Re: Floating point in kernel

From: Abramo Bagnara (abramo@alsa-project.org)
Date: Thu May 04 2000 - 15:27:53 EDT

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    "Richard B. Johnson" wrote:
    >
    > On Wed, 3 May 2000, Jeff Garzik wrote:
    >
    > > "B. James Phillippe" wrote:
    > > >
    > > > Hello,
    > > >
    > > > I know this is not legal. However, I'm reviewing some kernel module code
    > > > which basically does:
    > > >
    > > > unsigned long foo = some_other_ulong * 1.234;
    > >
    > > oof :) Reminds me of some fbdev code I saw a while back IIRC
    > >
    > > When you really want fractional numbers not literally floating point,
    > > you can always multiple both numbers to make them decimal, perform the
    > > math op, and then divide back down again.
    > >
    > > Jeff
    > >
    >
    > Correct. In the cited case, the code could read:
    >
    > foo = some_other_ulong * 1234 / 1000;
    >
    > The complier sometimes optimizes the wrong stuff so 1234/1000 might
    > get optimized to 1 which is not what you want. Therefore, you can
    > force the issue by doing:
    >
    > foo = some_other_ulong * 1234;
    > ^__ sequence point
    > foo /= 1000;
    >

    As far as I know this is not needed at all.
    * and / have the same precedence and associativity is specified from
    left to right so this is interpreted as:

    foo = (some_other_ulong * 1234) / 1000

    It's not guaranted that some_other_ulong * 1234 is evaluated before 1000
    but we are sure that multiplication is done before division.

    If I'm wrong I'd like to know ASAP as I've written bogus C code for 15
    years now ;-)) 

    -- 
Abramo Bagnara                       mailto:abramo@alsa-project.org

    Opera Unica
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    It sounds good!

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