Content

  1. Overview
  2. AST to Code
  3. Postfix to Code
  4. Three-Address Code
       1. SDT to TAC
   2. Conditional Statements
   3. Arrays
   4. Procedure Calls
   5. Reusing Temp Variables

Overview

  • Machine & language independent
  • Optional
  • Clean separation of front- & back-ends -> machine-independent code optimizations
  • e.g. AST, post-fix notation, three-address code, two-address code

AST to Code

  • Optimzation: combine common subtrees

Postfix to Code

Original: a := b * -c + b * -c
IR:       a b c uminus * b c uminus * +
Code:
          iload b
          iload c
          ineg
          imul
          ...
          iadd
          istore a
  • Easy to generate
  • Difficult to optimize

Three-Address Code

  • Assignment
        - `x := y op z`
    - `x := op y`
  • Indexed assignments
        - `x := y[i]`
    - `x[i] := y`
  • Pointer assignments
        - `x := &y`
    - `x := *y`
    - `*x := y`
  • Copy statements
        - `x := y`
  • Unconditional jumps
        - `goto label`
  • Conditional jumps
        - `if x relop y goto label`
  • Function calls
        - `param x... call p, n` `return y`

SDT to TAC

  • Synthesized attributes
        - `S.code`: TAC for S
    - `S.begin`: label to start of S || nil
    - `S.after`: label to end of S || nil
    - `E.place`: name holding value of E
    - `E.code`: TAC for S
  • Functions
        - `gen(...)`: code generation

Conditional Statements

  • Synthesized attributes
        - `E.truelist`: backpatch list for jumps on true
    - `E.falselist`: backpatch list for jumps on false
    - `M.quad`: location of current TAC
    - `S.nextlist`: backpatch list for jumps to next statement after `S` || nil
  • Functions
        - `makelist(i)`: create new list containing TAC location `i`
    - `merge(p1,p2)`: concatenate 2 lists
    - `backpatch(i,p)`: insert label `i` for each statement in `p`

Arrays

  • Synthesized attributes
        - `E.place`: temp holding value of E
    - `Elist.array`: array name
    - `Elist.place`: temp holding index value
    - `Elist.ndim`: number of array dimensions
    - `L.place`: lvalue (temp)
    - `L.offset`: index into array (temp) || null

Procedure Calls

Reusing Temp Variables

  • Modify newtemp() to keep a stack
  • Keep a counter c, init to 0
  • Decrement on use of $i in a 3-address statement
  • newtemp() returns $c++

References

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