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Why SSA Compilers?

(mcyoung.xyz)
185 points transpute | 1 comments | 22 Oct 25 20:13 UTC | HN request time: 0.26s | 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. titzer ◴[23 Oct 25 05:44 UTC] No.45678531[source]▶
> we’ve decided mutation is evil.

SSA isn't (primarily) concerned with memory, it's concerned with local variables. It completely virtualizes the storage of local variables--in fact, all intermediate computations. By connecting computations through dataflow edges and not storage, it removes ordering (except that induced by dependence edges) from consideration.

It is, after all, what CPUs do under the hood with register renaming! They are doing dynamic SSA, a trick they stole from us compiler people!