inline asm and multi-alternative constraints

[David Wohlferd]

On 11/6/2015 4:46 PM, Segher Boessenkool wrote:
> On Fri, Nov 06, 2015 at 03:29:43PM -0700, Jeff Law wrote:
>> It's never easy to predict whether or not something like this will be
>> contentious.  Worst case is you post, it's contentious, we iterate a bit
>> and reach some kind of resolution (ok, worst case is no resolution is
>> reached, but that doesn't happen to often).
>>
>> In this case I simply don't see a way to sensibly document those
>> modifiers without bringing in the implementation details of register
>> class preferencing, reload, IRA & LRA.  And once those details are
>> brought into the picture, everyone loses.

This very point is what made it clear to me that these flags should be 
removed.  If there is no practical way to describe to the target 
audience how they work or when to use them, they don't belong here.

I'll take partial credit for asking the questions that highlighted the 
problem, but Jeff taking the time to respond to them is what got us to 
the right answer here (thanks Jeff!).

>> I'm sure there's someone out there using '?' and '!' in a
>> multi-alternative asm constraint.  They may even read the docs and
>> complain and we can try to educate them why those modifiers are no
>> longer documented.
> Another reason why we shouldn't document such things is that it makes
> it harder to change (anything about) those things later, although they
> really are implementation details.

True.  In this case, they were already documented and the change was to 
remove them, something I hesitate to do.  But I think what Jeff checked 
in (thanks Jeff!) gives us the right answer for this page.

> The same goes for some constraints and almost all output modifiers.

Are you suggesting more doc changes?  Looking thru the pages you reference:

- Starting with 'modifiers', "=+&" and (reluctantly) "%" seem reasonable 
for inline asm.  But both "#*" seem sketchy.

- Under 'simple constraints', "mringX" all (more or less) make sense to 
me.  But "oV<>sp" are not things I can envision using.

- The 'machine constraints' for i386 (the only machine I know) all seem 
reasonable.  However for platforms that support autoincrement 
(powerpc?), apparently using "m" needs more docs (per 
https://gcc.gnu.org/ml/gcc/2008-03/msg01079.html).

Are these the things to which you are referring?  I've always assumed 
the parts that seem obscure here were due to my i386-centric view of the 
world.  Are some of them actually md-only?

There are other minor changes I'd make on some of these pages.  But 
mostly they are not worth it unless I'm doing something else there too.  
So if there's something here you think needs changing, let me know and 
I'll take a crack at it.

Other than that, I'll keep working my way thru the doc issues in the 
inline-asm bugs.  I've done what I can for 10396.

dw