Path: utzoo!utgpu!cunews!bnrgate!brchh104!brchs1!bnr.ca!rice.edu!sun-spots-request
From: luke@research.canon.oz.au (Luke Kendall)
Newsgroups: comp.sys.sun
Subject: Bug in C compiler(s)
Keywords: No Digest Subjects during Flush
Message-ID: <4115@brchh104.bnr.ca>
Date: 27 Jun 91 20:22:00 GMT
Sender: news@brchh104.bnr.ca
Organization: Sunspots, Flush Mode
Lines: 41
Approved: sun-spots@rice.edu
X-Original-Date:    Wed, 19 Jun 1991 02:57:25 GMT

A bug in the Sun and GNU C compilers.  The following tiny program does
not work:

main()
{
    int		i, shift;

    printf("%x\n", 0x10000 >> 1000);

    for (shift = 1, i = 0; i < 5; ++i, shift *= 10)
	printf("%x >> %d = %x\n", 0x10000, shift, 0x10000 >> shift);
}

The output of the program from the Sun C compiler (for a SPARC, SunOS 4.1):

100
10000 >> 1 = 8000
10000 >> 10 = 40
10000 >> 100 = 1000
10000 >> 1000 = 100
10000 >> 10000 = 1

The output of the program from the GNU C compiler (version 1.40):

0
10000 >> 1 = 8000
10000 >> 10 = 40
10000 >> 100 = 1000
10000 >> 1000 = 100
10000 >> 10000 = 1

Looking into the assembly code for the SPARC, it uses an `sra'
instruction (shift-right-arithmetic).  Unfortunately, this instruction
is documented as only working for numbers with the bottom 5 bits set.
In other words, both compilers are failing to mask the right operand
of the shift.
-- 
Luke Kendall, Senior Software Engineer.      | Net:   luke@research.canon.oz.au
Canon Information Systems Research Australia | Phone: +61 2 805 2914
P.O. Box 313 North Ryde, NSW, Australia 2113 | Fax:   +61 2 805 2929