Proposed doc update for Explicit Reg Vars 2/3

[Jeff Law]

On 10/12/2015 05:07 PM, David Wohlferd wrote:
>
> - There are unsourced, unsubstantiated reports that on some platforms,
> certain things might or might not work.
Back when this was written (circa 1996), this would have been considered 
helpful information -- having a system routine clobber a register 
unexpectedly actually did happen.  Been there, done the debugging, want 
to forget.

Since then, system software as gotten better.  And the details of what 
may have failed on some version of SunOS/Solaris or on the 3b1 just 
aren't that important anymore.  So I don't mind losing that information.

> - Eventually the compiler may work differently than it does now.
That is helpful.  It's a way signaling that things may change and that 
depending on the precise syntax and semantics may be unwise.

 From time to time, particularly with GCC extensions, it has been 
necessary to declare certain usage as invalid or to change the 
implementation is significantly visible ways.

We never like doing that, but it is sometimes unavoidable.  As a general 
rule, the more an extension exposes how GCC works internally, the more 
likely it has been to need significant changes over time.  (asms being 
the easiest example to cite).

So please consider keeping something which signals the semantics might 
change.

>
> As for the review, perhaps reading the html is easier than reading the
> patch
:-0  I think reviewing the HTML is easier.  Thanks for providing a link.

So for the example, "a5" is a particularly bad choice these days on the 
m68k as it's the PIC register.  It may be advisable to just use r<N> for 
some value of N and try to be processor agnostic here.

Segher's comments are valuable as well.  I think if you integrate his 
and mine the result ought to be ready to commit.

jeff