diff --git a/include/llvm/ADT/APFloat.h b/include/llvm/ADT/APFloat.h
index a83563a5a6f..9e8642bfaed 100644
--- a/include/llvm/ADT/APFloat.h
+++ b/include/llvm/ADT/APFloat.h
@@ -491,6 +491,22 @@ public:
   /// return true.
   bool getExactInverse(APFloat *inv) const;
 
+  /// \brief Returns the exponent of the internal representation of the APFloat.
+  ///
+  /// Because the radix of APFloat is 2, this is equivalent to floor(log2(x)).
+  friend APFloat logb(const APFloat &Arg) {
+    if (Arg.isZero() || Arg.isNaN())
+      return Arg;
+
+    if (Arg.isInfinity())
+      return APFloat::getInf(Arg.getSemantics());
+
+    APFloat Result(Arg.getSemantics(), std::abs(Arg.exponent));
+    if (Arg.exponent < 0)
+      Result.changeSign();
+    return Result;
+  }
+
 private:
 
   /// \name Simple Queries
@@ -617,6 +633,7 @@ private:
 /// This additional declaration is required in order to compile LLVM with IBM
 /// xlC compiler.
 hash_code hash_value(const APFloat &Arg);
+
 } // namespace llvm
 
 #endif // LLVM_ADT_APFLOAT_H
diff --git a/unittests/ADT/APFloatTest.cpp b/unittests/ADT/APFloatTest.cpp
index a4816efc214..8dd6f9f9931 100644
--- a/unittests/ADT/APFloatTest.cpp
+++ b/unittests/ADT/APFloatTest.cpp
@@ -2691,4 +2691,55 @@ TEST(APFloatTest, operatorOverloads) {
   EXPECT_TRUE(Two.bitwiseIsEqual(One * Two));
   EXPECT_TRUE(One.bitwiseIsEqual(Two / Two));
 }
+
+TEST(APFloatTest, logb) {
+  EXPECT_TRUE(
+      APFloat(APFloat::IEEEsingle, "0x0p+0")
+          .bitwiseIsEqual(logb(APFloat(APFloat::IEEEsingle, "0x1p+0"))));
+  EXPECT_TRUE(
+      APFloat(APFloat::IEEEsingle, "0x0p+0")
+          .bitwiseIsEqual(logb(APFloat(APFloat::IEEEsingle, "-0x1p+0"))));
+  EXPECT_TRUE(
+      APFloat(APFloat::IEEEsingle, "0x2Ap+0")
+          .bitwiseIsEqual(logb(APFloat(APFloat::IEEEsingle, "0x1p+42"))));
+  EXPECT_TRUE(
+      APFloat(APFloat::IEEEsingle, "-0x2Ap+0")
+          .bitwiseIsEqual(logb(APFloat(APFloat::IEEEsingle, "0x1p-42"))));
+
+  APFloat PInf = APFloat::getInf(APFloat::IEEEsingle, false);
+  APFloat MInf = APFloat::getInf(APFloat::IEEEsingle, true);
+  APFloat PZero = APFloat::getZero(APFloat::IEEEsingle, false);
+  APFloat MZero = APFloat::getZero(APFloat::IEEEsingle, true);
+  APFloat QNaN = APFloat::getNaN(APFloat::IEEEsingle, false);
+  APFloat SNaN = APFloat::getSNaN(APFloat::IEEEsingle, false);
+
+  EXPECT_TRUE(PInf.bitwiseIsEqual(logb(PInf)));
+  EXPECT_TRUE(PInf.bitwiseIsEqual(logb(MInf)));
+  EXPECT_TRUE(PZero.bitwiseIsEqual(logb(PZero)));
+  EXPECT_TRUE(MZero.bitwiseIsEqual(logb(MZero)));
+  EXPECT_TRUE(QNaN.bitwiseIsEqual(logb(QNaN)));
+  EXPECT_TRUE(SNaN.bitwiseIsEqual(logb(SNaN)));
+
+  APFloat PLargestValue = APFloat::getLargest(APFloat::IEEEsingle, false);
+  APFloat MLargestValue = APFloat::getLargest(APFloat::IEEEsingle, true);
+  APFloat PSmallestValue = APFloat::getSmallest(APFloat::IEEEsingle, false);
+  APFloat MSmallestValue = APFloat::getSmallest(APFloat::IEEEsingle, true);
+  APFloat PSmallestNormalized =
+      APFloat::getSmallestNormalized(APFloat::IEEEsingle, false);
+  APFloat MSmallestNormalized =
+      APFloat::getSmallestNormalized(APFloat::IEEEsingle, true);
+
+  EXPECT_TRUE(
+      APFloat(APFloat::IEEEsingle, "0x7Fp+0").bitwiseIsEqual(logb(PLargestValue)));
+  EXPECT_TRUE(
+      APFloat(APFloat::IEEEsingle, "0x7Fp+0").bitwiseIsEqual(logb(MLargestValue)));
+  EXPECT_TRUE(
+      APFloat(APFloat::IEEEsingle, "-0x7Ep+0").bitwiseIsEqual(logb(PSmallestValue)));
+  EXPECT_TRUE(
+      APFloat(APFloat::IEEEsingle, "-0x7Ep+0").bitwiseIsEqual(logb(MSmallestValue)));
+  EXPECT_TRUE(APFloat(APFloat::IEEEsingle, "-0x7Ep+0")
+                  .bitwiseIsEqual(logb(PSmallestNormalized)));
+  EXPECT_TRUE(APFloat(APFloat::IEEEsingle, "-0x7Ep+0")
+                  .bitwiseIsEqual(logb(MSmallestNormalized)));
+}
 }