👀 Views: 1 💬 Answers: 1 📅 Created: 2025-06-07

gcc optimization performance C

I'm performance testing and I'm relatively new to this, so bear with me. This might be a silly question, but I'm experiencing issues with GCC not optimizing loop unrolling in my C code, and it's significantly affecting performance. My code involves a nested loop where certain conditions determine whether to execute internal operations, and I noticed that the compiler's optimization flags don't seem to be having the intended effect. Here's a simplified version of the code that illustrates the question: ```c #include <stdio.h> void process(int *array, int size) { for (int i = 0; i < size; i++) { if (array[i] > 0) { array[i] *= 2; } else if (array[i] < 0) { array[i] += 1; } } } int main() { int data[100000]; for (int j = 0; j < 100000; j++) { data[j] = j - 50000; // Filling with both positive and negative numbers } process(data, 100000); return 0; } ``` When I compile this with `gcc -O3 -o my_program my_program.c`, I expected the compiler to apply loop unrolling for the `process` function, especially since it's a simple integer operation. However, using `perf` to analyze the program's performance shows that the loop executes much slower than expected, and the generated assembly code appears to still have the loop intact without unrolling. I've also tried using `#pragma GCC optimize("unroll-loops")` before the `process` function, but the performance remains the same. I’m running GCC version 11.2.0 on Ubuntu 20.04. I even experimented with `-funroll-loops` and `-fopt-info-vec` to get more insights, yet it still doesn't seem to optimize the loops effectively. Is there a way to force GCC to optimize this specific case, or am I missing some detail about the conditions that would prevent the compiler from applying such optimizations? Any insights into this would be greatly appreciated! My development environment is Ubuntu. I'm open to any suggestions. For reference, this is a production desktop app. Any ideas what could be causing this?