[PATCH] [AArch64] Make FP instructions optional

[Tim Northover]

  Hi Bernie,

  > Is there enough advantage in NEON-implies-FP to outweigh the loss of flexibility?

  Well, it's almost certainly going to be a completely untested
  combination. Other than that, I suppose not.

  > If you really want to model the architecture here then don't
  > you need a single feature controlling both FP and NEON?
  > The ARM-ARM today requires both or neither.

  I *think* the ARM ARM is referring to the ability to disable FP & NEON
  (not independently, as you note) so that any attempt to execute them
  will be UNDEFINED (e.g. to implement lazy context-switching).

  There's a separate issue of whether it's valid for a CPU to simply not
  implement one of them, and if so which combinations are allowed and
  where the line is drawn.

  My understanding (which is obviously out of date now) is that NEON was
  strongly encouraged but not mandated, but that FP was part of the base
  instruction-set. The NEON and basic instructions were always kept in
  separate documents and so on. But for some reason the beta ARM ARM
  doesn't annotate the variants right not.

  Is anything happening there?

  Cheers.

  Tim.

http://llvm-reviews.chandlerc.com/D2052