Reading binary file is slow

[gonsolo (Andreas Wendleder)]

@lukasa

Before we go further, I have to ask: are you compiling with optimisations turned on? (That is, have you passed either -O to swiftc or -c release to swift build or swift run ?) If you have not turned on optimisations in Swift it will be hilariously slow.

Yes, this was with -Ounchecked.

@lukasa

FileHandle.readHandleToEndOfFile is not an equivalent of a C++ iostream: for files at many sizes this will bring the entire file into memory at once.

That's via mmap, right? Do you have a better idea how to read binary files byte by byte? I couldn't find anything in the documentation.

@lukasa

Similarly, Data.convert is creating slices of Data for no reason: let view = self[index..<end] is entirely unnecessary, as you can instead ask for the offset of index from self.startIndex and then simply use self.withUnsafeBytes and load from the appropriate offset.

Ok, that was it. I changed that to load(fromByteOffset: index, as: T.self) and timings are down from 4.6s to 0.04s.

I would have assumed though that creating a view is not as expensive.

Thanks!