Optimization: Helping gcc with function attributes

[tqh]

I wanted to experiment with gcc's attribute((fastcall)) which makes the functions two first arguments being passed in registers. I added it to kernel functions I thought would be called a lot. The resulting OS seems to be very responsive, although no measurements have been done.

My patch is just for inspiration, it is ugly and I did not really care about compability or if I break any API. Providing it here for those interested. It is used A LOT in Mozilla sources btw although hidden behind macros. It might be useful along with other function attributes in critical parts.

Function attributes are described here: ​http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html

[tqh]

Ugly fastcall patch

[tqh]

attribute has two underscores in front and after that went missing in the submission.

[tqh]

Ugly fastcall patch against hrev25229

[korli]

Ingo, would you mind giving an eye to this enhancement ? It might be a bad idea on public functions though.

[tqh]

Not sure they provide much speedup. Although I still keep them around in my tree.

[bonefish]

This kind of optimization can make sense; I would wait with trying it a bit longer, though. "fastcall" saves only a few cycles by passing arguments via registers instead of via the stack, but we're still looking for optimization potential of several hundred percent for certain use cases.

At any rate I would use a macro instead of hardcoding the gcc attribute.

Assigning the ticket to myself, so it won't be forgotten...

[waddlesplash]

The x86_64 ABI has the first 8 (?) arguments passed in via registers, so this is only useful on 32-bit, right? I think ARM does the same too, so this may not be so valuable.