[arm64] Negative immediates and lack of default Neon support
marc.glisse at inria.fr
Wed Sep 24 05:54:01 UTC 2014
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.
More information about the gmp-bugs