[PATCH] Add an instcombine rule to optimize max(~a, ~b) to ~min(a, b) when profitable

[Sanjoy Das]

================
Comment at: lib/Transforms/InstCombine/InstCombineSelect.cpp:1182
@@ +1181,3 @@
+          if (BO->getOpcode() == Instruction::Add ||
+              BO->getOpcode() == Instruction::Sub)
+            return isa<Constant>(BO->getOperand(0)) ||
----------------
filcab wrote:
> We do this test here, but the calculation for Profitable only cares about Not.
> Do we really need this (or should we just check for this in the Porfitable calculation too)?
The `Profitable` check is intentionally more restrictive.  You don't gain or lose anything if you compute `(-1-Const) - A` instead of `Const + A`, so there is no penalty.  But the profitable check is specifically about figuring out if computing `~V` instead of `V` *gains* you anything -- if `V` was `xor P, -1` then you can now get away without actually computing the `xor`.  The profitability check has to be stronger than a break-even since this rule introduces a `not` and that extra work has to be justified.

http://reviews.llvm.org/D7821

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