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First off, props to the folks in the Swift Discord channel, especially the folks in #troubleshooting.

I was having trouble getting optimal performance out of a breadth-first search algorithm. It uses a bitvector to keep track of visited vertices (using a bitvector in this way tends to provide better cache coherency for large graphs).

Anyway, I borrowed code from Buckets-Swift/BitArray.swift at master · mauriciosantos/Buckets-Swift · GitHub and went to work.

Notice Line 201:

static let IntSize = MemoryLayout<Int>.size * 8

If we change this to

static let IntSize = Int.bitWidth

the BFS code speeds up by 40%. (This is with -O)

I don't understand why this should be the case. It seems a bit too magical to me, and I wonder where else I'm unintentionally giving up performance.

(After this change the Swift BFS is within 1.5x the Julia BFS, which itself is within 1.2x of C++.)

Could someone help me understand this?

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If you'd like to test this yourself, the code is at GitHub - sbromberger/SwiftGraphs: Graphs in Swift

and the data file can be downloaded from Dropbox - File Deleted (Warning: ~250 MB gzipped, ~495 MB uncompressed).

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By default, Swift globals are lazily initialized.

In this case, the compiler proved that Int.bitWidth was constant and avoid the lazy initialization overhead. In contrast, the compiler failed to prove that MemoryLayout<Int>.size * 8 is constant and therefore lazy initialization overhead was used.

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Could it ever be variable? If not, is this a candidate for optimization in the compiler itself?

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The result is "obviously" constant, but not to the compiler apparently. Please file a bug and we'll see what the optimization experts have to say. Thanks!

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Filed on @sbromberger's behalf, per request. [SR-7613] Missed constant propagation · Issue #50155 · apple/swift · GitHub

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If you want to look at this... warning nerd snipe.

I think this is most likely a global opt property but my memory might be wrong. But it could also be due to a phase ordering issue. I would run swiftc with the flag -Xllvm -sil-print-all. That will print out the result of each optimization pass on each function. I would look and see what pass gets rid of a, but not b after it runs. We have similar tests for stuff like this here:

And GlobalOpt is here:

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