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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Dave Rodgman' <dave.rodgman@arm.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
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CC:     "herbert@gondor.apana.org.au" <herbert@gondor.apana.org.au>,
        "davem@davemloft.net" <davem@davemloft.net>,
        Matt Sealey <Matt.Sealey@arm.com>,
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        "markus@oberhumer.com" <markus@oberhumer.com>,
        "minchan@kernel.org" <minchan@kernel.org>,
        "sergey.senozhatsky.work@gmail.com" 
        <sergey.senozhatsky.work@gmail.com>,
        "sonnyrao@google.com" <sonnyrao@google.com>,
        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        nd <nd@arm.com>
Subject: RE: [PATCH 2/8] lib/lzo: clean-up by introducing COPY16
Thread-Topic: [PATCH 2/8] lib/lzo: clean-up by introducing COPY16
Thread-Index: AQHUiKKcsNRVxoInU0ado7SP4WwMWKVoPj+w
Date:   Fri, 30 Nov 2018 12:27:58 +0000
Message-ID: <ccb719dd52354cfab8def75689d675af@AcuMS.aculab.com>
References: <20181130114715.27523-1-dave.rodgman@arm.com>
 <20181130114715.27523-3-dave.rodgman@arm.com>
In-Reply-To: <20181130114715.27523-3-dave.rodgman@arm.com>
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From: Dave Rodgman
> Sent: 30 November 2018 11:48
> From: Matt Sealey <matt.sealey@arm.com>
> 
> Most compilers should be able to merge adjacent loads/stores of sizes
> which are less than but effect a multiple of a machine word size (in
> effect a memcpy() of a constant amount). However the semantics of the
> macro are that it just does the copy, the pointer increment is in the
> code, hence we see
> 
>     *a = *b
>     a += 8
>     b += 8
>     *a = *b
>     a += 8
>     b += 8
> 
> This introduces a dependency between the two groups of statements which
> seems to defeat said compiler optimizers and generate some very strange
> sequences of addition and subtraction of address offsets (i.e. it is
> overcomplicated).
> 
> Since COPY8 is only ever used to copy amounts of 16 bytes (in pairs),
> just define COPY16 as COPY8,COPY8. We leave the definition to preserve
> the need to do unaligned accesses to machine-sized words per the
> original code intent, we just don't use it in the code proper.
> 
> COPY16 then gives us code like:
> 
>     *a = *b
>     *(a+8) = *(b+8)
>     a += 16
>     b += 16

You probably actually want:
	t1 = *b;
	t2 = *(b+8);
	*a = t1;
	*(a+8) = t2;
	a += 16;
	b += 16;

	David

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