tail call optimizations

[Mark Mitchell]

Richard --

  Thanks.

  For the record, Jeff, Richard, and I discussed this patch amongst
ourselves quite extensively.  One thing we agreed upon was that the
sibling call support would likely be removed once tree-based tail-call
elmimination is available, and once the C front-end can use it.

  In general, it's a long-term hope that we'll move towards a more
standard tree structure, and that front-ends will use that structure.
Then, some optimizations will be done there, before transforming trees
into to RTL.  Some optimizations (like this one) are relatively
trivially done on trees.

  However, we don't know when that kind of stuff will actually happen.
In the mean-time, this patch has considerable benefit.

--
Mark Mitchell                   mark@codesourcery.com
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