Shift-Reduce Conflicts in Parsing: Resolving Ambiguities in Grammar, Slides of Compiler Construction

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Compiler

Construction

Lecture 28

2 Stack Input ¤0 id – id × id $ s ¤0id4 – id × id $ r6 F→ id ¤0F3 – id × id $ r5 T→ F ¤0T2 – id × id $ s ¤0T2–5 id × id $ s

4 Stack Input ¤0 id – id × id $ s ¤0id4 – id × id $ r6 F→ id ¤0F – id × id $ r5 T→ F ¤0T2 – id × id $ s ¤0T2–5 id × id $ s

5 Action Goto id – × $ E T F 0 s4 1 2 3 1 acc 2 s5 r 3 r5 s6 r 4 r6 r6 r 5 s4 7 2 3 6 s4 8 3 7 r 8 r4 r

7 Stack Input ¤0 id – id × id $ s ¤0id4 – id × id $ r6 F→ id ¤0F3 – id × id $ r5 T→ F ¤0T2 – id × id $ s ¤0T2–5 id × id $ s

8 Stack Input ¤0T2–5id4 × id $ r6 F→ id ¤0T2–5F3 × id $ s ¤0T2–5F3× 6 id $ s ¤0T2–5F3×6id4 $ r6 F→ id ¤0T2–5F3×6F3 $ r5 T→ F ¤0T2–5F3×6T8 $^ r4 T→ F×T

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→β”) { stack.pop(2×|β|); s = stack.top(); stack.push(A); stack.push(Goto[s,A]); }

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;

13 Shift/Reduce Conflicts

 If a DFA states contains both

[X → α•aβ,b] and [Y → γ•,a]

 Then on input “a” we could

either

• shift into state [X → αa•β,b], or
• reduce with Y → γ

14 Shift/Reduce Conflicts

 If a DFA states contains both

[X → α•aβ,b] and [Y → γ•,a]

 Then on input “a” we could

either

• shift into state [X → αa•β,b], or
• reduce with Y → γ

16 Shift/Reduce Conflicts

 This is called a shift-reduce

conflict

 Typically, this is due to

ambiguities in the grammar

17 Shift/Reduce Conflicts

 This is called a shift-reduce

conflict

 Typically, this is due to

ambiguities in the grammar

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 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]