Predication

Question 1

What is predication?

Answer 1

We do the work of both the T and NT paths, and then throw away the wrong path

Question 2

When should we use predication?

Answer 2

When the cost is smaller! If branch prediction averages 25 cycles and predication costs 10, then predication is better. Example: small if-else statements

Question 3

What is If-Conversion?

Answer 3

the process a compiler uses to transform small if-statements to not have a branch (so that the hardware uses predication)

Question 4

How do we implement if-converesion?

Answer 4

Conditional move:
Ex. MOVZ Rd, Rs, Rt (move Rs into Rd if Rt is equal to 0)
This is not a branch instruction!

Question 5

What are the problems with conditional moves?

Answer 5

We need more registers and more instructions are executed

Question 6

How do we solve the problems of conditional moves?

Answer 6

Full predication (every instruction has a condition for it to be executed)

Question 7

How is full predication implemented?

Answer 7

Usually represented in the hardware by adding conditional bits to the instruction