[DOC Patch] Add sample for @cc constraint

[David Wohlferd]

 > I would like executable code that verifies that this feature is 
indeed working as intended.

First draft is attached.  It tests all 28 (14 conditions plus 14 
inverted).  I wasn't sure what to set for optimization (O2? O3? O0?), so 
I left the default.

It looks like even at O3 there are some missed optimizations here, but 
that's an issue for another day.

 > Is there any _actual_ problem here? Like, if you combine the output 
and the clobber you run into problems? Looks to me like an explicit "cc" 
clobber is just ignored on x86. We just need to make sure this stays 
working (testcases).

Today?  No.  You can clobber or not clobber and both will produce the 
exact same output.

But letting people program this two different ways guarantees that 
people *will* program it both ways.  And just because there isn't any 
definable reason to limit this today doesn't mean that there won't ever 
be.  But by then it will be 'too late' to change it because it "breaks 
existing code."

 >> 1) Leave this text in.
 >> 2) Remove the text and add the compiler check to v6.
 >> 3) Remove the text and add the compiler check to v7.
 >> 4) Leave the text in v6, then in v7: remove the text and add the 
compiler check.
 >> 5) (Reluctantly) remove the text and hope this never becomes a problem.

So, I've made my pitch, but it sounds like you want #5?

 > My question would be, can this information ever be relevant to users? 
They may notice that their code still works if they omit the "cc", but 
that's not really a habit we want to encourage.

People do this now without understanding how or why it works.

 > I think this is an internal implementation detail that doesn't 
necessarily even have to be documented.

One time it would matter is if people want to move from basic asm (which 
doesn't clobber "cc") to any type of extended asm (which always does).  
It /probably/ won't matter in that case (and may even make things 
better).  But it shouldn't be a secret.

dw
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