What are some kinds (examples?) of code that you find ICC to compile better than GCC? Like, what's a typical loop that ICC can vectorize but GCC can't? I always have the damnedest time pinpointing when and where these optimizations fire and I've pretty much given up on the compiler when it comes to them. Rather I just develop code as normal and then when it's done find the top 3 or 4 functions in gprof (or Shark or whatever) and vectorize those by hand. Either that or try every compiler you have available and pick the one that yields the best time, but in my experience it's not always Intel.
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..
