julia

37117492 - lowering: Keep track of which :enter correspond to which :leave (#51590)

lowering: Keep track of which :enter correspond to which :leave (#51590) This should be NFC and is intended to allow the optimizer to delete :enter statements (by replacing them with `nothing`), without leaving dangling `:leave`s around. This is accomplished by having `leave` take (a variable number of) `:enter` tokens (that are already being used by `:pop_exception`). The semantics are that a literal `nothing` or an SSAValue pointing to a `nothing` statement are ignored, and one exception handler is popped for each remaining argument. The actual value of the token is ignored, except that the verifier asserts that it belongs to an `:enter`. Note that we don't need to do the same for :pop_exception, because the token generated by an `:enter` is semantically only in scope for :pop_exception during its catch block. If we determine the `:enter` is dead, then its catch block is guaranteed to not be executed and will be deleted wholesale by cfg liveness. I was considering doing something fancier where :leave is changed back to taking an integer after optimization, but the case where the IR size is bigger after this change (when we are `:leave`ing many handlers) is fairly rare and likely not worth the additional complexity or time cost to do anything special. If it does show up in size benchmarks, I'd rather give `:leave` a special, compact encoding.