Re: AVR indirect_jump addresses limited to 16 bits

[Paul Schlie <schlie at comcast dot net>]

> From: Paul Schlie <schlie@comcast.net>
>> From: Marek Michalkiewicz <marekm@amelek.gda.pl>
>> Good question - I can't.  On the other hand, the manual says:
>> 
>> `indirect_jump'
>> An instruction to jump to an address which is operand zero.  *This pattern
>> name is mandatory on all machines.*
>> 
>> Why would it be mandatory if it was not truly needed?  If the manual is
>> correct, it seems this pattern is truly needed (not just an optional
>> optimization like some other patterns).
>> 
>> If it is impossible on the AVR, it could be implemented with invalid
>> assembler output (so we get an error if "impossible" ever happens).
>> But I'd like to be sure if this is really the case.  GCC is not only
>> a C compiler, perhaps indirect_jump is needed for some other language?
> 
> - I believe it's simply a vehicle to allow the target describe how to
>   jump indirectly to an address which may be required if the compiler
>   chooses to generate a static jump table mapped into presumably the
>   program's "text" section, therefore would guess the right thing to do
>   would be to (assuming operand-0 is a progmem reference) load from
>   progmem 2-words for 256K devices (or 1 word otherwise) into the
>   appropriate registers, then executes an extended jump instruction
>   (or regular jump otherwise).
> 
>   (as I'd hope the compiler would never map static jump tables that it
>    chooses to generate into the "data" section, but can't find any
>    description of under what circumstances it may generate/put them?)
> 
> (again, just my guess)

- or possibly GCC may try to be clever by jumping to no-return attribute
  function calls?