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

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

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.