[arm64] Negative immediates and lack of default Neon support

[Marc Glisse]

On Wed, 24 Sep 2014, Torbjörn Granlund wrote:

> There seem to be another problem with using clang for arm64, see the
> build failures at <https://gmplib.org/devel/tm-date.html>.

A piece of information I usually have a hard time finding is the compiler 
version, I don't think it gets printed at any time during the build. It 
might be hard, with all compilers having different syntax for that...

> Apparently, clang disables Neon instruction assembly by default.  I
> found that passing -mfpu=neon to the compiler helps.

Ah, I didn't have that problem on debian. The defaults are apparently not 
the same :-(

> This command line option is not recognised by gcc.  We therefore need to
> split out clang handling in configure.ac.
>
> A related problem is whether Neon is actually optional for armv8.  My
> understanding is that it is not optional, but I am not sure about that.
> If it is optional, then we need to move Neon assembly code into a
> arm64/neon subdirectory, and adjust configure.ac accordingly.

I though neon was compulsory on ARMv8-A but some googling shows 
conversations where the words 'disable' and 'flexibility' appear a lot.
Still, not enabling FP and neon by default seems like a questionable 
choice to me.

-- 
Marc Glisse