diff --git a/llvm/include/llvm/Support/KnownBits.h b/llvm/include/llvm/Support/KnownBits.h
--- a/llvm/include/llvm/Support/KnownBits.h
+++ b/llvm/include/llvm/Support/KnownBits.h
@@ -422,10 +422,12 @@
     return KnownBits(Zero.reverseBits(), One.reverseBits());
   }
 
-  /// Compute known bits for X & -X.
+  /// Compute known bits for X & -X. The name comes from the X86 BMI instruction
+  /// BLSI.
   KnownBits blsi() const;
 
-  /// Compute known bits for X ^ (X - 1).
+  /// Compute known bits for X ^ (X - 1). The name comes from the X86 BMI
+  /// instruction BLSMSK.
   KnownBits blsmsk() const;
 
   bool operator==(const KnownBits &Other) const {
diff --git a/llvm/lib/Support/KnownBits.cpp b/llvm/lib/Support/KnownBits.cpp
--- a/llvm/lib/Support/KnownBits.cpp
+++ b/llvm/lib/Support/KnownBits.cpp
@@ -624,20 +624,23 @@
 }
 
 KnownBits KnownBits::blsi() const {
-  KnownBits Known(getBitWidth());
-  unsigned Min = countMinTrailingZeros();
+  unsigned BitWidth = getBitWidth();
+  KnownBits Known(Zero, APInt(BitWidth, 0));
   unsigned Max = countMaxTrailingZeros();
-  Known.Zero.setLowBits(Min);
-  Known.Zero.setBitsFrom(Max + 0);
+  Known.Zero.setBitsFrom(std::min(Max + 1, BitWidth));
+  unsigned Min = countMinTrailingZeros();
+  if (Max == Min && Max < BitWidth)
+    Known.One.setBit(Max);
   return Known;
 }
 
 KnownBits KnownBits::blsmsk() const {
-  KnownBits Known(getBitWidth());
-  unsigned Min = countMinTrailingZeros();
+  unsigned BitWidth = getBitWidth();
+  KnownBits Known(BitWidth);
   unsigned Max = countMaxTrailingZeros();
-  Known.One.setLowBits(Min + 1);
-  Known.Zero.setBitsFrom(Max + 0);
+  Known.Zero.setBitsFrom(std::min(Max + 1, BitWidth));
+  unsigned Min = countMinTrailingZeros();
+  Known.One.setLowBits(std::min(Min + 1, BitWidth));
   return Known;
 }