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

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Date:   Mon, 17 May 2021 13:23:11 +1000
From:   Alexey Kardashevskiy <aik@...abs.ru>
To:     Segher Boessenkool <segher@...nel.crashing.org>,
        Masahiro Yamada <masahiroy@...nel.org>
Cc:     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



On 5/14/21 18:46, Segher Boessenkool wrote:
> Hi!
> 
> 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?


> 
>> __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.

> compilers have it as well.  _LITTLE_ENDIAN is not supported on all
> platforms (but it is if your compiler targets Linux, which you cannot
> necessarily rely on).  These macros are PowerPC-specific.
> 
> For GCC, for all targets, you can say
>    #if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> You do not need any of the other *ORDER__ macros in most cases.
> See "info cpp" for the sordid details.
> 
>> [2] powerpc-linux-gnu-gcc + -mlittle-endian    -> __LITTLE_ENDIAN__ is defined
> 
> 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) ;)


-- 
Alexey