1 / 12

COMPILERS

COMPILERS. Bhaskar Bagchi (11CS10058) Lecture Slides( 9 th Sept. 2013). LR Parsers. In LR Parser L stands for Left to right and R stands for Rightmost derivation LR Parsers are also known as Shift-Reduce Parser

ekram
Download Presentation

COMPILERS

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. COMPILERS Bhaskar Bagchi (11CS10058) Lecture Slides( 9th Sept. 2013)

  2. LR Parsers • In LR Parser L stands for Left to right and R stands for Rightmost derivation • LR Parsers are also known as Shift-Reduce Parser • Whenever we have a transition in the input symbol we either perform a SHIFT or perform a REDUCE. • For the shift and reduce operations we need an FSM, to construct which we need • State • Transition • A small augmentation in grammar

  3. Types of LR parser There are various types of LR parsers, namely • SLR (Simple LR) Parser • CLR (Canonical LR) Parser • LALR (look-Ahead LR) Parser • For all these parsers we need State transition machine • The construction of STM will be further discussed

  4. Construction of State Transition Machine • Augment the grammar G to G’ • Start symbol S • Introduce new non-terminal S’ S • Find CLOSURE() • Find GOTO()

  5. Construction of ITEMS form a production • Let there be a production A  XYZ • We can construct 4 items from this production, as listed below • A  .XYZ • A  X.YZ • A  XY.Z • A  XYZ.

  6. … ITEMS from production • Each of the item tell us to how much the input string has been recognized. • A  X.YZ implies that the prefix of the input parsed has been reduced to non-terminal X and the remaining string may be expected to match with YZ. • Similarly A  XY.Z says that XY has been recognized and Z is expected. • Also, A  .XYZ means that no prefix has been matched yet but it may match to XYZ.

  7. Construction of LR(0) automaton • In LR parser to know whether to shift or to reduce we need a parsing table, and in order to construct this parsing table we first need a State Transition Machine, the LR(0) automaton. • Each state of this automaton is a set of items • As mentioned earlier, to construct this automaton we need two functions: • CLOSURE() • GOTO()

  8. The CLOSURE() function • Helps us find state of the automaton. • Algorithm to find CLOSURE • Put all items of I0 in CLOSURE I0 • If there is a production of the form A  B.CD in I0 and there is a production of the form C  E then add the production C  .E to the CLOSURE I0 • Let we have an example grammar as follows • E  E + T | T • T  T * F | F • F  id

  9. The CLOSURE() function • First augment the grammar by introducing a new non terminal as E’  E • To find the state S0 add the item E’  .E to the state. Now for all productions in the augmented grammar G’ with E as its head, say, E  E + T | T, add the corresponding items E  .E + T and E  .T to the state S0 • Again we get an item with T appearing to the right of the dot, add the corresponding items. • Proceed similarly…

  10. The GOTO() function • GOTO(Ii , X)  Ii , where Ii is the current state(set of items), X is the terminal/non-terminal symbol and Ij is the next state. • If there is a production of the form A  A.XB in Ii and a production of the form A  AX.B in Ij then if the next input matches X in the state Ii then the GOTO() function returns Ij • Let the input string is of the form x1 x2 …xm xm=1 …xn-1 xn then x1 x2 …xm has already been matched to B and the remaining xm=1 …xn-1 xn can be matched to X.

  11. The GOTO() function • Let I0 be a start state for the above mentioned example grammar, then the GOTO() functions will be as follows: • GOTO(I0 , E)  I1 Item E’  E is available so we add the item E’  E. in I1 and take the CLOSURE of the items. So we get state I1 as E’E. and E’E. + T • GOTO(I0 , T)  I2 For the item E  .T and T  .T* F we add T  T. * F to I2 and find its CLOSURE. So, we get I2 as E  T. and TT. * F

  12. The GOTO() function • Similarly we get • GOTO(I0 , F)  I3 : T  F. • GOTO(I0 , id)  I4 : F  id. • Each of the above set of items I1 , I2 , I3 , I4 corresponding to new states of the LR(0) automaton. For each of these states we use GOTO() function for the terminals and non-terminals appearing in the items in the states and right shift the dot, i.e. match one symbol in at least one of the items present in the state.

More Related