[PATCH] Infer known bits from dominating conditions

[hfinkel at anl.gov]

In http://reviews.llvm.org/D7708#134110, @reames wrote:

> Hal - preliminary numbers on the dominance based approach make it look like that might be viable.  I need to cleanup my hacked implementation and fix at least one bug, but my initial impression is that it appears to be fast enough.  I'm a bit surprised by this, but it's what the data says.
>
> Worth discussing is whether we should actually incorporate both.  They seem to be good at different cases and maybe we should exploit that.  The use based approach is good for grabbing facts far from the use site (i.e. function entry guards), while the dominance approach is likely more powerful since it's not restricted to direct uses, but is probably best limited to a local scope.  Thoughts?


Yes, perhaps with a FIXME if there is some other "right" way to fix the problem. I've been thinking of something like LVI that keeps track of known bits. What do you think?


http://reviews.llvm.org/D7708

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