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Re: inline asm and multi-alternative constraints


On Fri, Nov 06, 2015 at 11:50:40PM -0800, David Wohlferd wrote:
> >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.

Output modifiers, not constraint modifiers -- things like "%X0" in
the output template.  Many are only useful in the machine description,
but some (like that 'X' for rs6000) are vital for asm as well.

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

Some are more useful on some targets than on others, sure.  But these
generic constraints are extremely unlikely to ever get a change; this
is not true for target constraints.  If we want to delete (or change
semantics of) some target constraint, and that constraint is documented,
we have to fear the ire of the one or two users of that constraint.

> - 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).

I think the manual (the "Simple Constraints" section) does describe it
all, but it could be clearer.  The PowerPC 'm' description does a good
job of explaining things further (for that target).

> 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?

Almost everything can be used in asm as well, but many constraints etc.
are much less useful there.

> 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.

Thanks again :-)


Segher


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