Proposed doc update for Explicit Reg Vars 2/3

[Jeff Law]

On 10/19/2015 03:42 PM, Segher Boessenkool wrote:
>> +After defining a global register variable, for the duration of
>> +the current compilation:
>
> It's probably better to say "for the current compilation unit"?  There now
> is LTO and whatnot.
Which raises the question, what happens for LTO?  Do we stream out 
enough information to know about global register variables and if we do, 
then its use as a global register variable implicitly covers anything 
that's visible at LTO time, right?


>
>> +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.

Jeff