questions; gcc builtins - IO scheduler - profiling

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Hi all,

since I have played a bit with the kernel sources these are some newbie
questions, that I couldn't find with trivial search engine usage;

a) It is possible to use gcc builtin functions in conjuction with some
gcc switches and the march to produce somewhat optimal code. For
instance function __builtin_popcount, together with the march=amdfam10
and -mabm produces a POPCNT assembly instruction which counts the bits
on word (machine word) in 1 instruction. For archs that does not support
the popcnt instruction  as far as I can tell gcc produces
/normal-expected/ code that does the same thing in a simple (using a
loop, shifting and counting Zero-Flags) or a more advanced way ().  Is
such programming practice condemned?

b) One of the coolest things in kernel is the different types of I/O
schedulers. I haven't exhaustively checked the source but is it possible
to have a differnet I/O scheduler per device? If for instance I have an
SSD and a normal hdd wouldn't be cooler to use noop I/O Sched for the
SSD  and anticipatory I/O Sched for the normal hdd?

c) Is there a way to profile *just* one specific module/function/group
of functions that run as a kernel thread for runtime performance? So far
I've (tried to) used (use) oprofile ( for some
profiling and it is quite disturbing ( can't see the tree for the forest
:P ).

Cheers :)
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