[AArch64] Implement initial SVE calling convention support
This patch adds initial support for the SVE calling convention such that
SVE types can be passed as arguments and return values to/from a
The SVE AAPCS states :
z0-z7 are used to pass scalable vector arguments to a subroutine, and to return scalable vector results from a function. If a subroutine takes arguments in scalable vector or predicate registers, or if it is a function that returns results in such registers, it must ensure that the entire contents of z8-z23 are preserved across the call. In other cases it need only preserve the low 64 bits of z8-z15, as described in §5.1.2. p0-p3 are used to pass scalable predicate arguments to a subroutine and to return scalable predicate results from a function. If a subroutine takes arguments in scalable vector or predicate registers, or if it is a function that returns results in these registers, it must ensure that p4-p15 are preserved across the call. In other cases it need not preserve any scalable predicate register contents.
SVE predicate and data registers are passed indirectly (i.e. spilled to the
stack and pass the address) if they exceed the registers used for argument
passing defined by the PCS referenced above. Until SVE stack support is merged
we can't spill SVE registers to the stack, so currently an llvm_unreachable is
used where we will eventually handle this.
Reviewed By: ostannard
Differential Revision: https://reviews.llvm.org/D65448