MAMathnerd314·Oct 26, 2023
Recently I was looking at how to do streaming I/O in Swift, reading a file line by line (but not the whole file at once). In C on Linux, this is one function, fgets, which does the buffering itself and uses SIMD-accelerated memchr to look for the newline. You call it with a file handle and it's done.
In Swift the best advice I found was https://forums.swift.org/t/read-text-file-line-by-line/28852...: use the C functions. I experimented with using FileHandle in Swift and manually scanning the return Data value for newlines, and found it added a 6x slowdown. The loop for scanning the newline does not optimize even the array access, it was calling a lookup function for every byte and there was of course no SIMD acceleration. Converting the Data value to a native Swift array did at least eliminate most of the lookup overhead, but from the pro
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file it seems like it still does bounds checking on every access, so it doesn't optimize to SIMD or anything like that.
So that's the issue - the Foundation library isn't a portable wrapper over the native system calls, it is a thick wad of gunk that adds significant overhead to even simple programs. There was an effort to make a better API, https://github.com/apple/swift-system/, but it seems to have stalled (or perhaps an improved cross-platform API was never its goal).