[RFC PATCH] ARM: fiq: Refactor {get,set}_fiq_regs() for Thumb-2

[Dave Martin]

On Thu, Apr 7, 2011 at 11:28 PM, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> On Thu, Apr 07, 2011 at 11:02:18AM +0100, Dave Martin wrote:
>> The main reason for that is to code the ->ARM_r8 in C (as in the
>> existing code).  I feel safer doing that than hard-coding an offset in
>> the assembler, though possibly that is overkill since if the layout of
>> struct pt_regs changes we are probably in trouble for all kinds of
>> other reasons anyway...
>
> It may be worth looking up exactly what's allowed with naked functions.
> A naked function tells the compiler to omit the function prologue and
> epologue, which effectively means you can't do very much other than
> inline asm in them.  So I think we're pretty safe from register
> allocation issues.
>
> If the compiler was to do register allocation for &regs->ARM_r8, and it
> landed up in r8, then that would not only break the code, but also break
> the ABI as the compiler would be unable to save the value of r8.
>
>> I could have used register variables with explicit register
>> assignments.  Alternatively, I could have added r8-r14 to the clobber
>> list -- but then the compiler would generate unnecessary save and
>> restore sequences for all those registers.
>
> I doubt it would for a naked function.  You're expected to handle
> that stuff yourself with such things.
>

It looks like your instinct is correct -- if I add those extra
registers to the clobber list, I get no save/resrote sequences, as you
suggest... but...