[PATCH V4 1/2] arm: mm: Introduce {pte,pmd}_isset and {pte,pmd}_isclear

[Russell King - ARM Linux]

On Fri, Jun 20, 2014 at 10:12:26AM +0100, Will Deacon wrote:
> On Mon, Jun 16, 2014 at 03:32:38PM +0100, Steve Capper wrote:
> > -#define pmd_young(pmd)		(pmd_val(pmd) & PMD_SECT_AF)
> > +#define pmd_isset(pmd, val)	((u32)(val) == (val) ? pmd_val(pmd) & (val) \
> > +						: !!(pmd_val(pmd) & (val)))
> 
> What does GCC generate for this as opposed to an unconditional !!?

For any constant "val", the result of (u32)(val) == (val) is also a
constant, so GCC should evaluate it at build time and not run time.

An unconditional !! would result in GCC doing the test and then
constructing code to turn it into a zero-or-one condition, and with
complicated tests, it can really add a lot of additional complexity,
even if it can properly optimise the simple case.

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