Bottom up Parsing - Compiler Construction - Lecture Notes, Study notes of Compiler Construction

Download Bottom up Parsing - Compiler Construction - Lecture Notes and more Study notes Compiler Construction in PDF only on Docsity! 63 Lecture 20 Bottom-up Parsing Bottom-up parsing is more general than top-down parsing. Bottom-up parsers handle a large class of grammars. It is the preferred method in practice. It is also called LR parsing; L means that tokens are read left to right and R means that the parser constructs a rightmost derivation. LR parsers do not need left-factored grammars. LR parsers can handle left-recursive grammars. LR parsing reduces a string to the start symbol by inverting productions.A derivation consists of a series of rewrite steps S>yw>n>.. > Int => Yn > sentence Each yj is a sentential form. If y contains only terminals, y is a sentence in L(G). Ify contains = 1 nonterminals, y is a sentential form. A bottom-up parser builds a derivation by working from input sentence back towards the start symbol S. Consider the grammar S > aABe A — Abc| b B > d The sentence abbcde can be reduced to S: abbcde aAbcde aAde aABe Ss These reductions, in fact, trace out the following right-most derivation in reverse: Ss => aABe => aAde => aAbcde => abbcde S => aBy = ayy > xy Sohail Aslam Compiler Construction Notes Set:3-63 64 rule: B= y Qa Y Es x y UY Terminals only Consider the grammar 1 E >E+(E) | int N The bottom-up parse of the string int + (int) + (int) would be int + (int) + (int) E + (int) + (int) E + (E) + (int) E + (int) E+ (E) E The consequence of an LR parser tracing a rightmost derivation in reverse is that given ay be a step of a bottom-up parse, assuming that next reduction is A > B then y is a string of terminals. The reason is that wAy > ay is a step in a rightmost derivation. This observation provides a strategy for building bottom up parsers: split the input string into two substrings. Right substring (a string of terminals) is as yet unexamined by parser and left substring has terminals and non-terminals. The dividing point is marked by a > (the > is not part of the string). Initially, all input is unexamined: P X1 X1. « . Xn. Shift-Reduce Parsing Bottom-up parsing uses only two kinds of actions: 1. Shift 2. Reduce Sohail Aslam Compiler Construction Notes Set:3-64