Swift needs a datatype that contains a fixed number of a given type; basically
a simple fixed-size array.
Motivation: I’ve been porting code for Monte Carlo Tree Search in my Go-playing
program from C++ to Swift. Performance is crucial for this code, as more
simulations lead to better play. After the initial port, the Swift code was
more than 10x slower than my C++ code. After several weeks of optimizing,
profiling, and digging through disassembly, I’ve gotten to within a factor of
2. Most of that gain came from using the ugly workaround of importing
fixed-size arrays from C.
My app is designed for a 19x19 (or smaller) Go board, not an arbitrary N x N
board, so I don’t want to pay the high cost of variable-size data structures in
the lowest levels of my app. Most apps are not like this, and most of my app is
not, but this kernel of my app needs to be fast. Heap allocations, reference
counting, and indirections all slow down the code. I just need a fixed size of
memory that I can access like an array, and Swift doesn’t let me do that.
Workaround: By importing an array from C, I can allocate a blob of memory on
the stack or include it in a struct. I can then use UnsafeRawPointer to access
that blob like an array (see details in SR-4548). This is ugly, but it works,
and it is much faster than using a Swift array. However, I’m stymied by
SR-4542, which causes mutability to spread like a plague through client code.
(SR-4542: Calling a function taking an UnsafeRawPointer forces the parameter to
be passed as inout, which means the method must be mutating.
UnsafeMutableRawPointer should require inout, UnsafeRawPointer should not.)
Proposal: UnsafeMutablePointer almost provides what I need. However, it can
only allocate memory on the heap, or it can take a given blob of memory and
interpret it as something else. What’s missing is a way to allocate typed
memory of a certain size on the stack or in a struct. For example, something
like this, with support for subscripts, limited to value types:
var foo = UnsafeMemory<Int64>(count: 6)
or
var bar = FixedSizeArray<UInt32>(repeating: 0, count: 380)
Alternatives:
(1) C arrays are currently imported as tuples, so extending tuples with
subscripts and adding a way to create tuples with a specific count of the same
type could address this need. However, I don’t think this fits well with the
concept of tuples.
(2) Changing the Array type to allow a fixed size could be considered in the
future. ‘count’ and ‘capacity’ would be fixed and only known to the compiler,
not stored with the data. However, I suspect the consequences would be
far-reaching, and thus I don’t see this happening soon.
An UnsafeMemory type would be a limited addition that fits in well with the
existing low-level Pointer module, and fills a gap in the capabilities of
Swift. The Pointer module helps implement low-level, performance-critical code,
and not being able to create data on the stack is a serious omission. Jumping
through C hoops is not a solution.
Anders Kierulf
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