linux-kernel - Re: [PATCH 3/7] [HACK] pass endianess flag to LTO linker

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Date:   Wed, 21 Feb 2018 10:48:10 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Nicolas Pitre <nico@...aro.org>, Andi Kleen <ak@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 3/7] [HACK] pass endianess flag to LTO linker

On Wed, Feb 21, 2018 at 9:37 AM, Ard Biesheuvel
<ard.biesheuvel@...aro.org> wrote:
> On 20 February 2018 at 21:59, Arnd Bergmann <arnd@...db.de> wrote:
>> We need some way to pass -mbig-endian to the linker during the
>> LTO link stage, otherwise we get a waning like
>>
>> arm-linux-gnueabi/bin/ld: arch/arm/lib/clearbit.o: compiled for a big endian system and target is little endian
>>
>> for each file we link in.
>>
>> There is probably a better method of passing that flag, I'm just
>> adding it to a different hack that I added earlier for x86 LTO
>> here.
>>
>
> In general, LTO requires that *all* C flags are passed to the linker.
> Given that linking now involves code generation, any C flag that
> affects code generation must be visible to the linker as well, which
> includes all the tweaks and overrides that we add per-file or
> per-directory. It is not clear to me how much of this is carried in
> the intermediate representation as metadata, but we should probably
> err on the side of caution here, and update the Kbuild routines to
> pass the complete value of KBUILD_CFLAGS (or whatever it is called) to
> ld as well.

It looks like we're just missing KBUILD_CPPFLAGS.

However, I wonder for the more general case what happens to files
that require non-standard CFLAGS. In some cases we turn off
some optimization step for a file, we might remove '-pg', or build for
a particular target architecture. Do we have to turn off -flto for any file
that requires this for correct behavior?

      Arnd