SSA from bytecode: imperative pearl

Ufuk made a comment that he enjoyed my description of SSA from bytecode in Aaron’s livestream with me. While filming the livestream—an intro to ZJIT internals—I got a little nervous that people would shy away from our 1900 line add_iseq_to_hir function. This function is responsible for converting CRuby’s stack based bytecode for the YARV virtual machine into a high-level SSA IR for ZJIT to analyze and optimize. It takes bytecode that looks roughly like this: push 3 push 4 add into SSA form, giving an SSA value to each stack slot, ish: v0 = const 3 v1 = const 4 v2 = add v0, v1 While our function is 2000 lines long because it handles a lot of specific opcodes and weird facts about those opcodes, the core of it is small. Here is that core. What if a stack, but virtual The bytecode assumes that when it is run, it will be operating on a stack. This means that every opcode has implicit stack inputs and implicit stack outputs. push 3, for example, has empty stack inputs and one stack output.…

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