30th May 2016
In Effect of Optimizer in gcc on Intel/AMD and Power8 I measured speed ratios between optimized and non-optimized C code of three on Intel/AMD, and eight on Power8 (PowerPC) for integer calculations. For floating-point calculations the factors were two and three, respectively.
Michael Larabel in GCC 6.1 Compiler Optimization Level Benchmarks: -O0 To -Ofast + FLTO measured various optimization flags of the newest GCC.
For a Poisson solver the speed ratio between optimized and non-optimized code was five.