[InstSimplify] simplify abs if operand is known non-negative

[spatel]

Summary

abs() should be rare enough that using value tracking is not going to be a compile-time cost burden, so use it to reduce a variety of potential patterns. We do this in DAGCombiner too.

[nikic]

Yes, we should be fine compile-time wise. The icmp + select representation of abs implicitly did this as well, because we fold comparisons against zero more aggressively than others.

llvm/lib/Analysis/InstructionSimplify.cpp

5260

We should also pass through Q.AC, Q.CxtI, Q.DT.

[spatel]

Patch updated:
Pass in more info from the SimplifyQuery to ValueTracking.
Not sure what it takes to show the benefit, so no test diffs, but if there are suggestions, let me know.

[nikic]

LG

In D85043#2188428, @spatel wrote:

Patch updated:
Pass in more info from the SimplifyQuery to ValueTracking.
Not sure what it takes to show the benefit, so no test diffs, but if there are suggestions, let me know.

Something like assume(x >= 0); abs(x) should do it.