RFC Exploratory Patch - Significant performance improvement

[Jerry DeLisle]

Hi all,

I submit the attached patch for review and comment.

With this patch, the test case in PR32489, fft257.f90 (attached) compiles in 
about 2 to 3 seconds.  Without the patch, I am unable to compile this test case 
without running out of memory and going into endless swapping.  I suppose it is 
possible that if I wait long enough it may complete, but my system starts to go 
unresponsive so I aborted the compile after about 5 minutes. ( I have 4 Gbyte 
system. )

Regardless, the patch improves on this with about a thousand fold improvement. 
Admittedly, its a rare test case, but perhaps this will help others with real 
world applications that are compiling slowly.

I have regression tested this with no failures. The tests for NULL in expr.c I 
added to avoid segfaults with string_compare_3.f90.  I don't like that fix, but 
it works.  I suspect we can further enhance the new function 
gfc_is_expandable_expr in resolve.c to work around this.

I also have successfully run Polyhedron 2005 tests.  I get one failure with 
aermod which I thinks existed before the patch.

The patch includes my previous patch for PR20923 since it is in my tree and I 
don't want to try the rest of the patch without it.

I invite you to explore this patch, not only to identify any other holes, but 
also to explore the implications of what this is doing.  Are the front-end 
results better for the middle end folks?  Do we get any better run time 
performance or only compile time improvement?  Are there other bugs lurking here 
and/or is the concept flawed?

Enjoy,

Jerry

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