Re: [PATCH v2 07/11] math64: New DIV_U64_ROUND_CLOSEST helper

[Geert Uytterhoeven]

Hi Pali,

On Fri, Jun 25, 2021 at 7:44 PM Pali Rohár <pali@xxxxxxxxxx> wrote:
> On Friday 25 June 2021 19:39:10 Geert Uytterhoeven wrote:
> > On Fri, Jun 25, 2021 at 5:50 PM Willy Tarreau <w@xxxxxx> wrote:
> > > On Fri, Jun 25, 2021 at 05:38:03PM +0200, Pali Rohár wrote:
> > > > On Friday 25 June 2021 17:22:31 Geert Uytterhoeven wrote:
> > > > > > +/*
> > > > > > + * DIV_U64_ROUND_CLOSEST - unsigned 64bit divide with 32bit divisor rounded to nearest integer
> > > > > > + * @dividend: unsigned 64bit dividend
> > > > > > + * @divisor: unsigned 32bit divisor
> > > > > > + *
> > > > > > + * Divide unsigned 64bit dividend by unsigned 32bit divisor
> > > > > > + * and round to closest integer.
> > > > > > + *
> > > > > > + * Return: dividend / divisor rounded to nearest integer
> > > > > > + */
> > > > > > +#define DIV_U64_ROUND_CLOSEST(dividend, divisor)       \
> > > > > > +       ({ u32 _tmp = (divisor); div_u64((u64)(dividend) + _tmp / 2, _tmp); })
> > > > >
> > > > > Given "dividend" should already be an unsigned 64-bit value, I don't
> > > > > think the cast to "u64" is needed. Similar macros in this file also
> > > > > don't have the cast.
> > > >
> > > > It is just to ensure that plus operation between dividend and _tmp is
> > > > evaluated in 64-bit context to prevent overflow. Just a case when user
> > > > calls this macro with 32-bit dividend param. As it is a macro (and not
> > > > inline function) type is not automatically enforced.
> > >
> > > I agree, a large u32 argument added to _tmp/2 could overflow and remain
> > > 32 bits, yielding an incorrect result. The cast is mandatory here (and
> > > will either emit no code, or be useful).
> >
> > Fair enough.
> > So we want to add a cast to DIV64_U64_ROUND_CLOSEST() above, too?
>
> For DIV64_U64_ROUND_CLOSEST() it is not needed. divisor is copied into
> u64 _tmp variable and therefore "(dividend) + _tmp / 2" is already
> evaluated in 64-bit context even when dividend is only 32-bit.

Thanks, I stand corrected.  Time to enter weekend mode...

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds