COMS W4115
Programming Languages and Translators
Lecture 13: Intermediate Representations
March 6, 2013

Lecture Outline

1. Syntax-directed translation
2. Variants of syntax trees
3. Three-address code
4. Semantic analysis

1. Syntax-Directed Translation

• Postfix translation schemes
• Translation schemes with actions inside productions
• Producing ASTs with top-down parsing
• Here is the L-attributed SDD based on an LL(1) grammar for translating arithmetic expressions into ASTs from Lecture 12.

   E → T A        { E.node = A.s;
                    A.i = T.node; }

   A → + T A1     { A1.i = Node('+', A.i, T.node);
                    A.s = A1.s; }

   A → ε          { A.s = A.i; }

   T → ( E )      { T.node = E.node; }

   T → id         { T.node = Leaf(id, id.entry); }
  

2. Variants of Syntax Trees

• Abstract syntax trees
• Directed acyclic graphs
• Algorithm 6.3: Value-number method for constructing a DAG (p. 361)

3. Three-Address Code

• Three-address instructions
• Representations for three-address code
• Records
• Quadruples
• Triples
• Static single-assignment form

4. Semantic Analysis

• Uses made of semantic information for a variable x:
• What kind of value is stored in x?
• How big is x?
• Who is responsible for allocating space for x?
• Who is responsible for initializing x?
• How long must the value of x be kept?
• If x is a procedure, what kinds of arguments does it take and what kind of return value does it have?
• Storage layout for local names

5. Practice Problems

1. Construct a DAG for the expression
((x + y) - ((x + y) * (x - y))) + ((x + y) * (x - y))
2. Translate the following assignments into (a) syntax trees, (b) quadruples, (c) triples, (d) three-address code:
1. x = a + -(b+c)
2. x[i] = y[i] + z[i]
3. x = f(y+1) + 2

6. Reading

• ALSU, Sections 6.1-6.3