Re: [PATCH v3] ARM: add get_user() support for 8 byte types
From: Russell King - ARM Linux
Date: Tue Jun 17 2014 - 09:36:36 EST
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.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/