linux-kernel - Re: [PATCH kernel v3] powerpc/makefile: Do not redefine $(CPP) for preprocessor

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Date:   Mon, 17 May 2021 14:10:32 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Alexey Kardashevskiy <aik@...abs.ru>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH kernel v3] powerpc/makefile: Do not redefine $(CPP) for preprocessor

Hi!

On Mon, May 17, 2021 at 01:23:11PM +1000, Alexey Kardashevskiy wrote:
> On 5/14/21 18:46, Segher Boessenkool wrote:
> >On Fri, May 14, 2021 at 11:42:32AM +0900, Masahiro Yamada wrote:
> >>In my best guess, the reason why powerpc adding the endian flag to CPP
> >>is this line in arch/powerpc/kernel/vdso64/vdso64.lds.S
> >>
> >>#ifdef __LITTLE_ENDIAN__
> >>OUTPUT_FORMAT("elf64-powerpcle", "elf64-powerpcle", "elf64-powerpcle")
> >>#else
> >>OUTPUT_FORMAT("elf64-powerpc", "elf64-powerpc", "elf64-powerpc")
> >>#endif
> >
> >Which is equivalent to
> >
> >#ifdef __LITTLE_ENDIAN__
> >OUTPUT_FORMAT("elf64-powerpcle")
> >#else
> >OUTPUT_FORMAT("elf64-powerpc")
> >#endif
> >
> >so please change that at the same time if you touch this :-)
> 
> "If you touch this" approach did not work well with this patch so sorry 
> but no ;)
> 
> and for a separate patch, I'll have to dig since when it is equal, do 
> you know?

Since 1994, when the three-arg version was introduced (the one-arg
version is from 1992).

> >>__LITTLE_ENDIAN__  is defined by powerpc gcc and clang.
> >
> >This predefined macro is required by the newer ABIs, but all older
> 
> That's good so I'll stick to it.

Great.

> >You can just write -mbig and -mlittle btw.  Those aren't available on
> >all targets, but neither are the long-winded -m{big,little}-endian
> >option names.  Pet peeve, I know :-)
> 
> I am looking the same guarantees across modern enough gcc and clang and 
> I am not sure all of the above is valid for clang 10.0.something (or 
> whatever we say we support) ;)

-mbig/-mlittle is supported in GCC since times immemorial.  Whether LLVM
supports it as well just depends on how good their emulation is, I have
no idea.


Segher