[arm64] Negative immediates and lack of default Neon support

[Torbjörn Granlund]

Marc Glisse <marc.glisse at inria.fr> writes:

  I get:
  clang: warning: argument unused during compilation: '-mfpu=neon'
  
  for every call to clang, but it works.
  
Well, they really do like pointless warnings over in clang land.

We cannot fix this without desupporting some clang releases, since
autoconf does not support having feature test results to be controlled
by warning output.

  By the way, for inline asm, the 'w' modifier you showed me is the
  official way and gcc is going to document that:
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63359

This is a pity, since it makes it inconsistent with other gcc ports, and
depending on gcc internals is not very clean.  Furthermore, this creates
an incompatibility trap between gcc and clang (which we just triggered,
of course).

-- 
Torbjörn
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