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cost model patch


I took the liberty to pick up Richard's original fvect-cost-model
patch and made some modification.

What has not changed:
1) option -ftree-vect-loop-version is removed;
2) three cost models are introduced: cheap, dynamic, and unlimited;
3) unless explicitly specified, cheap model is the default at O2 (e.g.
when -ftree-loop-vectorize is used with -O2), and dynamic mode is the
default for O3 and FDO
4) alignment based versioning is disabled with cheap model.

What has changed:
1) peeling is also disabled with cheap model;
2) alias check condition limit is reduced with cheap model, but not
completely suppressed. Runtime alias check is a pretty important
enabler.
3) tree if conversion changes are not included.

Does this patch look reasonable?

thanks,

David

Attachment: cost_model.txt
Description: Text document


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