lecture-Oct-8.md

Tuples and Garbage Collection

Review of the previous lectures

• 2-space copying collector

• copy via Cheney's algorithm (BFS with Queue in the ToSpace)

• data representation (root stack, tuple metadata with pointer mask)

• running example:

    (let ([v (vector 42)])
       (let ([w (vector v)])
          (let ([v^ (vector-ref w 0)])
             (vector-ref v^ 0))))
  

• review passes from last time

  • type-check: introduce HasType
  • shrink: update to generate HasType
  • expose-allocation (new): (vector ...) becomes collect-allocate-initialize
  • remove-complex-opera*:
    • collect, allocate, and global-value are complex
    • careful not to separate Prim from its surrounding HasType

explicate-control

minor changes to handle the new forms

uncover-locals

Collect the local variables and their types by inspecting assignment statements. Store in the info field of Program.

select-instructions (5.3.3)

Here is where we implement the new operations needed for tuples.

example: block9056

• vector-set! turns into movq with a deref in the target

    lhs = (vector-set! vec n arg);
  

  becomes

    movq vec', %r11
    movq arg', 8(n+1)(%r11)
    movq $0, lhs'
  

  what if we use rax instead:

    movq vec', %rax
    movq -16(%rbp), 8(n+1)(%rax)
    movq $0, lhs'
  
    movq vec', %rax
    movq -16(%rbp), %rax
    movq %rax, 8(n+1)(%rax)
    movq $0, lhs'
  

  We use r11 for temporary storage, so we remove it from the list of registers used for register allocation.

• vector-ref turns into a movq with deref in the source

    lhs = (vector-ref vec n);
  

  becomes

    movq vec', %r11
    movq 8(n+1)(%r11), lhs'
  

• allocate

  1. put the current free_ptr into lhs
  2. move the free_ptr forward by 8(len+1) (room for tag)
  3. initialize the tag (use bitwise-ior and arithmetic-shift) using the type information for the pointer mask.

  So

    lhs = (allocate len (Vector type ...));
  

  becomes

    movq free_ptr(%rip), lhs'
    addq 8(len+1), free_ptr(%rip)
    movq lhs', %r11
    movq $tag, 0(%r11)
  

• collect turns into a callq to the collect function.

  Pass the top of the root stack (r15) in register rdi and the number of bytes in rsi.

    (collect bytes)
  

  becomes

    movq %r15, %rdi
    movq $bytes, %rsi
    callq collect
  

allocate-registers

• Spill vector-typed variables to the root stack. Handle this in the code for assigning homes (converting colors to stack locations and registers.)

  Use r15 for the top of the root stack. Remove it from consideration by the register allocator.

• If a vector variable is live during a call to collect, make sure to spill it. Do this by adding interference edges between the call-live vector variables and the callee-saved registers. You'll need to pass the variable-type information as another parameter to build-interference.

  example: block9059: define vecinit9047 -> block9058: collect -> block9056: use vecinit9047

print-x89

• Move the root stack forward to make room for the vector spills.

• The first call to collect might happen before all the slots in the root stack have been initialized. So make sure to zero-initialize the root stack in the prelude!