From: Christian Groessler <cpg@aladdin.de>
To: gcc@gcc.gnu.org
Subject: signed/unsigned right shift
Date: Fri, 12 Mar 2004 21:25:00 -0000 [thread overview]
Message-ID: <87wu5pwyg7.fsf@aladdin.de> (raw)
Hello,
I have the following program which takes 2 unsigned short values,
multiplies them and shifts the result right.
This right shift gives a value with 1s bits at the high end. I think
it should give 0s there.
---- test program ---
unsigned int bug(long long uu, long long vv)
{
union {
unsigned long long ll;
unsigned short s[4];
} u, v;
u.ll = (unsigned long long)uu;
v.ll = (unsigned long long)vv;
printf("u.s[0] = %04x, v.s[1] = %04x\n", u.s[0], v.s[1]);
return (u.s[0] * v.s[1]) >> 16;
}
int main(void)
{
long long u;
long long v;
u = 0x8f669000ll;
v = 0xe4f06000ll;
printf("%08x\n", bug(u, v));
return 0;
}
---- test program ---
The output is
u.s[0] = 9000, v.s[1] = e4f0
ffff80c7
If I write the last line of the bug function like
return (unsigned)(u.s[0] * v.s[1]) >> 16;
it works like I expected and gives
u.s[0] = 9000, v.s[1] = e4f0
000080c7
I post this here since with 3 different compilers tried only gcc gives
this result.
Sorry if this behaviour is a C feature I shouldn't have asked here,
and if it is, please point me to the according section in the
standard.
regards,
chris
next reply other threads:[~2004-03-12 21:25 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-03-12 21:25 Christian Groessler [this message]
2004-03-12 22:24 ` Andreas Schwab
2004-03-12 22:37 ` Dale Johannesen
2004-03-14 18:09 Christian Groessler
2004-03-14 18:23 ` Ian Lance Taylor
2004-03-15 22:02 Christian Groessler
2004-03-15 22:14 ` Andreas Schwab
2004-03-15 22:18 ` Andreas Schwab
2004-03-15 22:17 ` Ian Lance Taylor
2004-03-15 22:24 Christian Groessler
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