Path: utzoo!attcan!uunet!contex!chris
From: chris@contex.UUCP (Chris Anderson)
Newsgroups: comp.unix.ultrix
Subject: Warning! Compiler Bug (DecSt.3100)
Summary: Unsigned compares + invalid optimization
Keywords: XNS
Message-ID: <473@contex.UUCP>
Date: 21 Nov 89 19:42:20 GMT
References: <458@tron.UUCP>
Organization: Xyvision Design Systems, Wakefield MA
Lines: 16

The following program, when compiled on the DecStation 3100,
produces d=0, when it should be d=1:

    unsigned a = 2, d;
    int b = 1, c = 1;

    d = (c >= (b - a + 1));

What happens is that the whole expression is promoted to unsigned as expected,
but then the comparison is changed to ">" and the +1 operation is
"optimized" out. (Note, this happens even without -O). Voila! Bad code.

Beware of unsigned IFs with +1 in them somewhere...

Chris Anderson     (contex!chris@uunet.uu.net)
Xyvision, Inc.