The AArch64 feature-flag code is currently limited to a maximum
of 64 features. This patch reworks it so that the limit can be
increased more easily. The basic idea is:
(1) Turn the ARM_FEATURE_FOO macros into an enum, with the enum
counting bit positions.
(2) Make the feature-list macros take an array index argument
(currently always 0). The macros then return the
aarch64_feature_set contents for that array index.
An N-element array would then be initialised as:
{ MACRO (0), ..., MACRO (N - 1) }
(3) Provide convenience macros for initialising an
aarch64_feature_set for:
- a single feature
- a list of individual features
- an architecture version
- an architecture version + a list of additional features
(2) and (3) use the preprocessor to generate static initialisers.
The main restriction was that uses of the same preprocessor macro
cannot be nested. So if a macro wants to do something for N individual
arguments, it needs to use a chain of N macros to do it. There then
needs to be a way of deriving N, as a preprocessor token suitable for
pasting.
The easiest way of doing that was to precede each list of features
by the number of features in the list. So an aarch64_feature_set
initialiser for three features A, B and C would be written:
AARCH64_FEATURES (3, A, B, C)
This scheme makes it difficult to keep AARCH64_FEATURE_CRYPTO as a
synonym for SHA2+AES, so the patch expands the former to the latter.
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