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

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Date:   Wed, 21 Feb 2018 14:00:33 +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 11:09 AM, Ard Biesheuvel
<ard.biesheuvel@...aro.org> wrote:
> On 21 February 2018 at 09:48, Arnd Bergmann <arnd@...db.de> wrote:
>> 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:
>
>> 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?
>>
>
> Excellent question. I think the problem is that the file boundary is
> becoming blurred with LTO, and so any option that is absolutely
> required to build a single file correctly may need to be applied to
> vmlinux as a whole. Whether that is the case for any particular option
> depends on which compiler pass(es) is (are) affected by the option,
> i.e., whether it is taken into account when creating the intermediate
> representation.

Newer compilers are able to change both the optimization and warning
flags per function using a #pragma or _Pragma() directive or
function attributes.
There has been some recent discussion about requiring a newer gcc
version, so if we do that, we could perhaps replace all the existing
CFLAGS overrides with those pragmas.

     Arnd