Previously, block.to_proc was called first, by vm_caller_setup_arg_block. kw.to_hash was called later inside CALLER_SETUP_ARG or setup_parameters_complex. This adds a splatkw instruction that is inserted before sends with ARGS_BLOCKARG and KW_SPLAT and without KW_SPLAT_MUT. This is not needed in the KW_SPLAT_MUT case, because then you know the value is a hash, and you don't need to call to_hash on it. The splatkw instruction checks whether the second to top block is a hash, and if not, replaces it with the value of calling to_hash on it (using rb_to_hash_type). As it is always before a send with ARGS_BLOCKARG and KW_SPLAT, second to top is the keyword splat, and top is the passed block.