Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!bloom-beacon!mit-eddie!uw-beaver!apollo!mrst!sdti!turner
From: turner@sdti.UUCP (Prescott K. Turner)
Newsgroups: comp.lang.c
Subject: Re: no noalias not negligible
Summary: test idea not standard
Message-ID: <275@sdti.UUCP>
Date: 27 May 88 03:59:00 GMT
References: <54080@sun.uucp> <1988May21.030207.25063@light.uucp>
Reply-To: turner@sdti.UUCP (Prescott K. Turner, Jr.)
Organization: Software Development Technologies, Sudbury MA
Lines: 21

In article <1988May21.030207.25063@light.uucp> bvs@light.UUCP (Bakul Shah) writes:
>In article <54080@sun.uucp> dgh%dgh@Sun.COM (David Hough) writes:
>>Anyway there is no portable way in draft ANSI C to say "this pointers are
>>guaranteed to have no aliases".
>
>How about adding a test before the for loop?  Something like:
>#define overlap(x,y,n)    (!(x + n <= y || y + n <= x))
>       if (overlap(dx, dy, n))
>               return complain("overlapping arrays\n");
>
>Now a smart compiler can figure out that dx, dy don't overlap ...

This test make non-standard assumptions.  In draft ANSI C,
those pointer comparisons are undefined (meaning your program might
crash and burn) unless "x" and "y" point within the same object.
Anyone know a machine where this could be experienced?
--
Prescott K. Turner, Jr.
Software Development Technologies, Inc.
375 Dutton Rd., Sudbury, MA 01776 USA        (617) 443-5779
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