Compiler Construction

1
Compiler Construction

• Sohail Aslam
• Lecture 32

2

Parse tree for 101
Number
List
Sign
List

Bit
List
1
Bit
Bit
0
1
3

attributed tree
val -5
Number
pos 0val 5
List
Sign
neg true
pos 1val 4
List
pos 0val 1
Bit

pos 2val 4
pos 1val 0
List
1
Bit
pos 2val 4
Bit
0
1
4

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

pos 2val 4
pos 1val 0
List
1
Bit
pos 2val 4
Bit
0
1
5

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

pos 2val 4
pos 1val 0
List
1
Bit
pos 2val 4
Bit
0
1
6

Together
val -5
Number
pos 0val 5
List
Sign
neg true
pos 1val 4
List
pos 0val 1
Bit

pos 2val 4
pos 1val 0
List
1
Bit
pos 2val 4
Bit
0
1
7

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

pos 2val 4
pos 1val 0
List
1
Bit
pos 2val 4
Bit
0
1
8

dependence graph
val -5
pos 0val 5
neg true
pos 1val 4
pos 0val 1

pos 2val 4
pos 1val 0
1
pos 2val 4
0
1
9

dependence graph
val -5
pos 0val 5
neg true
pos 1val 4
pos 0val 1

pos 2val 4
pos 1val 0
1
pos 2val 4
0
must be acyclic!
1
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
Problems

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

18
Problems

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

19
Problems

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

20
Problems

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

21
Problems

• 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( Actions,token reduce A?b)
• stack.pop(2?b)
• s stack.top()
• stack.push(A)
• stack.push(Gotos,A)
• else if( Actions,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( Actions,token reduce A?b)
• stack.pop(2?b)
• s stack.top()
• stack.push(A)
• stack.push(Gotos,A)
• else if( Actions,token shift i)
• stack.push(token) stack.push(i) token
  scanner.next()

29
LR(1) Skeleton Parser

• if( Actions,token reduce A?b)

stack.pop(2?b) s stack.top()
stack.push(A) stack.push(Gotos,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 ? Bit List.val ? Bit.val
List0 ? List1 Bit List0.val ? 2?List1.val Bit.val
Bit ? 0 Bit.val ? 0
Bit ? 1 Bit.val ? 1