netdev - Re: [PATCH iproute2] tc: fix warning in q

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Date:   Wed, 27 Nov 2019 09:25:37 -0800
From:   Brian Vazquez <brianvv@...gle.com>
To:     Maciej Żenczykowski <maze@...gle.com>
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        Brian Vazquez <brianvv.kernel@...il.com>,
        David Ahern <dsahern@...il.com>,
        Mahesh Bandewar <maheshb@...gle.com>,
        Linux NetDev <netdev@...r.kernel.org>,
        Leslie Monis <lesliemonis@...il.com>
Subject: Re: [PATCH iproute2] tc: fix warning in q_pie.c

On Wed, Nov 27, 2019 at 8:44 AM Maciej Żenczykowski <maze@...gle.com> wrote:
>
> > What compiler is this?
> > The type seems correct already.  The type of double / unsigned long is double.
> > And the conversion may give different answer.

I don't think this conversion will give a different answer, the
compiler already change the value from UINT64_MAX to 'UINT64_MAX + 1'
which is pow of 2 and can be represented precisely in a double.  This
change is just making that conversion explicit to avoid the warning.

>
> Probably some recent version of clang with -Wall.

It's clang 10

>
> That said, I think the warning/error is correct.
> UINT64 doesn't fit in double (which is also 64 bits, but includes sign
> and exponent) - you lose ~13 bits of precision.
> I'm not aware of a way to (natively) divide a double by a uint64
> without the loss (not that it really matters since the double doesn't
> have the requisite precision in the first place).
>
> Why do you think the conversion will give a different answer?
> Isn't this exactly what the compiler will do anyway?
> It's not like we have long double anymore...