Re: [PATCH] introduce lower_32_bits() macro

[H. Peter Anvin]

Andrew Morton wrote:
> > >   */
> > >  #define upper_32_bits(n) ((u32)(((n) >> 16) >> 16))
> > >  
> > > +/**
> > > + * lower_32_bits - return bits 0-31 of a number
> > > + * @n: the number we're accessing
> > > + */
> > > +#define lower_32_bits(n) ((n) & 0xffffffffULL)
> > > +
> > NAK.  These are assymmetric with regards to type, which is the *last* 
> > thing we want.
>
> Yes, it will convert a 32-bit expression into a 64-bit one.

Sort of.  upper_32_bits() takes a 32- or 64-bit expression, and delivers 
the upper 32 bits *as a 32-bit number*.  An equivalent expression would be:

	((u32)((u64)(n) >> 32))

... which might actually produce better code, for all I know.

Logically speaking, if we have a lower_32_bits() macro, it should also 
produce a 32-bit type as result.

	-hpa

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