Date: Tue, 4 Oct 2022 21:46:33 +0200 From: Hans Petter Selasky <hselasky@freebsd.org> To: Ravi Pokala <rpokala@freebsd.org>, src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-main@FreeBSD.org Subject: Re: 1024bb26337b - main - qdivrem: Predict division by zero as false. Message-ID: <3d8cc143-23ea-2ae9-8e08-dbc36b18369b@freebsd.org> In-Reply-To: <3D44E023-8220-4B65-89A1-B5B8BB77EBD5@panasas.com> References: <202210041153.294Br2JP011500@gitrepo.freebsd.org> <3D44E023-8220-4B65-89A1-B5B8BB77EBD5@panasas.com>
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On 10/4/22 18:38, Ravi Pokala wrote: > -----Original Message----- > From: <owner-src-committers@freebsd.org> on behalf of Hans Petter Selasky <hselasky@FreeBSD.org> > Date: 2022-10-04, Tuesday at 04:53 > To: <src-committers@FreeBSD.org>, <dev-commits-src-all@FreeBSD.org>, <dev-commits-src-main@FreeBSD.org> > Subject: git: 1024bb26337b - main - qdivrem: Predict division by zero as false. > > The branch main has been updated by hselasky: > > URL: https://cgit.FreeBSD.org/src/commit/?id=1024bb26337bdc6679af477977247e9155d502bc > > commit 1024bb26337bdc6679af477977247e9155d502bc > Author: Hans Petter Selasky <hselasky@FreeBSD.org> > AuthorDate: 2022-10-04 10:28:25 +0000 > Commit: Hans Petter Selasky <hselasky@FreeBSD.org> > CommitDate: 2022-10-04 11:51:06 +0000 > > qdivrem: Predict division by zero as false. > > Division by zero triggers an arithmetic exception and should not be very > common. Predict this. > > Looking at this change with a little more context, I see that (static volatile const unsigned int) 'zero' is set to 0... and then used as the denominator for division. Since these are all integers, I would expect that to cause a fatal divide-by-zero exception. And yet, there's more code after that divide-by-zero. > > What am I missing? > Hi, I know some rare CPU's continue executing after division by zero, but the ones the FreeBSD support doesn't. Maybe it's something we've inherited. I have no good explanation for that extra code. --HPS
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