Proposed doc update for Explicit Reg Vars 2/3

[Jeff Law]

On 10/20/2015 03:12 AM, Segher Boessenkool wrote:
> On Mon, Oct 19, 2015 at 11:14:30PM -0600, Jeff Law wrote:
>>>> +All global register variable declarations must precede all function
>>>> +definitions.  If such a declaration appears after function definitions,
>>>> +the declaration would be too late to prevent the register from being used
>>>> +for other purposes in the preceding functions.
>>>
>>> This isn't true anymore, not even with -fno-toplevel-reorder or -O0.
>> I've asked David to verify this.  However, if you have, then he doesn't
>> need to re-confirm.
>
> - - -
> int f(int a, int b) { int t = a + b; return t*t + a + b; }
> register int nope asm("9");
> - - -
>
> on PowerPC (GPR9 is the first one allocated, adjust for other archs).
Cool thanks.  I figured this would be the case due to the nature of the 
IPA framework.

jeff