From: night DOT walker AT tiscalinet DOT it
Message-ID: <3922AD5F.EB45FD4D@tiscalinet.it>
Date: Wed, 17 May 2000 16:32:00 +0200
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To: DJ Delorie <djgpp AT delorie DOT com>
Subject: Re: Re: Right shift
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Reply-To: djgpp AT delorie DOT com


>night DOT walker AT tiscalinet DOT it wrote:
>> Why right-shifting ">>" a negative signed int will first turn it
>> positive?

>Wrong question, sort of.
No. I wanted just to understand how does >> really works, so i used a
simple number.
Next time i will use 0x04, or 0000 0100, but would be the same thing...

>The behaviour of bitshift operations on negative integers is
>purposefully left implementation-defined by the C language definition.
>For short, that means you should not assume anything about the way it
>works.
>Bitshifting never was meant to be used for numbers (integers of
>any kind), really, it's for collections of bits.
>If you wanted / 2, write /2, not >>1. Trust your compiler to
>automatically translate that as >>1 if that's a safe and fast method
>for the case at hand. If you think you cannot trust your compiler even
>that far: why aren't you using a better one?

Yes, i know the platform-sensitivity of bit-shifting, but, in the case,
i'm working just on a little demo prog. Changing some divisions with
right-shifting DOES improve the speed (from 3/4 fps to 4/5 fps on my
P90) with no errors.
I will be glad to upload you the src (needs ALLEGRO lib).

>It's better for your programming skills if you do not know how this is
>implemented, so you don't make any silent assumption that will break
>if your program ever gets moved to another compiler, operating system
>or machine.

My prog works perfectly, but, as i do not make any silent assumpion, i
was wondering how the da mn thing works, so i made a little test prog...

About my prog skill: if i must improve (and i must) let me learn.

>Ignorance can protect you, sometimes.
I prefer knowledge (do you like aphorisms?)

NW