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From:   Masahiro Yamada <masahiroy@kernel.org>
Date:   Wed, 3 Mar 2021 17:33:16 +0900
Message-ID: <CAK7LNAS4Ri=K6M39hYU+17JVf0Z=hbRgSxuTdX5ZaVYLpmJRtA@mail.gmail.com>
Subject: Re: [PATCH 1/2] Makefile: Remove '--gcc-toolchain' flag
To:     Fangrui Song <maskray@google.com>
Cc:     Nathan Chancellor <nathan@kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
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Hi.

On Wed, Mar 3, 2021 at 6:44 AM Fangrui Song <maskray@google.com> wrote:
>
> Reviewed-by: Fangrui Song <maskray@google.com>
>
> Thanks for the clean-up!
> --gcc-toolchain= is an obsscure way searching for GCC installation prefixes (--prefix).
> The logic is complex and different for different distributions/architectures.
>
> If we specify --prefix= (-B) explicitly, --gcc-toolchain is not needed.


I tested this, and worked for me too.

Before applying this patch, could you please
help me understand the logic?


I checked the manual
(https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-b-dir)


-B<dir>, --prefix <arg>, --prefix=<arg>
    Add <dir> to search path for binaries and object files used implicitly

--gcc-toolchain=<arg>, -gcc-toolchain <arg>
    Use the gcc toolchain at the given directory


Hmm, this description is too concise
to understand how it works...


I use Ubuntu 20.10.

I use distro's default clang
located in /usr/bin/clang.

I place my aarch64 linaro toolchain in
/home/masahiro/tools/aarch64-linaro-7.5/bin/aarch64-linux-gnu-gcc,
which is not in my PATH environment.


From my some experiments,

clang --target=aarch64-linux-gnu -no-integrated-as \
--prefix=/home/masahiro/tools/aarch64-linaro-7.5/bin/aarch64-linux-gnu-  ...

works almost equivalent to

PATH=/home/masahiro/tools/aarch64-linaro-7.5/bin:$PATH \
clang --target=aarch64-linux-gnu -no-integrated-as ...


Then, clang will pick up aarch64-linux-gnu-as
found in the search path.

Is this correct?


On the other hand, I could not understand
what the purpose of --gcc-toolchain= is.


Even if I add --gcc-toolchain=/home/masahiro/tools/aarch64-linaro-7.5,
it does not make any difference, and it is completely useless.


I read the comment from stephenhines:
https://github.com/ClangBuiltLinux/linux/issues/78

How could --gcc-toolchain be used
in a useful way?


> On 2021-03-02, Nathan Chancellor wrote:
> >This is not necessary anymore now that we specify '--prefix=', which
> >tells clang exactly where to find the GNU cross tools. This has been
> >verified with self compiled LLVM 10.0.1 and LLVM 13.0.0 as well as a
> >distribution version of LLVM 11.1.0 without binutils in the LLVM
> >toolchain locations.
> >
> >Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> >---
> > Makefile | 4 ----
> > 1 file changed, 4 deletions(-)
> >
> >diff --git a/Makefile b/Makefile
> >index f9b54da2fca0..c20f0ad8be73 100644
> >--- a/Makefile
> >+++ b/Makefile
> >@@ -568,10 +568,6 @@ ifneq ($(CROSS_COMPILE),)
> > CLANG_FLAGS   += --target=$(notdir $(CROSS_COMPILE:%-=%))
> > GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)elfedit))
> > CLANG_FLAGS   += --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))
> >-GCC_TOOLCHAIN := $(realpath $(GCC_TOOLCHAIN_DIR)/..)
> >-endif
> >-ifneq ($(GCC_TOOLCHAIN),)
> >-CLANG_FLAGS   += --gcc-toolchain=$(GCC_TOOLCHAIN)
> > endif
> > ifneq ($(LLVM_IAS),1)
> > CLANG_FLAGS   += -no-integrated-as
> >
> >base-commit: 7a7fd0de4a9804299793e564a555a49c1fc924cb
> >--
> >2.31.0.rc0.75.gec125d1bc1
> >
> >--
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--
Best Regards

Masahiro Yamada