Proposed doc update for Explicit Reg Vars 3/3

[David Wohlferd]

Patch 3/3 is the update for the Local Register Variables page (attached).

This patch starts with a question.  Looking at bug 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64951 (register variable 
with template function) is this a bug that will be fixed? Or a 
limitation that should be doc'ed?  Both the current docs and the patch 
ignore this bug.

As with patch #2, this is primarily about reformatting/reorganizing.  
Although it also adds the limitation from asm labels re statics.

I was hoping to modify the text to say that local register variables can 
"only" be used to call Extended asm.  This would greatly simplify this 
section.  But there has been pushback on this (despite the fact that no 
one has really suggested any other reasonable use).  So I have softened 
this, and listed things from the existing docs that are explicitly not 
supported.

For people who find the HTML easier to review:

Here's the current text: 
https://gcc.gnu.org/onlinedocs/gcc/Local-Reg-Vars.html
And here's the new: 
http://limegreensocks.com/gcc/Local-Register-Variables.html

dw


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