Today in CS362: Translating Function and Procedure Calls

Admin
* Expect phase 4 of the project on Monday

Overview
* Background:
  + Format of stack frame
  + Strategy for translating expressions
  + Phases of procedure/function call/return
  + Key responsibilities
* Design decisions: Who is responsible for what?
* Details

Review: What goes in a stack frame?
* Parameters to function or procedure.
* Pointer to the previous stack frame.
* Room for return value.
* Local variables (maybe just part of the stack).
* Saved register values.
* Return address (potentially)
* Some extra space in case we can't count correctly in
  figuring out what goes where on the stack.

Review: Three strategies for translating expressions
* translate(Exp) returns code+location
* translate(Exp,result-location) returns code
* hybrid

Review: Caller and Callee

Push parameters
Push old frame pointer
Push return address
Push space for local variables
Push space for return value (in some cases)

Return to code
Restore old frame pointer

Observation: What our Pascal Compiler does makes it easier
  to read the caller.  Generates less code.
Counter observation: With some clever rearrangement, we
  could get by with one ADD instead of three.
Current technique does not require the caller to know about
  the number of local variables required by callee.

translate(ID(<exp1>,<exp2>,<exp3>),desired_location)
  translate(<exp1>,onstack)
  translate(<exp2>,onstack)
  translate(<exp3>,onstack)
  push return_address
  jump lookup(ID,address)
RETURN_ADDRESS:
  move return-register desired-location
  clean up the stack

....

code for procedure

Setup:
  push frame pointer
  frame-pointer = stackpointer
  push space for local variables
Body:
Cleanup: 
  stack-pointer = frame-pointer
  frame-pointer = pop
  JUMP pop