inline asm and multi-alternative constraints

[Richard Earnshaw]

On 07/11/15 09:23, Segher Boessenkool wrote:
> 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.
> 

They're not just useful, they're essential on AArch64 and ARM.  They're
needed, for example, to get the 32/64-bit register sizing correct.

R.

>> - 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
>