1. Compiler Construction Sohail Aslam Lecture 28

2. 2 StackInput ¤0¤0 id – id  id $ s4 ¤0 id 4 – id  id $ r6 F → id ¤0F3¤0F3 – id  id $ r5 T → F ¤0T2¤0T2 – id  id $ s5 ¤0T2–5¤0T2–5 id  id $ s4

3. 3 ActionGoto id–  $ETF 0s4123 1acc 2s5r3 3r5s6r5 4r6 5s4723 6 83 7r2 8r4

4. 4 StackInput ¤0¤0 id – id  id $ s4 ¤0 id 4 – id  id $ r6 F → id ¤0F¤0F – id  id $ r5 T → F ¤0T2¤0T2 – id  id $ s5 ¤0T2–5¤0T2–5 id  id $ s4

5. 5 ActionGoto id–  $ETF 0s4123 1acc 2s5r3 3r5s6r5 4r6 5s4723 6 83 7r2 8r4

6. 6 StackInput ¤0¤0 id – id  id $ s4 ¤0 id 4 – id  id $ r6 F → id ¤0F3¤0F3 – id  id $ r5 T → F ¤0T2¤0T2 – id  id $ s5 ¤0T2–5¤0T2–5 id  id $ s4

7. 7 StackInput ¤0¤0 id – id  id $ s4 ¤0 id 4 – id  id $ r6 F → id ¤0F3¤0F3 – id  id $ r5 T → F ¤0T2¤0T2 – id  id $ s5 ¤0T2–5¤0T2–5 id  id $ s4

8. 8 StackInput ¤0 T 2 – 5 id 4  id $ r6 F → id ¤0T2–5F3¤0T2–5F3  id $ s6 ¤0T2–5F36¤0T2–5F36 id $ s4 ¤0 T 2 – 5 F 3  6 id 4 $ r6 F → id ¤0T2–5F36F3¤0T2–5F36F3 $ r5 T → F ¤0T2–5F36T8¤0T2–5F36T8 $ r4 T → F  T

9. 9 StackInput ¤0T2–5T2¤0T2–5T2 $ r3 E → T ¤0 T 2 – 5 E 7 $ r2 E → T–E ¤0E1¤0E1 $ accept

10. 10 LR(1) Skeleton Parser stack.push(dummy); stack.push(0); done = false; token = scanner.next(); while (!done) { s = stack.top(); if( Action[s,token] == “reduce A →  ”) { stack.pop(2  |  |); s = stack.top(); stack.push( A ); stack.push(Goto[s, A ]); }

11. 11 else if( Action[s,token] == “shift i”){ stack.push(token); stack.push(i); token = scanner.next(); } else if(Action[s,token] == “accept” && token == “ $ ” ) done = true; else report error and recover; } report success;

12. 12 Shift/Reduce Conflicts  If a DFA states contains both [ X →  a  b ] and [ Y →  a ]  Then on input “ a ” we could either shift into state [ X →  a  b ], or reduce with Y → 

13. 13 Shift/Reduce Conflicts  If a DFA states contains both [ X →  a  b ] and [ Y →  a ]  Then on input “ a ” we could either shift into state [ X →  a  b ], or reduce with Y → 

14. 14 Shift/Reduce Conflicts  If a DFA states contains both [ X →  a  b ] and [ Y →  a ]  Then on input “ a ” we could either shift into state [ X →  a  b ], or reduce with Y → 

15. 15 Shift/Reduce Conflicts  If a DFA states contains both [ X →  a  b ] and [ Y →  a ]  Then on input “ a ” we could either shift into state [ X →  a  b ], or reduce with Y → 

16. 16 Shift/Reduce Conflicts  This is called a shift-reduce conflict  Typically, this is due to ambiguities in the grammar

17. 17 Shift/Reduce Conflicts  This is called a shift-reduce conflict  Typically, this is due to ambiguities in the grammar

18. 18 Shift/Reduce Conflicts Classic example: dangling else stmt → if E then stmt | if E then stmt else stmt

19. 19 Shift/Reduce Conflicts We will have DFA state containing [ stmt → if E then stmt , else ] [ stmt → if E then stmt  else stmt, x ]

20. 20 Shift/Reduce Conflicts If else follows, we can shift [ stmt → if E then stmt  else stmt, x ] or reduce [ stmt → if E then stmt , else ]

21. 21 Shift/Reduce Conflicts If else follows, we can shift [ stmt → if E then stmt  else stmt, x ] or reduce [ stmt → if E then stmt , else ]

22. 22 Shift/Reduce Conflicts [ stmt → if E then stmt , else ] [ stmt → if E then stmt  else stmt, x ] Typical action is shift so that else matches with most recent if