Lesson 6--Branches

Question 1

Types of branches

Answer 1

• -Unconditional (ends of loops, etc.)
• -Indirect (indirect jumps, case statements, etc.)
• -Conditional (if then else, loops, etc.)
• -Function (function calls)

Question 2

How to mitigate branch delays

Answer 2

Speculation: Predicting branches, executing in anticipation.
Predication: Removing branches, converting code into one that depends on outcome of a condition for a commit.

Question 3

3 Components of a branch

Answer 3

• -Whether to take the branch
• -Where to go
• -When to go

Question 4

Two Step Branching

Answer 4

• -separates comparison(whether) from branch instructions(where/when)
• -Store the comparison results in branch/condition registers
• -Delay can be filled with useful operation between compare and branch

Question 5

Three Step Branching

Answer 5

• -Decouple the address computation from the branch
• -Branch target register stores the results of the target address and comparison
• -The compiler can move the compare and offset computations in any order

Question 6

True or False: Unbundling increases code size.

Answer 6

True: 2 step leads to 14% code growth. 3 step leads to 28% growth.

Question 7

True or False: In a VLIW architecture with unbundled branches, the branch targets must be computed once for all clusters

Answer 7

False. The branch will be executed in one cluster, not all.

Question 8

True or False: The branch condition is computed in one cluster and broadcast to all other clusters

Answer 8

True

Question 9

True or False: If the branch condition is taken, each cluster transfers control to one branch target.

Answer 9

False. Clusters only need to be notified of which branch is taken, but they do not affect control of other clusters.

Question 10

True or False: All branch targets correspond to the same logical block

Answer 10

True.

Question 11

Describe conservative techniques used in speculation

Answer 11

Unless legality of reordering can be proven, always serialize operations

Question 12

Speculation

Answer 12

Executes branch ahead based on probabilistic assumptions, and fixes the execution if speculation is wrong.

Question 13

Control speculation

Answer 13

Removes control dependence

Question 14

How many additional instructions are generated for 2 step branching

Answer 14

1

Question 15

How many additional instructions are generated for 3 step branching

Answer 15

2

Question 16

Are 2 or 3 step branches more common for embedded systems?

Answer 16

2 step.