linux-kernel - Re: [PATCH v3] ARM: add get_user() support for 8 byte types

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Date:	Tue, 17 Jun 2014 14:36:20 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Daniel Thompson <daniel.thompson@...aro.org>
Cc:	Rob Clark <robdclark@...il.com>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Arnd Bergmann <arnd.bergmann@...aro.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	patches@...aro.org, linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH v3] ARM: add get_user() support for 8 byte types

On Tue, Jun 17, 2014 at 02:28:44PM +0100, Daniel Thompson wrote:
> On 17/06/14 12:09, Russell King - ARM Linux wrote:
> > On Tue, Jun 17, 2014 at 11:17:23AM +0100, Daniel Thompson wrote:
> >> ... at this point there is a narrowing cast followed by an implicit
> >> widening. This results in compiler either ignoring r3 altogether or, if
> >> spilling to the stack, generating code to set r3 to zero before doing
> >> the store.
> > 
> > In actual fact, there's very little difference between the two
> > implementations in terms of generated code.
> > 
> > The difference between them is what happens on the 64-bit big endian
> > narrowing case, where we use __get_user_4 with your version.  This
> > adds one additional instruction.
> 
> Good point.
> 
> 
> > and 64-bit narrowed to 32-bit:
> > 
> >         str     lr, [sp, #-4]!
> > -       mov     ip, r0
> > +       mov     r3, r0
> >         mov     r0, r1
> >  #APP
> >  @ 275 "t-getuser.c" 1
> > -       bl      __get_user_8
> > +       bl      __get_user_4
> >  @ 0 "" 2
> > -       str     r2, [ip, #0]
> > +       str     r2, [r3, #0]
> >         ldr     pc, [sp], #4
> 
> The later case avoids allocating r3 for the __get_user_x and should
> reduce register pressure and, potentially, saves a few instructions
> elsewhere (one of my rather large test functions does demonstrate this
> effect).
> 
> I don't know if we care about that. If we do I'm certainly happy to put
> a patch together than exploits this (whilst avoiding the add in the big
> endian case).

No need - the + case is your version, the - case is my version.  So your
version wins on this point. :)

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