This is essentially a long-belated follow-up to Arnold's #12606.
The key observation here is that the enum-tag-single-payload witnesses
are strictly more powerful than the XI witnesses: you can simulate
the XI witnesses by using an extra case count that's <= the XI count.
Of course the result is less efficient than the XI witnesses, but
that's less important than overall code size, and we can work on
fast-paths for that.
The extra inhabitant count is stored in a 32-bit field (always present)
following the ValueWitnessFlags, which now occupy a fixed 32 bits.
This inflates non-XI VWTs on 32-bit targets by a word, but the net effect
on XI VWTs is to shrink them by two words, which is likely to be the
more important change. Also, being able to access the XI count directly
should be a nice win.
From what I see the only fields are DATA_VALUE_WITNESS which
all have type size_t. I converted them to use the target-dependent
`StoredSize`. While I was around I fixed also isValueInline()
to do the right thing (it was using ValueBuffer instead of
TargetValueBuffer) and all the getters for the data value witnesses.
<rdar://problem/41546568>
And update the existential container's initializeWithTake implementation
in the runtime. After only allowing bitwise takable values in the
inline buffer we can use memcpy to move existential container values.
rdar://31414907
SR-343
So far single payload enums were implemented in terms of runtime functions which
internally emitted several calls to value witnesses.
This commit adds value witnesses to get and store the enum tag side stepping the
need for witness calls as this information is statically available in many cases
/// int (*getEnumTagSinglePayload)(const T* enum, UINT_TYPE emptyCases)
/// Given an instance of valid single payload enum with a payload of this
/// witness table's type (e.g Optional<ThisType>) , get the tag of the enum.
/// void (*storeEnumTagSinglePayload)(T* enum, INT_TYPE whichCase,
/// UINT_TYPE emptyCases)
/// Given uninitialized memory for an instance of a single payload enum with a
/// payload of this witness table's type (e.g Optional<ThisType>), store the
/// tag.
A simple 'for element in array' loop in generic code operating on a
ContigousArray of Int is ~25% faster on arm64.
rdar://31408033
