[gfortran] Fix NULL reference types.

[Nathan Sidwell]

Richard Kenner wrote:
>     I suppose that we could provide the same information I wanted to convey 
>     with REFERENCE_TYPE to the optimizer in some other way: a bit on 
>     VAR_DECLs, PARM_DECLs, FIELD_DECLs, etc. that says "never NULL".
> 
> And INDIRECT_REF, which is somewhat of the harder one.  That's why I
> think types are the best approach.

I don't think it's needed here.  Value propagation can determine the
thing being indirected will be non-null -- that'll be something the
propagator has determined, or it'll be a VAR, PARM or FIELD decl with the
non-null bit valid.

You will need it on function return values though. That might be via a
function-pointer, so it needs to be held in the FUNCTION_TYPE node
somewhere.

My understanding is that you'll need the cannot-be-null information at
the boundary of the map that the value propagator works with.  That
boundary is global variables, function parameters (of the function being
optimized) and function return values (of non-inlined function it is
calling).

nathan

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