Compiler ILP

Question 1

How can compilers help with ILP?

Answer 1

1. Reduce dependency chains
2. Move instructions around so that processor can see independent instructions soon (instead of just seeing long dependency chains)

Question 2

How can a compiler reduce dependency chains?

Answer 2

Tree Height Reduction - Not always possible to do this => relies on associativity of addition, not all operations are associative

Question 3

What are the techniques to make instructions easier for a processor to find?

Answer 3

1. Instruction Scheduling
2. Loop Unrolling
3. Trace Scheduling

Question 4

What is instruction scheduling?

Answer 4

Finding instructions that can be done in place of stalls (i.e. find instructions that the processor can do while it is waiting on a result). Can combine with if-conversion to interleave two potions of branch.

Question 5

What are the pros of loop unrolling?

Answer 5

Reduces the number of instructions and can combine with instruction scheduling to reduce CPI

Question 6

What are the cons of loop unrolling?

Answer 6

Causes code bloat and we won’t be able to use it if we don’t know the number of iterations ahead of time or if the number of iterations is not a multiple of N

Question 7

What are the pros/cons of function inlining?

Answer 7

(+) eliminate call/return overhead, can allow for better scheduling (just more instructions to use)
(-) code bloat