Branch Prediction and Compiler Optimization Quiz Quiz

Explore the fundamentals of branch prediction and compiler optimization, focusing on control flow, performance enhancements, and code transformation techniques. This quiz challenges your understanding of dynamic prediction, speculative execution, and how compilers improve execution efficiency.

1. Static vs. Dynamic Branch Prediction

   In branch prediction, what is the key distinction between static and dynamic prediction techniques when optimizing control flow instructions?

   1. Static prediction changes with every program execution, while dynamic prediction does not.
   2. Static prediction uses fixed rules, whereas dynamic prediction relies on runtime behavior.
   3. Static prediction always results in incorrect predictions, unlike dynamic prediction.
   4. Static prediction requires speculative execution, dynamic does not.

   Explanation: Static branch prediction makes decisions based on predefined rules, such as always predicting branches as taken or not taken, while dynamic prediction monitors the actual runtime behavior to adapt predictions based on past outcomes. The second option is incorrect because it reverses the concept; static is unchanging, dynamic adapts. The third statement is false as neither always results in incorrect predictions, and the fourth incorrectly associates speculative execution solely with static prediction.

2. Compiler Loop Unrolling

   When a compiler applies loop unrolling as an optimization, which of the following is a primary intended benefit?

   1. Increasing the overall number of iterations required by a loop.
   2. Reducing the number of branch instructions and improving instruction-level parallelism.
   3. Making the code longer to reduce cache locality.
   4. Forcing all variables to use global scope.

   Explanation: Loop unrolling reduces the number of iterations by doing more work per loop, which decreases the number of branch instructions and allows for greater instruction-level parallelism, boosting performance. The second option is incorrect because unrolling doesn't increase iterations. The third is wrong because longer code doesn't necessarily reduce cache locality—it can even harm it in some cases, but it's not the primary intent. The fourth option misrepresents loop unrolling as affecting variable scope, which it does not.

3. Speculative Execution

   Why is speculative execution used in conjunction with branch prediction in modern processors?

   1. To ensure every branch is resolved before proceeding any further.
   2. To always re-execute instructions after every branch outcome.
   3. To reduce instruction cache size by skipping unnecessary code.
   4. To execute predicted paths ahead of time, minimizing pipeline stalls caused by branches.

   Explanation: Speculative execution allows the processor to continue working on predicted paths during branch evaluation, which helps minimize costly pipeline stalls if the prediction is correct. The second choice is incorrect because instructions are not always re-executed after each branch. The third option is the opposite of speculative execution, which doesn't wait for every branch to resolve. The fourth is unrelated, as speculative execution doesn't directly affect instruction cache size.

4. Branch Target Buffer (BTB)

   What role does a Branch Target Buffer (BTB) play in dynamic branch prediction?

   1. It stores variable values during function calls for optimization.
   2. It caches the target addresses of branch instructions to speed up prediction and fetching.
   3. It increases the number of pipeline stages to handle more instructions.
   4. It swaps out unused instructions from main memory.

   Explanation: A Branch Target Buffer predicts and remembers the target addresses of recently executed branches, which helps processors quickly fetch and decode the next instruction without delay if a branch is predicted as taken. The second option wrongly describes a variable stack, not a BTB. The third is unrelated to BTB functionality, as increasing pipeline stages is not its job. The fourth confuses BTB with memory management, which it is not.

5. If-Conversion Optimization

   Which statement correctly describes the compiler optimization known as if-conversion?

   1. It reduces the size of function call stacks by inlining functions.
   2. It transforms conditional branches into branch-free code using conditional instructions.
   3. It converts all integer variables into floating-point format.
   4. It delays the evaluation of arithmetic expressions until they are needed.

   Explanation: If-conversion removes explicit branch instructions in favor of conditional execution, reducing the negative impact of branch mispredictions on performance. The second option is unrelated, describing type casting rather than optimization. The third is a feature of function inlining, not if-conversion. The fourth confuses if-conversion with lazy evaluation, which is a different concept.