proposal for compilation unil wide alias analyis

[David Edelsohn]

>>>>> Mark Mitchell writes:

Mark> The Laffer curve argument (we can remove so much code in the optimizers 
Mark> so quickly that generating the code will actually be faster) seems to me 
Mark> like it is one that should be proven experimentally before we commit to it.

	The argument that running some simple optimizations at -O0
decreases the amount of intermediate code and improves the compilation
speed of a compiler is proven experimentally by the IBM XLC Compiler.
That is exactly the technique that XLC uses, with extremely careful,
empirical tests of which fine-grained optimizations should be enabled to
produce the best compilation performance[1].  We need to test that such a
technique has a beneficial effect on GCC, but there is no question that
the technique can be effective.

	As a side note, I believe that Apple Computer, which is very vocal
about compilation speed, generally considers XLC a good example of a
compiler with fast compilation speed at -O0.

David

[1] The decision to not enable a particular XLC optimization at -O0 that
affects where XLC looks for function parameters (probably something
related to dead store elimination) is the reason that XLC looks in the
stack parameter block at -O0, which prompted the need for the -mxl-call
compatibility option for GCC on AIX.  I am pointing this out to emphasize
the detailed cost-benefit analysis that XLC uses when choosing which
optimizations to enable at -O0.