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

[Sergey Kubushyn]

On Thu, 24 Mar 2016, Tom Rini wrote:

> On Thu, Mar 24, 2016 at 08:50:03AM +0100, Albert ARIBAUD wrote:
>> Hello Tom,
>>
>> On Wed, 23 Mar 2016 17:36:17 -0400, Tom Rini <trini at konsulko.com> 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
>>
>> (ok, that explains what you mean by AEABI functions -- those are
>> actually not functions defined by the AEABI, but functions that the GCC
>> folks prefixed with __aeabi.)
>
> No.  For reference,
> http://infocenter.arm.com/help/topic/com.arm.doc.ihi0043d/IHI0043D_rtabi.pdf
> and chapter 4 is all about the support library.  We are entirely in our
> right to do either of (a) use the compiler-provided library (b) provide
> our own implementation of what we need.  The kernel opts for (b) and I
> would like us to follow that as well, consistently, rather than ad-hoc.

Second that. By having _EVERYTHING_ coming from U-Boot we are taking care of
_ANY_ possible mismatch between what we are building and pre-built toolchain
libraries. Soft float in ARM U-Boot and VFP hard float in most i.MX6/7
toolchains is just one of such prominent examples.

U-Boot is a standalone program not supposed to coexist with any external
applications i.e. it is totally self-sufficient, not living in some kind of
system environment so it makes perfect sense for it not to use _ANY_
external parts in the final binary.

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