lkml.org 
[lkml]   [2014]   [Jun]   [17]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [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. :)

--
FTTC broadband for 0.8mile line: now at 9.7Mbps down 460kbps up... slowly
improving, and getting towards what was expected from it.


\
 
 \ /
  Last update: 2014-06-17 16:21    [W:0.065 / U:0.332 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site