linux-kernel - Re: [PATCH 2/2] Makefile: Only specify '--prefix=' when building with clang + GNU as

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Message-ID: <CAKwvOdmR_p-zbrTUmbObmCVKBcuNLpg_V3NqLeYsEK4xNHfYOA@mail.gmail.com>
Date:   Tue, 2 Mar 2021 14:09:01 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Fangrui Song <maskray@...gle.com>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH 2/2] Makefile: Only specify '--prefix=' when building with
 clang + GNU as

On Tue, Mar 2, 2021 at 2:02 PM Fangrui Song <maskray@...gle.com> wrote:
>
> On 2021-03-02, Nathan Chancellor wrote:
> >When building with LLVM_IAS=1, there is no point to specifying
> >'--prefix=' because that flag is only used to find the cross assembler,
> >which is clang itself when building with LLVM_IAS=1. All of the other
> >tools are invoked directly from PATH or a full path specified via the
> >command line, which does not depend on the value of '--prefix='.
> >
> >Sharing commands to reproduce issues becomes a little bit easier without
> >a '--prefix=' value because that '--prefix=' value is specific to a
> >user's machine due to it being an absolute path.
> >
> >Signed-off-by: Nathan Chancellor <nathan@...nel.org>
>
> Reviewed-by: Fangrui Song <maskray@...gle.com>
>
> clang can spawn GNU as (if -f?no-integrated-as is specified) and GNU
> objcopy (-f?no-integrated-as and -gsplit-dwarf and -g[123]).

But -g get's set via CONFIG_DEBUG_INFO and -gsplit-dwarf by
DEBUG_INFO_SPLIT.  So if we say:
$ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang LLVM_IAS=1

So cross compile, use clang, use the integrated assembler (ie. with
this change, don't set --prefix), with either of the two above
configs, which objcopy get's exec'd?

>
> With LLVM_IAS=1, these cases are ruled out.



-- 
Thanks,
~Nick Desaulniers