[PATCH] D142271: [ValueTracking] Add KnownBits patterns `xor(x, x - 1)` and `and(x, -x)` for knowing upper bits to be zero

[Craig Topper via Phabricator via llvm-commits]

craig.topper added inline comments.


================
Comment at: llvm/lib/Analysis/ValueTracking.cpp:1094
+          match(I->getOperand(1), m_Neg(m_Specific(I->getOperand(0))))) {
+        unsigned MinBit = std::min(Known.One.countTrailingZeros(),
+                                   Known2.One.countTrailingZeros());
----------------
I'm not sure about this min. The `-x` operand will potentially find less trailing bits due to the recursion limit in computeKnownBits. It has one more node to walk through. Using the tailing zeros bits based on which operand is `x` seems more reliable.


Repository:
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  https://reviews.llvm.org/D142271/new/

https://reviews.llvm.org/D142271