linux-kernel - Re: [PATCH] ext4: simplify 32bit calculation of lblk

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Message-ID: <CA+tQmHAwRNxvH+BBA60BdSoVYxK+NzzNyP6TW2Y_gsaWAhu9iQ@mail.gmail.com>
Date:   Wed, 5 Apr 2023 20:47:47 +0800
From:   chi wu <wuchi.zero@...il.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        "tytso@....edu" <tytso@....edu>,
        "adilger.kernel@...ger.ca" <adilger.kernel@...ger.ca>,
        "ojaswin@...ux.ibm.com" <ojaswin@...ux.ibm.com>,
        "ritesh.list@...il.com" <ritesh.list@...il.com>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ext4: simplify 32bit calculation of lblk

David Laight <David.Laight@...lab.com> 于2023年4月5日周三 19:43写道：
>
> From: chi wu
> > Sent: 05 April 2023 09:48
> >
> > Christoph Hellwig <hch@...radead.org> 于2023年4月5日周三 13:40写道：
> > >
> > > On Mon, Apr 03, 2023 at 09:53:04PM +0800, wuchi wrote:
> > > > -                     if (block > ext_block)
> > > > -                             return ext_pblk + (block - ext_block);
> > > > -                     else
> > > > -                             return ext_pblk - (ext_block - block);
> > > > +                     return ext_pblk + ((signed long long)block - (signed long long)ext_block);
> > >
> > > And what exactly is the value add here, except for turning an easy
> > > to parse statement into a complex expression using casts?
> > >
> > Yes，it will be more complex. the original intention is to reduce the
> > conditional branch.
>
> What is wrong with just:
>         return ext_pblk + block - ext_block;
> (64bit + 32bit - 32bit)
>
oh, It's my fault. I am trapped in that ext_pblk + block may overflow,
but it is ok here. thanks.
>         David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
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