linux-kernel - RE: [PATCH 2/8] lib/lzo: clean-up by introducing COPY16

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Message-ID: <ccb719dd52354cfab8def75689d675af@AcuMS.aculab.com>
Date:   Fri, 30 Nov 2018 12:27:58 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Dave Rodgman' <dave.rodgman@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
CC:     "herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
        "davem@...emloft.net" <davem@...emloft.net>,
        Matt Sealey <Matt.Sealey@....com>,
        "nitingupta910@...il.com" <nitingupta910@...il.com>,
        "markus@...rhumer.com" <markus@...rhumer.com>,
        "minchan@...nel.org" <minchan@...nel.org>,
        "sergey.senozhatsky.work@...il.com" 
        <sergey.senozhatsky.work@...il.com>,
        "sonnyrao@...gle.com" <sonnyrao@...gle.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        nd <nd@....com>
Subject: RE: [PATCH 2/8] lib/lzo: clean-up by introducing COPY16

From: Dave Rodgman
> Sent: 30 November 2018 11:48
> From: Matt Sealey <matt.sealey@....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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