[PATCH] D14315: [ValueTracking] Recognize that and(x, add (x, -1)) clears the low bit

[Philip Reames via llvm-commits]

reames added inline comments.

================
Comment at: lib/Analysis/ValueTracking.cpp:1092
@@ +1091,3 @@
+      APInt KnownZero3(BitWidth, 0), KnownOne3(BitWidth, 0);
+      computeKnownBits(Y, KnownZero3, KnownOne3, DL, Depth + 1, Q);
+      if (KnownOne3.countTrailingOnes() > 0)
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sanjoy wrote:
> This is minor:
> 
> I think (but perhaps @majnemer can confirm this) instcombine canonicalizes operands (for commutative operations) so that the LHS is more complex than the RHS, so maybe there is not much value in checking for both "x & (x - 1)" and "(x - 1) & x" -- instcombine will transform the former to the latter.
I don't think it will in this case.  I believe the definition of complexity is on the type of the operand (constant, non-constant, etc..), not the form of the operand.  


http://reviews.llvm.org/D14315