Why Does Changing a Loop Counter from 32-bit to 64-bit Dramatically Impact _mm_popcnt_u64 Performance on Intel CPUs?

Replacing a 32-bit loop counter with 64-bit introduces crazy performance deviations with _mm_popcnt_u64 on Intel CPUs

Problem Summary

The performance of a popcount benchmark varied drastically when the loop counter variable was changed from 32-bit unsigned to 64-bit unsigned, despite the change not appearing to affect the basic operation of the loop.

Question

1. Why is there such a performance difference between using a 32-bit and 64-bit loop counter?
2. How can replacing a non-constant buffer size with a constant value lead to slower code?
3. How does adding the 'static' keyword to the buffer size variable make the 64-bit loop faster?

Answer

1. The performance difference is due to a false data dependency in the popcnt instruction on Intel CPUs.

When the loop counter is 32-bit, the popcnt instructions in each loop iteration are executed independently, allowing for parallel execution. However, when the loop counter is 64-bit, a false data dependency is introduced between the popcnt instructions, making it impossible for them to execute in parallel. This dependency is caused by the destination register for the popcnt instruction being reused for the next iteration, creating an artificial dependency that limits the performance.

2. Replacing a non-constant buffer size with a constant value can slow down the code because it prevents the compiler from performing some optimizations.

With a constant buffer size, the compiler knows the exact size of the buffer at compile time, which can allow for more efficient memory access patterns and instruction scheduling. However, with a non-constant buffer size, the compiler has to assume a worst-case scenario, which can lead to less optimized code.

3. Adding the 'static' keyword to the buffer size variable makes the 64-bit loop faster because it makes the buffer size a compile-time constant, allowing the compiler to perform additional optimizations.

By making the buffer size a compile-time constant, the compiler can more aggressively optimize the memory access patterns and instruction scheduling, resulting in faster code.

Lessons Learned

Even small changes in a loop can have a significant impact on performance due to unexpected dependencies or compiler optimizations. It is important to understand these dependencies and how they affect performance to write efficient code.

The above is the detailed content of Why Does Changing a Loop Counter from 32-bit to 64-bit Dramatically Impact _mm_popcnt_u64 Performance on Intel CPUs?. For more information, please follow other related articles on the PHP Chinese website!