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Date: Wed, 12 Mar 2003 01:55:38 +0000
From: Jamie Lokier <jamie@shareable.org>
To: Andrew Morton <akpm@digeo.com>
Cc: Linus Torvalds <torvalds@transmeta.com>, george@mvista.com,
       felipe_alfaro@linuxmail.org, cobra@compuserve.com,
       linux-kernel@vger.kernel.org
Subject: Re: Runaway cron task on 2.5.63/4 bk?
Message-ID: <20030312015538.GA17149@bjl1.jlokier.co.uk>
References: <20030311144448.4d9ee416.akpm@digeo.com> <Pine.LNX.4.44.0303111458390.1709-100000@home.transmeta.com> <20030311150913.20ddb760.akpm@digeo.com>
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Andrew Morton wrote:
> > However, gcc is unable to do-the-right-thing and generate 32x32->64 
> > multiplies, or 32x64->64 multiplies, even though those are both a _lot_ 
> > faster than the full 64x64->64 case.
> 
> 2.95.3 and 3.2.1 seem to do it right?
> 
> long a;
> long b;
> long long c;
> 
> void foo(void)
> {
> 	c = a * b;
> }

Your code is wrong for this test.  It does a 32x32->32 multiply, and
then sign extends the result to 64 bits.

The correct test has "c = (long long) a * b;".

-- Jamie

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