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

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Message-ID: <CANP3RGctgy98FsyeHq+aVk2S=N8ndY0Y+qMkZUhTB=26H_Y3Rg@mail.gmail.com>
Date:   Wed, 27 Nov 2019 08:44:46 -0800
From:   Maciej Żenczykowski <maze@...gle.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     Brian Vazquez <brianvv@...gle.com>,
        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

> What compiler is this?
> The type seems correct already.  The type of double / unsigned long is double.
> And the conversion may give different answer.

Probably some recent version of clang with -Wall.

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...