In C++, compiler optimization can reduce runtime but sometimes complicates debugging or alters behavior in edge cases. What are the best practices for balancing optimization activation (e.g., -O2 vs. -O3) while maintaining code reliability? What criteria do you use to decide which optimization level to apply in production projects?
How does compiler optimization affect the performance of C++ code?
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In my latest computer vision library project, I started compiling with `-O2` because we needed the code to be reasonably fast while still being able to debug with gdb without too many issues. During performance testing, I found that certain critical loops—especially those processing thousands of pixels per frame—were still consuming a lot of CPU. I then tried `-O3` only on those specific files using `target_compile_options`, and noticed a 12% improvement in processing time without any overflow errors or unexpected behavior. To maintain reliability, I configured CMake so that unit tests always run with `-O0` and `-Wall -Wextra`; if any test fails only in the optimized version, I revert to `-O2` and analyze with `-fsanitize=address,undefined`. In production, my approach is: use `-O2` as the default because it offers a good balance between speed and stability; enable `-O3` only in critical modules after validating that edge-case tests (boundary values, corrupted inputs, etc.) still pass, and always keep a debug configuration without optimizations for intensive debugging.