2015-03-03
Code Generation (Ch. 5-6)

Chapter 6: Intermediate-Code Generation

"intermediate-code" generation, not "intermediate code generation"
  - emits an intermediate/internal representation (IR)

directed acyclic graph (DAG) form of syntax tree
  - saves storage space in the presence of common subexpressions
  - unnecessary for our project

three-address code (3AC or TAC)
  - maximum of three locations ("addresses") involved in each operation
    - name (from source code)
    - constant
    - temporary variable (compiler-generated)
  - at most one operator on the right side
    - assignment: =
      - simple copy
      - indexed assignment
      - single-op assignment
        - unary ops:  - !
        - binary ops: + - * / % < > <= >= && ||
    - unconditional jump
    - conditional jump (single variable)
    - parameter passing
    - procedure call

representation:
  - quadruple: (op, result, arg1, arg2)
  - triple:    (op, arg1, arg2)
    - temporary result variables are given implicit names (indices)

static single-assignment (SSA) form
  - every variable is only assigned once
  - basically, 3AC w/ subscripts
  - use φ-function to handle control flow sensitivity

Ex. 6.2.1:
        +
      a   -
          +
         b c

    3AC:  t1 = b + c
          t2 = -t1
          t3 = a + t2

    quads (t1, +, b, c )
          (t2, -, t1,  )
          (t3, +, a, t2)

    triples 0: (+, b, c )
            1: (-, 0,   )
            2: (+, a, 1 )