I've done a bit of assembly programming for IA-32, none recently and zero compiler development other than simple parsers, but from what I know optimizing with newer instructions like SSEx is only part of there story. There's also optimizing for out of order instruction execution (OoOIE), or instruction reordering. Even on a single core, when optimized done properly it can yield 4x or more increase in processing speed, avoiding CPU stalls, which is likely happening a lot given the descriptions people have provided in this thread.
Obviously properly using SSEx instructions and optimizing for OoOIE is a non trivial undertaking. I think probably the best way to test FPC out is to stop writing speed tests against other languages, and to view the CPU instructions generated for a few different loop types, including trunc()/floor() functions. Next rewrite those parts in assembler, include SSE whatever instructions and pepper in OoOIE, and finally test the speed difference. My instincts tell me if the compiler could then generate both SSEx and OoOIE properly it could automatically applied in many places, resulting in a huge speed up for FPC programs. But for me, the main advantage of Free Pascal is that it generates native language, leading to easier reuse of C style APIs such as Cairo/GTK/SDL, and also allowing for the PME (properties, methods, events) plus all the other nice features Free Pascal offers. Most of the time my programs are either idle waiting for user input, or idle waiting for other APIs or in the case of OpenGL the graphics hardware to complete. As such the execution speed of the pascal portion of my programs isn't as important.
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