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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Tue, 2 Mar 2021 14:40:04 -0800
Message-ID: <CAKwvOd=MXsvTBXvyqDXo8Fr1+-UdqnDGY8JzrD-wSxdQbvHJ5g@mail.gmail.com>
Subject: Re: [PATCH 2/2] Makefile: Only specify '--prefix=' when building with
 clang + GNU as
To:     Nathan Chancellor <nathan@kernel.org>
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>,
        Fangrui Song <maskray@google.com>
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On Tue, Mar 2, 2021 at 2:09 PM Nick Desaulniers <ndesaulniers@google.com> wrote:
>
> On Tue, Mar 2, 2021 at 2:02 PM Fangrui Song <maskray@google.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@kernel.org>
> >
> > Reviewed-by: Fangrui Song <maskray@google.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?

Ok, I spoke to Fangrui more offline, and probably misread his
response. From our chat:
```
Fangrui:
objcopy is only used for GNU as assembled object files
With integrated assembler, the object file streamer creates .o and
.dwo simultaneously
With GNU as, two objcopy commands are needed to extract .debug*.dwo to
.dwo files &&& another command to remove .debug*.dwo sections
```

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

I ran this series through a mix of LLVM=1 vs CC=clang, LLVM_IAS=1 vs
unset, CROSS_COMPILE vs not, without issue.

>
> >
> > With LLVM_IAS=1, these cases are ruled out.
>
>
>
> --
> Thanks,
> ~Nick Desaulniers


-- 
Thanks,
~Nick Desaulniers