ICC's vectorization is nearly useless. I've run it on thousands of lines of basic DSP code and gotten almost nothing--at best a single bad autovectorization.
The reasons ICC are better are many but unrelated to vectorization: one optimization I noticed is that it will compile a set of code that depends heavily on aliasing concerns twice and branch to which code path depending on whether the relevant pointers alias or not. This branch is usually predictable, since the pointers in reality will probably never alias, but it has to abide by the C spec.
There's probably a few dozen more things like this that add up to make it a few percent better than GCC. Though GCC is so buggy and many of its heuristics (especially inlining and storing array/struct elements in registers) so utterly hackneyed that beating it is not extraordinarily difficult..
The reasons ICC are better are many but unrelated to vectorization: one optimization I noticed is that it will compile a set of code that depends heavily on aliasing concerns twice and branch to which code path depending on whether the relevant pointers alias or not. This branch is usually predictable, since the pointers in reality will probably never alias, but it has to abide by the C spec.
There's probably a few dozen more things like this that add up to make it a few percent better than GCC. Though GCC is so buggy and many of its heuristics (especially inlining and storing array/struct elements in registers) so utterly hackneyed that beating it is not extraordinarily difficult..