Very fast LR parsing

LR parsers can be made to run 6 to 10 times as fast as the best table-interpretive LR parsers. The resulting parse time is negligible compared to the time required by the remainder of a typical compiler containing the parser. A parsing speed of 1/2 million lines per minute on a computer similar to a VAX 11/780 was achieved, up from an interpretive speed of 40,000 lines per minute. A speed of 240,000 lines per minute on an Intel 80286 was achieved, up from an interpretive speed of 37,000 lines per minute. The improvement is obtained by translating the parser's finite state control into assembly language. States become code memory addresses. The current input symbol resides in a register and a quick sequence of register-constant comparisons determines the next state, which is merely jumped to. The parser's push-down stack is implemented directly on a hardware stack. The stack contains code memory addresses rather than the traditional state numbers. The strongly-connected components of the directed graph induced by the parser's terminal and nonterminal transitions are examined to determine a typically small subset of the states that require parse-time stack-overflow-check code when hardware does not provide the check automatically. The increase in speed is at the expense of space: a factor of 2 to 4 increase in total table size can be expected, depending upon whether syntactic error recovery is required.