Compiler Construction Sohail Aslam Lecture 32

2 Number Sign List Bit – 1 List Bit 1 0 Parse tree for – 101

3 Number Sign List Bit – 1 val: -5 List Bit 1 0 attributed tree pos: 0 val: 5 pos: 0 val: 1 pos: 1 val: 4 pos: 1 val: 0 pos: 2 val: 4 neg: true

4 Number Sign List Bit – 1 val: -5 List Bit 1 0 Inherited Attributes pos: 0 val: 5 pos: 0 val: 1 pos: 1 val: 4 pos: 1 val: 0 pos: 2 val: 4 neg: true

5 Number Sign List Bit – 1 val: -5 List Bit 1 0 Synthesized Attributes pos: 0 val: 5 pos: 0 val: 1 pos: 1 val: 4 pos: 1 val: 0 pos: 2 val: 4 neg: true

6 Number Sign List Bit – 1 val: -5 List Bit 1 0 Together pos: 0 val: 5 pos: 0 val: 1 pos: 1 val: 4 pos: 1 val: 0 pos: 2 val: 4 neg: true

7 Number Sign List Bit – 1 val: -5 List Bit 1 0 peel away parse tree pos: 0 val: 5 pos: 0 val: 1 pos: 1 val: 4 pos: 1 val: 0 pos: 2 val: 4 neg: true

8 – 1 val: dependence graph pos: 0 val: 5 pos: 0 val: 1 pos: 1 val: 4 pos: 1 val: 0 pos: 2 val: 4 neg: true

9 – 1 val: dependence graph pos: 0 val: 5 pos: 0 val: 1 pos: 1 val: 4 pos: 1 val: 0 pos: 2 val: 4 neg: true must be acyclic!

10 Evaluation Methods Dynamic methods  Build the parse tree  Build the dependence graph  Topological sort the graph  Define attributes in topological order

11 Evaluation Methods Dynamic methods  Build the parse tree  Build the dependence graph  Topological sort the graph  Define attributes in topological order

12 Evaluation Methods Dynamic methods  Build the parse tree  Build the dependence graph  Topological sort the graph  Define attributes in topological order

13 Evaluation Methods Dynamic methods  Build the parse tree  Build the dependence graph  Topological sort the graph  Define attributes in topological order

14 Evaluation Methods Dynamic methods  Build the parse tree  Build the dependence graph  Topological sort the graph  Define attributes in topological order

15 Evaluation Methods Rule-based (treewalk)  Analyze attribute rules at compiler-generation time  Determine a fixed (static) ordering  Evaluate nodes in that order

16 Evaluation Methods Oblivious (passes, dataflow)  Ignore rules and parse tree  Pick a convenient order (at design time) and use it

17 ProblemsProblems Attribute grammars have not achieved widespread use due to a myraid of problems

18 ProblemsProblems  non-local computation  traversing parse tree  storage management for short-lived attributes  lack of high-quality inexpensive tools

19 ProblemsProblems  non-local computation  traversing parse tree  storage management for short-lived attributes  lack of high-quality inexpensive tools

20 ProblemsProblems  non-local computation  traversing parse tree  storage management for short-lived attributes  lack of high-quality inexpensive tools

21 ProblemsProblems  non-local computation  traversing parse tree  storage management for short-lived attributes  lack of high-quality inexpensive tools

22 Ad-Hoc Analysis  In rule-based evaluators, a sequence of actions are associated with grammar productions

23 Ad-Hoc Analysis  Organizing actions required for context-sensitive analysis around structure of the grammar leads to powerful, albeit ad-hoc, approach which is used on most parsers

24 Ad-Hoc Analysis  A snippet of code (action) is associated with each production that executes at parse time  In top-down parsers, the snippet is added to the appropriate parsing routine

25 Ad-Hoc Analysis  A snippet of code (action) is associated with each production that executes at parse time  In top-down parsers, the snippet is added to the appropriate parsing routine

26 Ad-Hoc Analysis  In a bottom-up shift-reduce parsers, the actions are performed each time the parser performs a reduction.

27 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 ]); } else if( Action[s,token] == “shift i”){ stack.push(token); stack.push(i); token = scanner.next(); }

28 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 ]); } else if( Action[s,token] == “shift i”){ stack.push(token); stack.push(i); token = scanner.next(); }

29 LR(1) Skeleton Parser if( Action[s,token] == “reduce A →  ”) { stack.pop(2  |  |); s = stack.top(); stack.push( A ); stack.push(Goto[s, A ]); } invoke the code snippet

30 Productions Code snippet Number → Sign List Number.val ← – Sign.val  List.val Sign → + Sign.val ← 1 Sign → –Sign.val ← –1 List → BitList.val ← Bit.val List 0 → List 1 Bit List 0.val ← 2  List 1.val + Bit.val Bit → 0 Bit.val ← 0 Bit → 1 Bit.val ← 1