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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Dataflow analysis in the compiler to avoid runtime NULL checks","articleBody":"`LOAD_FAST` accounts for [14.6%](https://github.com/faster-cpython/ideas/blob/main/stats.md) of all bytecodes executed. Including superinstructions brings this up to 14.6+4.7+4.6+2.4+0.9 = 27.1%\r\n\r\nhttps://github.com/python/cpython/blob/202ed2506c84cd98e9e35621b5b2929ceb717864/Python/ceval.c#L1864-L1872\r\n\r\nWe can turn this NULL-check into an assertion in many cases, where we can determine at compile time that the local variable is already initialized. Preliminary tests show that almost all `LOAD_FAST` instructions can be statically analyzed to be loading already-initialized variables.\r\n\r\nThe one twist is handling `del frame.f_locals[\"x\"]` or `frame.f_lineno = 17`, where previously-safe loads could become unsafe. For now, we can just replace all the `LOAD_FAST (no null check)` with `LOAD_FAST_CHECK` in that particular code object.\r\n\r\nSee also https://github.com/faster-cpython/ideas/discussions/306\n\n\u003c!-- gh-pr-number: gh-99075 --\u003e\n* PR: gh-99075\n\u003c!-- /gh-pr-number --\u003e\n","author":{"url":"https://github.com/sweeneyde","@type":"Person","name":"sweeneyde"},"datePublished":"2022-05-23T18:32:38.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/93143/cpython/issues/93143"}