Index: llvm/trunk/include/llvm/ADT/StringRef.h
===================================================================
--- llvm/trunk/include/llvm/ADT/StringRef.h
+++ llvm/trunk/include/llvm/ADT/StringRef.h
@@ -557,6 +557,14 @@
     /// string is well-formed in the given radix.
     bool getAsInteger(unsigned Radix, APInt &Result) const;
 
+    /// Parse the current string as an IEEE double-precision floating
+    /// point value.  The string must be a well-formed double.
+    ///
+    /// If \p AllowInexact is false, the function will fail if the string
+    /// cannot be represented exactly.  Otherwise, the function only fails
+    /// in case of an overflow or underflow.
+    bool getAsDouble(double &Result, bool AllowInexact = true) const;
+
     /// @}
     /// @name String Operations
     /// @{
Index: llvm/trunk/lib/Support/StringRef.cpp
===================================================================
--- llvm/trunk/lib/Support/StringRef.cpp
+++ llvm/trunk/lib/Support/StringRef.cpp
@@ -8,6 +8,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/ADT/StringRef.h"
+#include "llvm/ADT/APFloat.h"
 #include "llvm/ADT/APInt.h"
 #include "llvm/ADT/Hashing.h"
 #include "llvm/ADT/edit_distance.h"
@@ -595,6 +596,18 @@
   return false;
 }
 
+bool StringRef::getAsDouble(double &Result, bool AllowInexact) const {
+  APFloat F(0.0);
+  APFloat::opStatus Status =
+      F.convertFromString(*this, APFloat::rmNearestTiesToEven);
+  if (Status != APFloat::opOK) {
+    if (!AllowInexact || Status != APFloat::opInexact)
+      return true;
+  }
+
+  Result = F.convertToDouble();
+  return false;
+}
 
 // Implementation of StringRef hashing.
 hash_code llvm::hash_value(StringRef S) {
Index: llvm/trunk/unittests/ADT/StringRefTest.cpp
===================================================================
--- llvm/trunk/unittests/ADT/StringRefTest.cpp
+++ llvm/trunk/unittests/ADT/StringRefTest.cpp
@@ -852,6 +852,27 @@
   }
 }
 
+struct GetDoubleStrings {
+  const char *Str;
+  bool AllowInexact;
+  bool ShouldFail;
+  double D;
+} DoubleStrings[] = {{"0", false, false, 0.0},
+                     {"0.0", false, false, 0.0},
+                     {"-0.0", false, false, -0.0},
+                     {"123.45", false, true, 123.45},
+                     {"123.45", true, false, 123.45}};
+
+TEST(StringRefTest, getAsDouble) {
+  for (const auto &Entry : DoubleStrings) {
+    double Result;
+    StringRef S(Entry.Str);
+    EXPECT_EQ(Entry.ShouldFail, S.getAsDouble(Result, Entry.AllowInexact));
+    if (!Entry.ShouldFail)
+      EXPECT_EQ(Result, Entry.D);
+  }
+}
+
 static const char *join_input[] = { "a", "b", "c" };
 static const char join_result1[] = "a";
 static const char join_result2[] = "a:b:c";