[U-Boot] [PATCH 5/5] lib: Enable private libgcc by default

[Marek Vasut]

On 03/24/2016 12:47 AM, Sergey Kubushyn wrote:
> On Thu, 24 Mar 2016, Marek Vasut wrote:
> 
>> On 03/24/2016 12:08 AM, Tom Rini wrote:
>>> On Wed, Mar 23, 2016 at 04:02:07PM -0700, Sergey Kubushyn wrote:
>>>> On Wed, 23 Mar 2016, Tom Rini wrote:
>>>>
>>>>> On Wed, Mar 23, 2016 at 06:08:45PM +0100, Albert ARIBAUD wrote:
>>>>>> Hello Tom,
>>>>>>
>>>>>> On Wed, 23 Mar 2016 09:22:38 -0400, Tom Rini <trini at konsulko.com>
>>>>>> wrote:
>>>>>>> On Wed, Mar 23, 2016 at 01:53:35PM +0100, Albert ARIBAUD wrote:
>>>>>>>> Hello Marek,
>>>>>>>>
>>>>>>>> On Sun, 20 Mar 2016 17:15:34 +0100, Marek Vasut <marex at denx.de>
>>>>>>>> wrote:
>>>>>>>>> This patch decouples U-Boot binary from the toolchain on
>>>>>>>>> systems where
>>>>>>>>> private libgcc is available. Instead of pulling in functions
>>>>>>>>> provided
>>>>>>>>> by the libgcc from the toolchain, U-Boot will use it's own set
>>>>>>>>> of libgcc
>>>>>>>>> functions. These functions are usually imported from Linux
>>>>>>>>> kernel, which
>>>>>>>>> also uses it's own libgcc functions instead of the ones
>>>>>>>>> provided by the
>>>>>>>>> toolchain.
>>>>>>>>>
>>>>>>>>> This patch solves a rather common problem. The toolchain can
>>>>>>>>> usually
>>>>>>>>> generate code for many variants of target architecture and
>>>>>>>>> often even
>>>>>>>>> different endianness. The libgcc on the other hand is usually
>>>>>>>>> compiled
>>>>>>>>> for one particular configuration and the functions provided by
>>>>>>>>> it may
>>>>>>>>> or may not be suited for use in U-Boot. This can manifest in
>>>>>>>>> two ways,
>>>>>>>>> either the U-Boot fails to compile altogether and linker will
>>>>>>>>> complain
>>>>>>>>> or, in the much worse case, the resulting U-Boot will build,
>>>>>>>>> but will
>>>>>>>>> misbehave in very subtle and hard to debug ways.
>>>>>>>>
>>>>>>>> I don't think using private libgcc by default is a good idea.
>>>>>>>>
>>>>>>>> U-Boot's private libgcc is not a feature of U-Boot, but a fix
>>>>>>>> for some
>>>>>>>> cases where a target cannot properly link with the libgcc
>>>>>>>> provided by
>>>>>>>> the (specific release of the) GCC toolchain in use. Using
>>>>>>>> private libgcc
>>>>>>>> to other cases than these does not fix or improve anything; those
>>>>>>>> other cases were working and did not require any fix in this
>>>>>>>> respect.
>>>>>>>
>>>>>>> This isn't true, exactly.  If using clang for example everyone
>>>>>>> needs to
>>>>>>> enable this code.  We're also using -fno-builtin -ffreestanding
>>>>>>> which
>>>>>>> should limit the amount of interference from the toolchain.  And
>>>>>>> we get
>>>>>>> that.
>>>>>>
>>>>>> You mean clang does not produce self-sustained binaries?
>>>>>
>>>>> clang does not provide "libgcc", so there's no -lgcc providing all of
>>>>> the functions that are (today) in:
>>>>> _ashldi3.S _ashrdi3.S _divsi3.S  _lshrdi3.S _modsi3.S _udivsi3.S
>>>>> _umodsi3.S div0.S  _uldivmod.S
>>>>> which aside from __modsi3 and __umodsi3 are all __aeabi_xxx
>>>>
>>>> There is also _udivmoddi4 pulled from libgcc for 64-bit division
>>>> since we
>>>> switched to 64-bit all around ARM. It comes from clock calculations for
>>>> video, e.g. from drivers/video/ipu_common.c for i.MX6.
>>>
>>> Well, this is an example of why we both don't want libgcc ever nor do we
>>> want to overly expand what we do offer.  In this case isn't it an
>>> example of something that should be using lldiv/do_div/etc?
>>
>> I haven't seen the _udivmoddi4 emitted in my tests. Linux's libgcc copy
>> also doesn't implement the function. Which toolchain do you use and
>> which target did you compile?
> 
> I'm using my own armv7hl-linux-gnueabi toolchain built for hard float.
> Linux
> arm libgcc does have arch/arm/lib/div64.S file that provides __do_div64()
> function that is used by do_div() from include/asm/div64.h for 32-bit ARM
> platform. Sure, arm64 has neither div64.h nor div64.S. We _DO_ have div64.h
> (that is totally different from what Linux provides) but no div64.S in
> arch/arm/lib.

In that case, we should just import div64.S from Linux on arm32 and be
done with it ? Since we now have all the necessary macros thanks to the
first four patches in this series, that should be trivial.

What do you think? I can bake a patch real quick, so you can test it ?

Best regards,
Marek Vasut