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Re: CSE not combining equivalent expressions.
On Thursday 18 January 2007 09:31, Jeffrey Law wrote:
> I haven't followed this thread that closely, but it seems to me this
> could be done in the propagation engine.
>
> Basically we keep track of the known zero, sign bit copies and known
> nonzero bits for SSA names, then propagate them in the obvious ways.
> Basically replicating a lot of what combine & cse do in this area,
> but at the tree level. It's something I've always wanted to see
> implemented, but never bothered to do...
I had this implemented at one point (2 years ago??) and I could not
show any real benefit. There were almost no opportunities for this
kind of optimization in GCC itself or in some benchmarks I looked at.
There appear to be more bit operations in RTL, so perhaps it is a
better idea to implement a known-bits propagation pass for RTL, with
the new dataflow engine.
Gr.
Steven