Implement Loop Unrolling and Measure the Speedup

Trade Code Size for Latency Hiding

Loop unrolling duplicates the body to cut branch instructions per iteration and expose instruction-level parallelism. Compilers unroll at -O3; manual #pragma unroll or partial unrolling helps when the compiler is conservative about trip counts.

Manual Unroll Example

Process four elements per iteration with a cleanup loop for remainder.

c
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Unrolling too far blows icache and register pressure
Pair with SIMD for 4x or 8x logical unroll
-funroll-loops lets GCC decide automatically
Measure on target CPU; unroll depth is not portable

Working Through the Exercise

Keep the relevant documentation open while you implement. When your output disagrees with the reference, trace one failing case by hand before changing random lines.

Trace first: walk one failing input step by step
Read the manual: skim the official doc for the mechanism named in this task
Compare baselines: save measurements from the prior step so speedups are honest

Why for this exercise

You will implement manual loop unrolling on a reduction loop and compare against compiler-unrolled assembly. This exercise requires reporting when unrolling helped and when it regressed.

Implement loop unrolling manually and compare performance to the baseline loop.

Requirements:

Implement a baseline sum loop (no unrolling)
Implement a 2x unrolled version
Implement a 4x unrolled version
Implement an 8x unrolled version
Process an array of at least 1 million elements
Ensure all versions produce the same result
Measure and print execution time for each version
Show speedup relative to baseline
Explain why larger unroll factors may not always help
Show compiler-generated assembly difference conceptually

Test:

Show all versions produce same result
Show timing comparison
Show speedup percentages
Explain optimal unroll factor

Trade Code Size for Latency Hiding

Manual Unroll Example

Process four elements per iteration with a cleanup loop for remainder.

c
Loading…

Unrolling too far blows icache and register pressure
Pair with SIMD for 4x or 8x logical unroll
-funroll-loops lets GCC decide automatically
Measure on target CPU; unroll depth is not portable

Working Through the Exercise

Keep the relevant documentation open while you implement. When your output disagrees with the reference, trace one failing case by hand before changing random lines.

Trace first: walk one failing input step by step
Read the manual: skim the official doc for the mechanism named in this task
Compare baselines: save measurements from the prior step so speedups are honest

Why for this exercise

You will implement manual loop unrolling on a reduction loop and compare against compiler-unrolled assembly. This exercise requires reporting when unrolling helped and when it regressed.

Implement loop unrolling manually and compare performance to the baseline loop.

Requirements:

Implement a baseline sum loop (no unrolling)
Implement a 2x unrolled version
Implement a 4x unrolled version
Implement an 8x unrolled version
Process an array of at least 1 million elements
Ensure all versions produce the same result
Measure and print execution time for each version
Show speedup relative to baseline
Explain why larger unroll factors may not always help
Show compiler-generated assembly difference conceptually

Test:

Show all versions produce same result
Show timing comparison
Show speedup percentages
Explain optimal unroll factor

Topic Overview

Trade Code Size for Latency Hiding

Manual Unroll Example

Working Through the Exercise

Why for this exercise

Implementation Task

Progressive Hints

Resources

Open Source

Implement Loop Unrolling and Measure the Speedup

Topic Overview

Trade Code Size for Latency Hiding

Manual Unroll Example

Working Through the Exercise

Why for this exercise

Implementation Task

Progressive Hints

Resources

Open Source