Path: utzoo!utgpu!utstat!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!bloom-beacon!mcgill-vision!mouse
From: mouse@mcgill-vision.UUCP (der Mouse)
Newsgroups: comp.lang.c
Subject: Re: Yet another pcc botch
Keywords: Compiler bugs
Message-ID: <1415@mcgill-vision.UUCP>
Date: 1 Feb 89 08:44:39 GMT
References: <3310@cbnews.ATT.COM>
Organization: McGill University, Montreal
Lines: 27

In article <3310@cbnews.ATT.COM>, lvc@cbnews.ATT.COM (Lawrence V. Cipriani) writes:
> This program demonstrates a bug that is in some versions of pcc:

> main() { float f,g; unsigned short u,v; short s,t;
>          u = 250;    s = 250;
>          v = 100;    t = 100;
>          f = t - s;  g = v - u;
>          printf("f = %f, g = %f\n", f, g);       }
[code compressed -dM]

> When compiled and executed the output is:
> f = -150.000000, g = 65386.000000
> The correct value for g is -150.000000.

That depends on the promotion rules in use.  Using the "unsignedness
preserving" rules, the correct value for g is MAXUINT-149.  Using the
"value preserving" rules, it depends again: if short is the same size
as int, the correct result is still MAXUINT-149; if short is smaller
than int, the correct result is -150.  Your result looks like
MAXUINT-149 on a machine with 16-bit ints, of course - or a buggy
compiler.  This may in fact indicate a bug in your compiler, but you
haven't given enough information to convince me of it.

					der Mouse

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