On Sat, 7 Apr 2007 15:16:17 -0700 (PDT) Christoph Lameter <[email protected]> wrote:
> On Fri, 6 Apr 2007, Andrew Morton wrote:
>
> > Did you investigate
> >
> > static inline int page_tail(struct page *page)
> > {
> > return ((page->flags & (PG_compound|PG_tail)) == (PG_compound|PG_tail));
> > }
>
> The usual test_bit that we are using there uses a volatile reference
> so these wont be combined if I check them separately.
>
> A working example of the above would be much uglier:
>
> static inline int page_tail(struct page *page)
> {
> return ((page->flags & ((1L << PG_compound)|(1L << PG_tail))) ==
> ((1L << PG_compound)|(1L << PG_tail)));
> }
>
> May be this can be cleaned up somehow.
It might generate better code to do
unsigned long compound;
compound = page->flags & (1 << PG_compound);
if (PG_compound > PG_tail)
return compound & (page->flags << (PG_compound - PG_tail));
else
return compound & (page->flags << (PG_tail - PG_compound));
ie: get the PG_compound flag into `compound', then bitwise-and that with
the PG_tail flag, after shifting it into PG_compound' slot. The return
value will be zero if either bit is clear, (1<<PG_compound) if both are
set. The `if (PG_compound > PG_tail)' will be swallowed by the compiler.
The compiler should turn it all into
(page->flags & N) & (page->flags << M)
Which may or may not be better than (page->flags & N == N), dunno.
Probably not - if the compiler's any good it won't save a branch, I
suspect.
Which is all a ton of fun, but this subversion of the architecture's
freedom to use volatile, memory barriers etc is a worry. We do the same in
page_alloc.c, of course...
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