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

[Albert ARIBAUD]

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. 

Also, libgcc is not a standalone project that can be frozen, forked or
improved freely; it is an internal component of the GCC toolchain. No
standard defines what libgcc is or should be, and we have no control
over the 'contract' between GCC-emitted code and libgcc. The GCC
project may decide to change that contract at any time, and produce a
new toolchain and a new libgcc. Using our private libgcc by default
will cause all targets to break for no good reason. We've already been
bitten by internal GCC changes on which we were dependent; adding more
such dependency is not the way to go IMO.

If we truly fear that GCC is *generally* unable to properly build our
targets due to its libgcc, then we should not only "snapshot and fix"
libgcc; we should "snapshot and fix" the whole GCC toolchain, to make
sure we keep a consistent copy of it. I don't think that would be a
viable move.

And if we don't believe that GCC is generally unable to properly build
U-Boot, then we should always use it as provided unless it is provably
buggy, in which case if a private libgcc is a fix, then by all means we
should use it.

And whenever we find that a GCC toolchain is provably buggy, we should
raise a bug, either to the toolchain provider if the issue is only with
a given binary release (e.g. mismatched or badly supported endianness),
or to the GCC project if the bug is inherent to GCC (e.g. generation
of non-supported opcodes for a given arch/cpu).

Amicalement,
-- 
Albert.