wrote:
>
> When CROSS_COMPILE is set (e.g. aarch64-linux-gnu-), if
> $(CROSS_COMPILE)elfedit is found at /usr/bin/aarch64-linux-gnu-elfedit,
> GCC_TOOLCHAIN_DIR will be set to /usr/bin/.  --prefix= will be set to
> /usr/bin/ and Clang as of 11 will search for both
> $(prefix)aarch64-linux-gnu-$needle and $(prefix)$needle.
>
> GCC searchs for $(prefix)aarch64-linux-gnu/$version/$needle,
> $(prefix)aarch64-linux-gnu/$needle and $(prefix)$needle. In practice,
> $(prefix)aarch64-linux-gnu/$needle rarely contains executables.
>
> To better model how GCC's -B/--prefix takes in effect in practice, newer
> Clang (since
> https://github.com/llvm/llvm-project/commit/3452a0d8c17f7166f479706b293caf6ac76ffd90)
> only searches for $(prefix)$needle. Currently it will find /usr/bin/as
> instead of /usr/bin/aarch64-linux-gnu-as.
>
> Set --prefix= to $(GCC_TOOLCHAIN_DIR)$(CROSS_COMPILE)
> (/usr/bin/aarch64-linux-gnu-) so that newer Clang can find the
> appropriate cross compiling GNU as (when -no-integrated-as is in
> effect).
>
> Cc: stable@vger.kernel.org
> Reported-by: Nathan Chancellor <natechancellor@gmail.com>
> Signed-off-by: Fangrui Song <maskray@google.com>
> Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
> Tested-by: Nathan Chancellor <natechancellor@gmail.com>
> Tested-by: Nick Desaulniers <ndesaulniers@google.com>

Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

> Link: https://github.com/ClangBuiltLinux/linux/issues/1099
> ---
> Changes in v2:
> * Updated description to add tags and the llvm-project commit link.
> * Fixed a typo.
>
> Changes in v3:
> * Add Cc: stable@vger.kernel.org
> ---
>  Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/Makefile b/Makefile
> index 0b5f8538bde5..3ac83e375b61 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -567,7 +567,7 @@ ifneq ($(shell $(CC) --version 2>&1 | head -n 1 | grep clang),)
>  ifneq ($(CROSS_COMPILE),)
>  CLANG_FLAGS    += --target=$(notdir $(CROSS_COMPILE:%-=%))
>  GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)elfedit))
> -CLANG_FLAGS    += --prefix=$(GCC_TOOLCHAIN_DIR)
> +CLANG_FLAGS    += --prefix=$(GCC_TOOLCHAIN_DIR)$(CROSS_COMPILE)
>  GCC_TOOLCHAIN  := $(realpath $(GCC_TOOLCHAIN_DIR)/..)
>  endif
>  ifneq ($(GCC_TOOLCHAIN),)
> --
> 2.28.0.rc0.105.gf9edc3c819-goog
>


-- 
Thanks,
~Nick Desaulniers