diff --git a/lib/Target/X86/X86TargetMachine.cpp b/lib/Target/X86/X86TargetMachine.cpp
index 7344116e14a..9a9ea245f70 100644
--- a/lib/Target/X86/X86TargetMachine.cpp
+++ b/lib/Target/X86/X86TargetMachine.cpp
@@ -249,25 +249,10 @@ X86TargetMachine::getSubtargetImpl(const Function &F) const {
                      : (StringRef)TargetFS;
 
   SmallString<512> Key;
-  Key.reserve(CPU.size() + FS.size());
-  Key += CPU;
-  Key += FS;
-
-  // FIXME: This is related to the code below to reset the target options,
-  // we need to know whether or not the soft float flag is set on the
-  // function before we can generate a subtarget. We also need to use
-  // it as a key for the subtarget since that can be the only difference
-  // between two functions.
-  bool SoftFloat =
-      F.getFnAttribute("use-soft-float").getValueAsString() == "true";
-  // If the soft float attribute is set on the function turn on the soft float
-  // subtarget feature.
-  if (SoftFloat)
-    Key += FS.empty() ? "+soft-float" : ",+soft-float";
-
-  // Keep track of the key width after all features are added so we can extract
-  // the feature string out later.
-  unsigned CPUFSWidth = Key.size();
+  // The additions here are ordered so that the definitely short strings are
+  // added first so we won't exceed the small size. We append the
+  // much longer FS string at the end so that we only heap allocate at most
+  // one time.
 
   // Extract prefer-vector-width attribute.
   unsigned PreferVectorWidthOverride = 0;
@@ -275,7 +260,7 @@ X86TargetMachine::getSubtargetImpl(const Function &F) const {
     StringRef Val = F.getFnAttribute("prefer-vector-width").getValueAsString();
     unsigned Width;
     if (!Val.getAsInteger(0, Width)) {
-      Key += ",prefer-vector-width=";
+      Key += "prefer-vector-width=";
       Key += Val;
       PreferVectorWidthOverride = Width;
     }
@@ -288,16 +273,35 @@ X86TargetMachine::getSubtargetImpl(const Function &F) const {
         F.getFnAttribute("min-legal-vector-width").getValueAsString();
     unsigned Width;
     if (!Val.getAsInteger(0, Width)) {
-      Key += ",min-legal-vector-width=";
+      Key += "min-legal-vector-width=";
       Key += Val;
       RequiredVectorWidth = Width;
     }
   }
 
-  // Extracted here so that we make sure there is backing for the StringRef. If
-  // we assigned earlier, its possible the SmallString reallocated leaving a
-  // dangling StringRef.
-  FS = Key.slice(CPU.size(), CPUFSWidth);
+  // Add CPU to the Key.
+  Key += CPU;
+
+  // Keep track of the start of the feature portion of the string.
+  unsigned FSStart = Key.size();
+
+  // FIXME: This is related to the code below to reset the target options,
+  // we need to know whether or not the soft float flag is set on the
+  // function before we can generate a subtarget. We also need to use
+  // it as a key for the subtarget since that can be the only difference
+  // between two functions.
+  bool SoftFloat =
+      F.getFnAttribute("use-soft-float").getValueAsString() == "true";
+  // If the soft float attribute is set on the function turn on the soft float
+  // subtarget feature.
+  if (SoftFloat)
+    Key += FS.empty() ? "+soft-float" : "+soft-float,";
+
+  Key += FS;
+
+  // We may have added +soft-float to the features so move the StringRef to
+  // point to the full string in the Key.
+  FS = Key.substr(FSStart);
 
   auto &I = SubtargetMap[Key];
   if (!I) {