Index: include/llvm/IR/Attributes.h
===================================================================
--- include/llvm/IR/Attributes.h
+++ include/llvm/IR/Attributes.h
@@ -292,6 +292,10 @@
   /// \brief Return true if attribute exists at the given index.
   bool hasAttributes(unsigned Index) const;
 
+  /// \brief Equivalent to hasAttribute(AttributeSet::FunctionIndex, Kind) but
+  /// may be faster.
+  bool hasFnAttribute(Attribute::AttrKind Kind) const;
+
   /// \brief Return true if the specified attribute is set for at least one
   /// parameter or for the return value.
   bool hasAttrSomewhere(Attribute::AttrKind Attr) const;
Index: include/llvm/IR/Function.h
===================================================================
--- include/llvm/IR/Function.h
+++ include/llvm/IR/Function.h
@@ -199,7 +199,7 @@
 
   /// @brief Return true if the function has the attribute.
   bool hasFnAttribute(Attribute::AttrKind Kind) const {
-    return AttributeSets.hasAttribute(AttributeSet::FunctionIndex, Kind);
+    return AttributeSets.hasFnAttribute(Kind);
   }
   bool hasFnAttribute(StringRef Kind) const {
     return AttributeSets.hasAttribute(AttributeSet::FunctionIndex, Kind);
@@ -207,6 +207,8 @@
 
   /// @brief Return the attribute for the given attribute kind.
   Attribute getFnAttribute(Attribute::AttrKind Kind) const {
+    if (!hasFnAttribute(Kind))
+      return Attribute();
     return AttributeSets.getAttribute(AttributeSet::FunctionIndex, Kind);
   }
   Attribute getFnAttribute(StringRef Kind) const {
@@ -215,6 +217,8 @@
 
   /// \brief Return the stack alignment for the function.
   unsigned getFnStackAlignment() const {
+    if (!hasFnAttribute(Attribute::StackAlignment))
+      return 0;
     return AttributeSets.getStackAlignment(AttributeSet::FunctionIndex);
   }
 
Index: lib/IR/AttributeImpl.h
===================================================================
--- lib/IR/AttributeImpl.h
+++ lib/IR/AttributeImpl.h
@@ -214,6 +214,11 @@
 private:
   LLVMContext &Context;
   unsigned NumAttrs; ///< Number of entries in this set.
+  /// Bitset with a bit for each available attribute Attribute::AttrKind.
+  uint64_t AvailableFunctionAttrs;
+  static_assert(Attribute::EndAttrKinds
+                <= sizeof(AvailableFunctionAttrs)*CHAR_BIT,
+                "Too many attributes");
 
   // Helper fn for TrailingObjects class.
   size_t numTrailingObjects(OverloadToken<IndexAttrPair>) { return NumAttrs; }
@@ -229,7 +234,7 @@
 public:
   AttributeSetImpl(LLVMContext &C,
                    ArrayRef<std::pair<unsigned, AttributeSetNode *> > Attrs)
-      : Context(C), NumAttrs(Attrs.size()) {
+      : Context(C), NumAttrs(Attrs.size()), AvailableFunctionAttrs(0) {
 
 #ifndef NDEBUG
     if (Attrs.size() >= 2) {
@@ -242,6 +247,21 @@
 #endif
     // There's memory after the node where we can store the entries in.
     std::copy(Attrs.begin(), Attrs.end(), getTrailingObjects<IndexAttrPair>());
+
+    // Initialize AvailableFunctionAttrs summary bitset.
+    if (NumAttrs > 0) {
+      static_assert(AttributeSet::FunctionIndex == ~0u,
+                    "FunctionIndex should be biggest possible index");
+      const std::pair<unsigned, AttributeSetNode *> &Last = Attrs.back();
+      if (Last.first == AttributeSet::FunctionIndex) {
+        const AttributeSetNode *Node = Last.second;
+        for (AttributeSetNode::iterator I = Node->begin(), E = Node->end();
+             I != E; ++I) {
+          if (!I->isStringAttribute())
+            AvailableFunctionAttrs |= ((uint64_t)1) << I->getKindAsEnum();
+        }
+      }
+    }
   }
 
   /// \brief Get the context that created this AttributeSetImpl.
@@ -271,6 +291,12 @@
     return getNode(Slot)->second;
   }
 
+  /// \brief Return true if the AttributeSetNode for the FunctionIndex has an
+  /// enum attribute of the given kind.
+  bool hasFnAttribute(Attribute::AttrKind Kind) const {
+    return AvailableFunctionAttrs & ((uint64_t)1) << Kind;
+  }
+
   typedef AttributeSetNode::iterator iterator;
   iterator begin(unsigned Slot) const { return getSlotNode(Slot)->begin(); }
   iterator end(unsigned Slot) const { return getSlotNode(Slot)->end(); }
Index: lib/IR/Attributes.cpp
===================================================================
--- lib/IR/Attributes.cpp
+++ lib/IR/Attributes.cpp
@@ -999,6 +999,10 @@
   return ASN && ASN->hasAttributes();
 }
 
+bool AttributeSet::hasFnAttribute(Attribute::AttrKind Kind) const {
+  return pImpl && pImpl->hasFnAttribute(Kind);
+}
+
 bool AttributeSet::hasAttrSomewhere(Attribute::AttrKind Attr) const {
   if (!pImpl) return false;