Comment by nickelpro

People keep saying "O3 has bugs," but that's not true. At least no more bugs than O2. It did and does more aggressively expose UB code, but that isn't why people avoid O3.

You generally avoid O3 because it's slower. Slower to compile, and slower to run. Aggressively unrolling loops and larger inlining windows bloat code size to the degree it impacts icache.

The optimization levels aren't "how fast do you want to code to go", they're "how aggressive do you want the optimizer to be." The most aggressive optimizations are largely unproven and left in O3 until they are generally useful, at which point they move to O2.