[gfortran] Fix NULL reference types.

[Mark Mitchell]

Daniel Jacobowitz wrote:

>>RTH's complaint about Fortran and debugging is interesting.  Although, I 
>>don't think Fortran has a notion of "reference type" in the same way 
>>that C++ does.  It might be that DWARF 2/3 already provides some way of 
>>describing optional arguments that does not rely on their type being 
>>REFERENCE_TYPE.  It might also be that we could adjust the 
>>debug-generators to generate reference types even for optional arguments.
>>    
>>
>
>The DWARF2 we output really should describe them as being reference
>types.  The debugger needs to know to pass pointers instead of values,
>and to display values instead of pointers; at the debug info level,
>that's what the distinction between a pointer and a reference is for.
>  
>
OK.

>I've got no opinion on whether that should be a tree type or a flag on
>a pointer type.  It seems that keeping the distinction to the tree
>optimizers is not useful, aside from the possible non-NULL flag.
>  
>
It sounds like maybe a flag on never-NULL DECLs is the way to go.

And I hereby apologize to Paul for steering him wrong with respect to 
changing gfortran!

-- 
Mark Mitchell
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