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Re: Optimization of offset computations

To: Bernd Schmidt <bernds at pathia dot cygnus dot co dot uk>
Subject: Re: Optimization of offset computations
From: Richard Henderson <rth at cygnus dot com>
Date: Mon, 29 Nov 1999 19:56:53 -0800
Cc: Richard Kenner <kenner at vlsi1 dot ultra dot nyu dot edu>, gcc-patches at gcc dot gnu dot org
References: <9911271403.AA23120@vlsi1.ultra.nyu.edu> <Pine.LNX.4.04.9911291633370.29329-100000@pathia.cygnus.co.uk>

On Mon, Nov 29, 1999 at 04:35:54PM +0000, Bernd Schmidt wrote:
> C standard question: is it legal to optimize
> 
>     unsigned int a = 65536;
>     unsigned int b;
> 
>     {
>          b = (a * 65536) / 8;
>     }
> 
> to
> 
>     unsigned int a = 65536;
>     unsigned int b;
> 
>     {
>          b = (a * 8192);
>     }

No.  Unsigned arithmetic has well-defined overflow.


r~

References:
- Optimization of offset computations
  - From: Richard Kenner
- Re: Optimization of offset computations
  - From: Bernd Schmidt

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