https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61333
--- Comment #8 from Dominique d'Humieres <dominiq at lps dot ens.fr> --- Some comments: original shell: 1:1.86:2.9 + -Ofast : 1:1.37:1.8 (gcc 4.10.0 r210749). Does this mean that there is a problem with -Ofast and -fopenmp? The Wallclock time are: original shell: 46.49s:25.83s:16.02s + -Ofast : 7.82s:5.72s:4.21s Estimating an Amdahl's law: s+p/n (s serial time, p parallel time, n number of threads), based on n=1 and 2, gives original shell: s= 5.17s, p=41.32s, time for n=4: 15.50s, + -Ofast : s=3.63s, p=4.19s, time for n=4: 4.68s. This crude estimate shows that the serial time is only slightly improved with -Ofast while the parallel one is an order of magnitude faster with it. Could you give the Wallclock time for the different cases in comment 0?
