SPEC CPU 2006 and 2017 are widely recognized CPU benchmark suites designed to evaluate the performance of system processors, memories, and compilers across various platforms. This paper tries to explore why ICC has significantly higher runtime efficiency than GCC on some benchmarks by analyzing the specific optimizations performed by ICC through various methods, aiming to figure out the reasons behind its high scores as well as the generality and portability of these optimizations, like loop fusion, SCEV and parallelization, structure peeling, vectorization, loop tiling, etc. The optimized GCC is more than 5.8% and 47% faster than the original GCC 12.3 for the rate metric on SPEC CPU 2017 and 2006, respectively. The performance gap between the optimized GCC and ICC on SPEC CPU 2006 has been reduced from 36.4% to 4.2% (speed metric) and from 51.8% to 3.0% (rate metric). It also surpassed ICC for both rate and speed metrics on SPEC CPU 2017, on an Intel Core i9-12900KF CPU.