Flag -qopenmp makes serial code slower when using -O3

Hello,

we are doing some experiments, comparing the execution time of C code compiled with and without the option -parallel (icc 18.0.0) and we noticed that when we compile code that has no OpenMP pragmas with the flag -O3, if we also use the flag -qopenmp, the execution time is slower that when we do not use it. If we use -O2 instead of -O3, there is no performance difference, in the code we tested.

Could you give some insight on what might be happening? Is -qopenmp preventing optimizations that would otherwise be applied when using -O3?

Thanks,

João Bispo