A question about combining constraints

[Joern Rennecke]

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.
And there are subtle register priority implications of having merged or
separate alternatives.