1 Your Task

Fill in a pipeline diagram for a small function’s full execution.

Place you answer in the online tool https://kytos.cs.virginia.edu/coa2-s2019/pld.php.

2 Guidelines

Consider the following code:

long count_odd(long *arr, long len) {
    long ans = 0;
    for(long i=0; i<len; i+=1)
        ans += arr[i]&1;
    return ans;
}

which has been compiled to (numbers inserted for ease of reference)

    count_odd:
0.      xorl    %eax, %eax
1.      testq   %rsi, %rsi
2.      jle .LBB0_2
    .LBB0_1:
3.      movq    (%rdi), %rcx
4.      andl    $1, %ecx
5.      addq    %rcx, %rax
6.      addq    $8, %rdi
7.      decq    %rsi
8.      jne .LBB0_1
    .LBB0_2:
9.      retq

Assume that this is run on a five-stage pipeline, where

• every instruction must go through all five stages in order

• each stage may handle at most one instruction per cycle

• decode needs to stall until values (program register and condition code) are available, but only needs values by the end of its cycle.

  • e.g., if a memory read is followed by an operation on the read value, the decode stage of the operation can be co-scheduled with the memory stage of the read.

    read	F	D	E	M	W
    use		F	D	D	E	M	W

  • e.g., if a computation is followed by an operation on the computed value, the decode stage of the operation can be co-scheduled with the execute stage of the computation.

    compute	F	D	E	M	W
    use		F	D	E	M	W

• computation not available until the end of execute includes

  • results of arithmetic, logic, and address computation
  • condition code creation from test, compare, arithmetic, and logic
  • condition code comparison for conditional moves and jumps

  e.g., if a conditional jump is poorly speculated, the correct instruction will be fetched after the jump completes its execute stage.

  jump	F	D	E	M	W
  guess1		F	D
  guess2			F
  correct				F	D	E	M	W

• conditional jumps are speculated using the “backwards-taken forwards-not-taken” branch prediction heuristic

You’re asked to expand this assuming

1. you start with the first instruction after count_odd is called
2. the first instruction of count_odd does not need to stall
3. arr pointing to the array {1, 2}
4. len stores the value 2
5. the retq is the last instruction you need to handle