https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109587
--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> --- At -O2 we get: size: 26-3, last_iteration: 2-2 Loop size: 26 Estimated size after unrolling: 245 Not unrolling loop 1: size would grow. With -O3: size: 20-4, last_iteration: 2-2 Loop size: 20 Estimated size after unrolling: 170 /app/example.cpp:8:29: optimized: loop with 16 iterations completely unrolled (header execution count 63136016) Exit condition of peeled iterations was eliminated. Last iteration exit edge was proved true. Forced exit to be taken: if (0 != 0) Well yes -O3 is known to cause issues like this. I had thought it was documented saying that sometimes -O3 might cause performance issues over -O2 but I can't find that documentation either.
