A few weeks ago I posted this:

Which, if you’re not interested in watching a video right now, is a proof-of-concept LLVM to Excel spreadsheet compiler.

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If I have an aligned machine-word-sized variable (Int) and I store to it from Thread A, then I know Thread B might see the old value instead of the new value (because of per-processor caching, or the compiler “hoisting” a load to earlier in the function). But there’s no way, on a modern processor, that Thread B sees a mix of the old and new value, right? That can only happen with wider values, or unaligned values, that the code may update non-atomically, right?

This question is paraphrased from the Swift forums, though I’m not linking to it cause it’s in the middle of a larger thread and it’s something people might reasonably ask anyway. My response, lightly edited, is below; it is Swift-oriented but also applies to C, C++, and Rust.

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It’s April 1, and that means it’s both April Fools’ Day and the anniversary of the founding of Apple Inc. While this year is a sober one due to current events, I think a lot of people still appreciate what people are creating and sharing to keep spirits up, whether that be music or art or…impractical programming projects. And while pranks on April Fools’ seem less and less fun¹, obvious jokes and whimsy, not at anyone’s expense, are still something I believe in…and even better if they actually work.

Last year I implemented the world’s best code visualizer. This year I decided to seriously attempt something that I’d thought about in the past: getting a Swift program to run on Mac OS 9.

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