A multi-entry CFG design conundrum | Max Bernstein

The ZJIT compiler compiles Ruby bytecode (YARV) to machine code. It starts by transforming the stack machine bytecode into a high-level graph-based intermediate representation called HIR. We use a more or less typical1 control-flow graph (CFG) in HIR. We have a compilation unit, Function, which has multiple basic blocks, Block. Each block contains multiple instructions, Insn. HIR is always in SSA form, and we use the variant of SSA with block parameters instead of phi nodes. Where it gets weird, though, is our handling of multiple entrypoints. See, YARV handles default positional parameters (but not default keyword parameters) by embedding the code to compute the defaults inside the callee bytecode. Then callers are responsible for figuring out what offset in the bytecode they should start running the callee, depending on the amount of arguments the caller provides.2 In the following example, we have a function that takes two optional positional parameters a and b. If neither is provided, we start at offset 0000. If just a is provided, we start at offset 0005. If both are provided, we can start at offset 0010.