On Thu, Mar 30, 2006 at 04:58:25PM -0800, Christoph Lameter wrote:
> On Thu, 30 Mar 2006, Luck, Tony wrote:
>
> > > Also some higher level functions may want to have the mode passed to them
> > > as parameters. See f.e. include/linux/buffer_head.h. Without the
> > > parameters you will have to maintain farms of definitions for all cases.
> >
> > But if any part of the call chain from those higher level functions
> > down to these low level functions is not inline, then the compiler
> > won't be able to collapse out the "switch (mode)" ... so we'd end up
> > with a ton of extra object code.
>
> Correct. But such bitops are typically defined to be inline.
That's doesn't seem to be the point that Tony was making. To illustrate
it let's add a practical example:
static inline void clear_bit_mode(int bit, unsigned long *ptr, int mode)
{
case (mode) {
...
}
}
void foo(blah blah, int mode)
{
... complex function ...
clear_bit_mode(bit, ptr, mode);
...
}
void bar(blah blah)
{
foo(blah, MODE_BARRIER);
}
In this case, the compiler can not optimise the unnecessary code from
the clear_bit_mode because the mode argument quite definitely is not a
constant known at compile time. Only if 'foo' was a static inline
would it be known.
--
Russell King
Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/
maintainer of: 2.6 Serial core
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