I’ve had this idea floating around my head for a little while, and I’m not sure if it’s either really interesting or totally absurd. Sorry if it’s not time for ideas like this yet. It’s not really a “proposal”, but it would be ABI-related I think.
So, the idea: The compiler can generate variations of functions where it statically substitutes a type for a placeholder name. Would it also be possible to statically generate a variation of a function if a parameter is a certain value? I have some code which scans a sequence of bytes. The bytes may be in UTF-8, UTF-16 or UTF-32, and any endianness flavour thereof. Fortunately, the scanner is looking for a ASCII-compatible characters which have the same value (albeit a different size) in each encoding, so this can be implemented as a generic function based on the type of the CodeUnit (UInt8/16/32 respectively). Something roughly like this: > @_specialize(UInt8) > @_specialize(UInt16) > @_specialize(UInt32) > func scanCharacters<U:UnsignedInteger>(from bytes: [UInt8], > hasMismatchedEndianness: Bool) -> ... { > > var byteIterator = bytes.makeIterator() > while let nextByte = byteIterator.next() { > > var codeUnit : U > // The compiler will statically optimise this branching away > because generics. > if size(of: U.self) == 1 { > codeUnit = numericCast(nextByte) > } > else { > codeUnit = consumeCodeUnit(withInitialByte: nextByte, > source: byteIterator) > // Even when marking this _slowPath(), there is a > significant overhead. > if hasMismatchedEndianness { > codeUnit = codeUnit.byteSwapped > } > } > > // Process the code-unit > } > } However, we still have a problem with this endianness flag. All it requires is that after we consume an entire CodeUnit from the buffer, that we byte-swap it before checking its value. I don’t want to duplicate the entire function code to deal with this one variation in parameter value, but this is a very hot code-path and the branching overhead is significant. So I would like to tell the compiler that we branch a lot on `hasMismatchedEndianness`, so it can generate and optimise variations of the function while keeping the abstraction level high and the maintenance burden low. There are lots of contexts where this could be useful - not just your typical Boolean switches, but Optionals, too. Even general value-types with specific values that are heavily branched against could benefit from these kinds of optimisations. Thoughts? Would something like this be possible/valuable? _______________________________________________ swift-evolution mailing list swift-evolution@swift.org https://lists.swift.org/mailman/listinfo/swift-evolution