RE: [PATCH v5 01/38] minmax: Add in_range() macro

[David Laight]

From: Andrew Morton
> Sent: 11 July 2023 00:14
> To: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> Cc: linux-arch@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH v5 01/38] minmax: Add in_range() macro
> 
> On Mon, 10 Jul 2023 21:43:02 +0100 "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx> wrote:
> 
> > Determine if a value lies within a range more efficiently (subtraction +
> > comparison vs two comparisons and an AND).  It also has useful (under
> > some circumstances) behaviour if the range exceeds the maximum value of
> > the type.
> >
> > Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> > --- a/include/linux/minmax.h
> > +++ b/include/linux/minmax.h
> > @@ -158,6 +158,32 @@
> >   */
> >  #define clamp_val(val, lo, hi) clamp_t(typeof(val), val, lo, hi)
> >
> > +static inline bool in_range64(u64 val, u64 start, u64 len)
> > +{
> > +	return (val - start) < len;
> > +}
> > +
> > +static inline bool in_range32(u32 val, u32 start, u32 len)
> > +{
> > +	return (val - start) < len;
> > +}
> > +
> > +/**
> > + * in_range - Determine if a value lies within a range.
> > + * @val: Value to test.
> > + * @start: First value in range.
> > + * @len: Number of values in range.
> > + *
> > + * This is more efficient than "if (start <= val && val < (start + len))".
> > + * It also gives a different answer if @start + @len overflows the size of
> > + * the type by a sufficient amount to encompass @val.  Decide for yourself
> > + * which behaviour you want, or prove that start + len never overflow.
> > + * Do not blindly replace one form with the other.
> > + */
> > +#define in_range(val, start, len)					\
> > +	sizeof(start) <= sizeof(u32) ? in_range32(val, start, len) :	\
> > +		in_range64(val, start, len)
> 
> There's nothing here to prevent callers from passing a mixture of
> 32-bit and 64-bit values, possibly resulting in truncation of `val' or
> `len'.
> 
> Obviously caller is being dumb, but I think it's cost-free to check all
> three of the arguments for 64-bitness?
> 
> Or do a min()/max()-style check for consistently typed arguments?

Just use integer promotions to extend everything to 'unsigned long long'.

#define in_range(val, start, len) ((val) + 0ull - (start)) < (len))

If all the values are unsigned 32bit the compiler will discard
all the zero extensions.

If values might be signed types (with non-negative values)
you might want to do explicit ((xxx) + 0u + 0ul + 0ull) to avoid
any potentially expensive sign extensions.

	David

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