Re: [Xen-devel] [PATCH] ARM: convert all "mov.* pc, reg" to "bx reg" for ARMv6+

[Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>]

On Tue, Jul 01, 2014 at 04:58:24PM +0100, Russell King wrote:
> ARMv6 and greater introduced a new instruction ("bx") which can be used
> to return from function calls.  Recent CPUs perform better when the
> "bx lr" instruction is used rather than the "mov pc, lr" instruction,
> and this sequence is strongly recommended to be used by the ARM
> architecture manual (section A.4.1.1).
> 
> We provide a new macro "ret" with all its variants for the condition
> code which will resolve to the appropriate instruction.
> 
> Rather than doing this piecemeal, and miss some instances, change all
> the "mov pc" instances to use the new macro, with the exception of
> the "movs" instruction and the kprobes code.  This allows us to detect
> the "mov pc, lr" case and fix it up - and also gives us the possibility
> of deploying this for other registers depending on the CPU selection.
> 
> Signed-off-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxxx>
Works fine on my ARM-v7M machine:

$ objdump -d vmlinux | grep -E 'mov.*pc'
8c000022:       46e7            mov     pc, ip
8c000028:       46ef            mov     pc, sp
8c000b26:       46af            movne   pc, r5
8c0037e0:       4687            mov     pc, r0
8c0828d2:       4697            mov     pc, r2
8c1213ee:       469f            mov     pc, r3

Also boots fine.

Tested-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>

but I wonder what is the reason to not use bx if \reg != lr.

Thanks
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |

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