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Why SSA Compilers?

(mcyoung.xyz)
160 points transpute | 1 comments | 22 Oct 25 20:13 UTC | HN request time: 0.204s | source
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zachixer ◴[22 Oct 25 22:09 UTC] No.45675783[source]▶
Every time I see a clean SSA explainer like this, I’m reminded that the “simplicity” of SSA only exists because we’ve decided mutation is evil. It’s not that SSA is simpler — it’s that we’ve engineered our entire optimization pipeline around pretending state doesn’t exist.

It’s a brilliant illusion that works… until you hit aliasing, memory models, or concurrency, and suddenly the beautiful DAG collapses into a pile of phi nodes and load/store hell.

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1. toast0 ◴[22 Oct 25 22:37 UTC] No.45676050[source]▶
> pretending state doesn’t exist.

As a fan of a functional language, immutability doesn't mean state doesn't exist. You keep state with assignment --- in SSA, every piece of state has a new name.

If you want to keep state beyond the scope of a function, you have to return it, or call another function with it (and hope you have tail call elimination). Or, stash it in a mutable escape hatch.