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From: Nagesh_Pabbisetty.Henr801C@xerox.COM
Newsgroups: comp.windows.x
Subject: Re: right shifting negative numbers considered harmful
Message-ID: <"24-Jan-91..9:49:57.EST".*.Nagesh_Pabbisetty.Henr801C@Xerox.com>
Date: 24 Jan 91 14:49:57 GMT
Sender: daemon@athena.mit.edu (Mr Background)
Organization: The Internet
Lines: 45



      Dear X server hackers,
      
      My copy of the ANSI C spec says that when right shifting E1 >> E2, "If
      E1 has a signed type, whether the right shift is logical (fill with
      zeros) or arithmetic (fill with a copy of the sign bit) is
      implementation-defined." (section 3.3.7)
      
      Consider the following fragment of code from the MIARCOCTANTSHIFT macro
      in mizerarc.h  
      
      ...
		  d = b - (a >> 1) - d + (k3 >> 3); \
      ...
      
      this is called out from the cfb arc drawing routines.  Note that
      variable a can be negative.
      
      I have the misfortune to be running on an architecture without an
      arithmetic right shift instruction, and so my compiler has exercised
      it's "implementation-defined" option.  
      
      In the interests of portability, can someone arrange to fix this
      apparent ANSI non-compliance in the standard distribution?  I'm using
      R4.  Perhaps it is already fixed in R5.  Does anybody know?
      
      many thanks.
      
      Andy Lowe
      andy@nth.com


I am glad(;-) to see that I am not the only one who feels that way. 
I got bitten by this in the arc drawing code. 
It took us quite a while to locate and change right shifts to divides (:-(


- nagesh (pabbiset@arisia.xerox.com)
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