Class 29: More control structures

Held Monday, November 16, 1998

Notes

• Read chapter 8 for Wednesday.
• Because of some special requests, I have not finished grading the makeups. I'll get them back to you on Wednesday.
• Please finish the type checkers by Friday. No exceptions.
• I'm working on a new version of the final project, and will have it ready by Friday (or possibly Wednesday, depending on how tomorrow goes).
• How many of you plan to miss the class before Thanksgiving break?
• Don't forget that we'd discussed having ``end of term'' presentations. I will ask you to come up with topics in the next week or so.

More control structures

Case statements

• What's the best way to translate a case statement?
• It's certainly possible to do a series of conditionals
  • Taken to the logical extreme, this may be absurd
• If the legal values in the case statement are in a reasonable range, then it may be possible to do a form of ``computed goto''
  • In effect, you store labels in an array, and use the choice value to index into that array
• Consider

    switch (x) {
      case 1 : ...
      case 2 : ...
      case 3 : ...
      case 4 : ...
      default: ...
    }
  

• This might be translated into

    CJUMP LT x 1 def
    CJUMP GT  4 def
    JUMP x (c1,c2,c3,c4)
  c1:
    code for case 1
    JUMP end
  c2: 
    code for case 2
    JUMP end
  c3: 
    code for case 3
    JUMP end
  c4: 
    code for case 4
    JUMP end
  def
    code for default case
    JUMP end
  end: 
    whew!
  

• If you don't have contiguous ranges, it may also be helpful to do a form of binary search. For example, consider

  switch (exp) {
    case 1..10: ...
    case 100..120: ...
    case 1000..2000: ...
    ...
  }
  

While loops

• These should be straightforward
  • Generate labels for the top (before the test), the body (after the test, but before any auxiliary code), and the end;
  • Translate the test, and fill in the appropriate branches;
  • Tranlsate the body;
  • Add a branch to the top;
  • Put it all together, filling in the labels

For loops

• Left as an exercise for the reader (we'll still discuss them in class, but I'd prefer not to have notes available in advance).

Function calls

• Function calls are relatively easy (given the CALL operation).
• You'll need to pass a static link as well as all of the arguments.
• [We may need to review what happens during function calls.]

History

• Created Friday, November 13, 1998 as a mostly blank page.
• Filled in details on Monday, November 16, 1998.