LNO Branch merge proposal

[David Edelsohn]

>>>>> Diego Novillo writes:

Diego> While it would be very cool to have LNO integrated, the fact that none
Diego> of the optimizers are triggered by default may be a bit of a problem. 

	High-level loop optimizations and auto-vectorization are not
enabled by default in most compilers.  We are trying to improve GCC
performance on benchmarks, but we are not trying to transform GCC into a
benchmark compiler when invoked under normal operating conditions.

Diego> We have had a history of technology preview flags that ended up
Diego> abandoned or are limping along, not quite finished.  From my personal
Diego> experience, branches are wonderful for this type of situation.  We have
Diego> new cool stuff that we can break at will and reshape without paying
Diego> attention to what goes on in mainline.

	What technology previews in GCC with long-term commitments of
support for developers have been abandoned?

Diego> The other questions are the same we asked of tree-ssa: what about
Diego> compile and run time performance?  memory consumption?  We already have
Diego> a problem with that in tree-ssa.  How much worse/better is it on LNO? 
Diego> How does SPEC look like?  Are we beating mainline?  Are we close to ICC?

	GCC performance with LNO is closeR to ICC, but no new features are
going to get GCC close to ICC from day one.

	LNO will not be enabled by default, so it cannot have any impact
on compile time, run time, or memory consumption.  Only diskspace.  High
level loop transformations are complicated and expensive.  One enables
them selectively at very high optimization for applications that will
benefit from those transformations and are willing to pay the compile time
price. 

	Merging LNO into mainline is very low risk and will not impact the
release schedule.  And merging it in will accelerate the development of
GCC itself and of LNO features.  Is there some reason to discourage rapid
improvement in GCC performance?

	Many GCC developers have been looking for increased contribution
of technology to GCC.  It would be a shame to discourage those efforts and
pass up an opportunity.

David