[ValueTracking] Add better support for ConstantRange(Shl)

1) If LHS is constant:
    - The low bits of the LHS is set, the lower bound is non-zero
    - The upper bound can be capped at popcount(LHS) high bits
2) If RHS is constant:
    - The upper bound can be capped at (Width - RHS) high bits

1 files changed, 13 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index e186431..9b29d64 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -8553,7 +8553,20 @@ static void setLimitsForBinOp(const BinaryOperator &BO, APInt &Lower,
           Lower = *C;
           Upper = C->shl(ShiftAmount) + 1;
         }
+      } else {
+        // If lowbit is set, value can never be zero.
+        if ((*C)[0])
+          Lower = APInt::getOneBitSet(Width, 0);
+        // If we are shifting a constant the largest it can be is if the longest
+        // sequence of consecutive ones is shifted to the highbits (breaking
+        // ties for which sequence is higher). At the moment we take a liberal
+        // upper bound on this by just popcounting the constant.
+        // TODO: There may be a bitwise trick for it longest/highest
+        // consecutative sequence of ones (naive method is O(Width) loop).
+        Upper = APInt::getHighBitsSet(Width, C->popcount()) + 1;
       }
+    } else if (match(BO.getOperand(1), m_APInt(C)) && C->ult(Width)) {
+      Upper = APInt::getBitsSetFrom(Width, C->getZExtValue()) + 1;
     }
     break;