On Thu, Mar 22, 2018 at 09:41:57AM -0700, Matthew Wilcox wrote:
> On Thu, Mar 22, 2018 at 09:22:31AM -0700, Alexander Duyck wrote:
> > You could just use the pfc->mask here instead of size - 1 just to
> > avoid having to do the subtraction more than once assuming the
> > compiler doesn't optimize it.
>
> Either way I'm assuming a compiler optimisation -- that it won't reload
> from memory, or that it'll remember the subtraction. I don't much care
> which, and I'll happily use the page_frag_cache_mask() if that reads better
> for you.
Looks like it does reload from memory if I make that change. Before:
37e7: c7 43 08 ff 7f 00 00 movl $0x7fff,0x8(%rbx)
37ee: b9 00 80 00 00 mov $0x8000,%ecx
37f3: be ff 7f 00 00 mov $0x7fff,%esi
37f8: ba 00 80 00 00 mov $0x8000,%edx
...
380b: 01 70 1c add %esi,0x1c(%rax)
After:
37e7: c7 43 08 ff 7f 00 00 movl $0x7fff,0x8(%rbx)
37ee: b9 00 80 00 00 mov $0x8000,%ecx
37f3: ba 00 80 00 00 mov $0x8000,%edx
...
3806: 8b 73 08 mov 0x8(%rbx),%esi
3809: 01 70 1c add %esi,0x1c(%rax)
Of course, it's shorter because it's fewer bytes to reload from memory
than it is to put a 32-bit immediate in the instruction stream, but
it's one additional memory reference (cache-hot, of course). I don't
really care because it's the cold path.
