Class 26: Translating Expressions, Continued

Held Wednesday, April 4, 2001

Summary

Today we continue our considerations of the steps involved in translating expressions by visiting the compound data types, arrays and records.

Notes

• Are there questions on the type checker?
  • I know that Rachel reported that the type checker was the worst part of their effort; however, they were dealing with a different language.
• In case you haven't figured it out yet, I'm providing project advice primarily in an "on demand" fashion; I'm unlikely to grade your projects until the end of the term.
  • Having looked at the British system, I feel less bad about these only end-of-semester evaluations.

Overview

• Status
• About records and arrays
• Translating record fields
• Translating array references
• Translating other kinds of expressions

Records and Arrays

• Compound values, like records and arrays are more complicated than basic values because they occupy more memory and hence cannot fit in a register.
• There are many ways to handle these bigger things, most of which involve storing the base address in a temporary.
• It may help to distinguish between values stored on the stack and values stored in the heap or "static" memory.

Assigning Records

• Probably can't be done with the same instruction as integer or real copies; you need to copy an area of memory, rather than a single value.
• You may have to implement a memcopy helper if one isn't built in to your assembly.

Translating Record Components

• Example: r.a or r.b
• Generate the code to build the record. You should end up with a memory reference for the start of the record.
• Add the offset for the field. This gives you the address of the start of the field.

Translating Array Selections

• Example: A[i] or A[i+3]
• Similar to records, except that the offset is given by a formula that involves the size of the stored values, the base index, and the desired index.
• Yes, you'll need to look in the symbol table.
• Yes, here's a case in which you'll probably want to store a reference to the desired location in a temporary.

Translating Other Expressions

• Relational opeartors are often better handled in the appropriate context (e.g., in translating conditionals).
• Function calls are complicated enough that we'll return to them next week.