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Re: A question about combining constraints
On 12/11/2010 15:32, Joern Rennecke wrote:
> Quoting Dave Korn <dave.korn.cygwin@gmail.com>:
>
>> By "combine constraints", you mean "Omit all the commas between
>> alternatives, mash them together in a single string, and expect GCC
>> to permute
>> all the possible combinations"?
>>
>> I didn't know that was possible at all. I thought GCC would
>> interpret those
>> as multi-letter constraint names.
>
> Actually, you need to add one or more register constraints to input
> memory constraints to enable optional input reload generation.
Is that documented anywhere? I couldn't find it in the constraints chapters
of the internals manual.
> And there are subtle register priority implications of having merged or
> separate alternatives.
Sorry, is this separate from the thing about appending register constraints
to memory ones or part of it? Again, no documentation is apparent that
suggests or mentions such a concept as "merged" alternatives.
cheers,
DaveK