[arm64] Negative immediates and lack of default Neon support
tg at gmplib.org
Wed Sep 24 08:42:38 UTC 2014
Marc Glisse <marc.glisse at inria.fr> writes:
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
yourmachine$ ssh shell.gmplib.org
shell$ ssh leg
leg$ clang -v
Debian clang version 3.4.2-8 (tags/RELEASE_34/dot2-final) (based on LLVM 3.4.2)
> 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 :-(
My install is based on Debian. (Install 14 of
> 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
Still, not enabling FP and neon by default seems like a questionable
choice to me.
We at least need to pass -mfpu=neon to clang since it is sometimes
required and else presumably harmless. We can defer any neon code
splitting until we know that's needed.
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