[PATCH] powerpc/mm: use _raw variant of page table accessors

[Michael Ellerman]

On Wed, 2016-06-01 at 15:13 -0700, Ram Pai wrote:
> On Tue, May 31, 2016 at 04:29:42PM +0530, Aneesh Kumar K.V wrote:
> > diff --git a/arch/powerpc/include/asm/book3s/64/pgtable-4k.h b/arch/powerpc/include/asm/book3s/64/pgtable-4k.h
> > index 71e9abced493..9db83b4e017d 100644
> > --- a/arch/powerpc/include/asm/book3s/64/pgtable-4k.h
> > +++ b/arch/powerpc/include/asm/book3s/64/pgtable-4k.h
> > @@ -31,7 +31,7 @@ static inline int pgd_huge(pgd_t pgd)
> >  	 * leaf pte for huge page
> >  	 */
> >  	if (radix_enabled())
> > -		return !!(pgd_val(pgd) & _PAGE_PTE);
> > +		return !!(pgd_raw(pgd) & cpu_to_be64(_PAGE_PTE));
> 
> pgd_raw() will not do the endian swapping.
> But instead cpu_to_be64(_PAGE_PTE) will now do the endian swapping. So does it
> really optimize anything? i tend to think it just moves the endian
> swapping overhead from one place to the other. no?

> Is cpu_to_be64(constant) faster than cpu_to_be64(variable)  ?

Yes, the compiler will do it at compile time for constants, or anything it can
prove is a constant.

You can also see from the asm dump that Aneesh posted that it drastically
improves the generated code.

cheers