[PATCH v3] ARM: add get_user() support for 8 byte types
Daniel Thompson
daniel.thompson at linaro.org
Tue Jun 17 06:28:44 PDT 2014
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).
Daniel.
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