Index: include/clang/Basic/Attr.td
===================================================================
--- include/clang/Basic/Attr.td
+++ include/clang/Basic/Attr.td
@@ -864,6 +864,13 @@
   let Documentation = [Undocumented];
 }
 
+def AssumeAligned : InheritableAttr {
+  let Spellings = [GCC<"assume_aligned">];
+  let Subjects = SubjectList<[ObjCMethod, Function]>;
+  let Args = [ExprArgument<"Alignment">, ExprArgument<"Offset", 1>];
+  let Documentation = [AssumeAlignedDocs];
+}
+
 def NoReturn : InheritableAttr {
   let Spellings = [GCC<"noreturn">, Declspec<"noreturn">];
   // FIXME: Does GCC allow this on the function instead?
Index: include/clang/Basic/AttrDocs.td
===================================================================
--- include/clang/Basic/AttrDocs.td
+++ include/clang/Basic/AttrDocs.td
@@ -154,6 +154,30 @@
 Marks a function as releasing a capability.
   }];
 }
+
+def AssumeAlignedDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+Use ``__attribute__((assume_aligned(<alignment>[,<offset>]))`` on a function
+declaration to specify that the return value of the function (which must be a
+pointer type) has the specified offset, in bytes, from an address with the
+specified alignment. The offset is taken to be zero if omitted.
+
+.. code-block:: c++
+
+  // The returned pointer value has 32-byte alignment.
+  void *a() __attribute__((assume_aligned (32)));
+
+  // The returned pointer value is 4 bytes greater than an address having
+  // 32-byte alignment.
+  void *b() __attribute__((assume_aligned (32, 4)));
+
+Note that this attribute provides information to the compiler regarding a
+condition that the code already ensures is true. It does not cause the compiler
+to enforce the provided alignment assumption.
+  }];
+}
+
 def EnableIfDocs : Documentation {
   let Category = DocCatFunction;
   let Content = [{
Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -1952,6 +1952,9 @@
 def warn_attribute_return_pointers_only : Warning<
   "%0 attribute only applies to return values that are pointers">,
   InGroup<IgnoredAttributes>;
+def warn_attribute_return_pointers_refs_only : Warning<
+  "%0 attribute only applies to return values that are pointers or references">,
+  InGroup<IgnoredAttributes>;
 def err_attribute_no_member_pointers : Error<
   "%0 attribute cannot be used with pointers to members">;
 def err_attribute_invalid_implicit_this_argument : Error<
Index: include/clang/Sema/Sema.h
===================================================================
--- include/clang/Sema/Sema.h
+++ include/clang/Sema/Sema.h
@@ -2711,8 +2711,11 @@
 
   void checkUnusedDeclAttributes(Declarator &D);
 
-  /// Determine if type T is a valid subject for a nonnull attribute.
-  bool isValidNonNullAttrType(QualType T);
+  /// Determine if type T is a valid subject for a nonnull and similar
+  /// attributes. By default, we look through references (the behavior used by
+  /// nonnull), but if the second parameter is true, then we treat a reference
+  /// type as valid.
+  bool isValidPointerAttrType(QualType T, bool RefOkay = false);
 
   bool CheckRegparmAttr(const AttributeList &attr, unsigned &value);
   bool CheckCallingConvAttr(const AttributeList &attr, CallingConv &CC, 
@@ -7338,6 +7341,11 @@
   void AddAlignedAttr(SourceRange AttrRange, Decl *D, TypeSourceInfo *T,
                       unsigned SpellingListIndex, bool IsPackExpansion);
 
+  /// AddAssumeAlignedAttr - Adds an assume_aligned attribute to a particular
+  /// declaration.
+  void AddAssumeAlignedAttr(SourceRange AttrRange, Decl *D, Expr *E, Expr *OE,
+                            unsigned SpellingListIndex);
+
   // OpenMP directives and clauses.
 private:
   void *VarDataSharingAttributesStack;
Index: lib/CodeGen/CGCall.cpp
===================================================================
--- lib/CodeGen/CGCall.cpp
+++ lib/CodeGen/CGCall.cpp
@@ -3144,75 +3144,92 @@
   // lexical order, so deactivate it and run it manually here.
   CallArgs.freeArgumentMemory(*this);
 
-  switch (RetAI.getKind()) {
-  case ABIArgInfo::InAlloca:
-  case ABIArgInfo::Indirect:
-    return convertTempToRValue(SRetPtr, RetTy, SourceLocation());
+  RValue Ret = [&] {
+    switch (RetAI.getKind()) {
+    case ABIArgInfo::InAlloca:
+    case ABIArgInfo::Indirect:
+      return convertTempToRValue(SRetPtr, RetTy, SourceLocation());
 
-  case ABIArgInfo::Ignore:
-    // If we are ignoring an argument that had a result, make sure to
-    // construct the appropriate return value for our caller.
-    return GetUndefRValue(RetTy);
+    case ABIArgInfo::Ignore:
+      // If we are ignoring an argument that had a result, make sure to
+      // construct the appropriate return value for our caller.
+      return GetUndefRValue(RetTy);
 
-  case ABIArgInfo::Extend:
-  case ABIArgInfo::Direct: {
-    llvm::Type *RetIRTy = ConvertType(RetTy);
-    if (RetAI.getCoerceToType() == RetIRTy && RetAI.getDirectOffset() == 0) {
-      switch (getEvaluationKind(RetTy)) {
-      case TEK_Complex: {
-        llvm::Value *Real = Builder.CreateExtractValue(CI, 0);
-        llvm::Value *Imag = Builder.CreateExtractValue(CI, 1);
-        return RValue::getComplex(std::make_pair(Real, Imag));
-      }
-      case TEK_Aggregate: {
-        llvm::Value *DestPtr = ReturnValue.getValue();
-        bool DestIsVolatile = ReturnValue.isVolatile();
+    case ABIArgInfo::Extend:
+    case ABIArgInfo::Direct: {
+      llvm::Type *RetIRTy = ConvertType(RetTy);
+      if (RetAI.getCoerceToType() == RetIRTy && RetAI.getDirectOffset() == 0) {
+        switch (getEvaluationKind(RetTy)) {
+        case TEK_Complex: {
+          llvm::Value *Real = Builder.CreateExtractValue(CI, 0);
+          llvm::Value *Imag = Builder.CreateExtractValue(CI, 1);
+          return RValue::getComplex(std::make_pair(Real, Imag));
+        }
+        case TEK_Aggregate: {
+          llvm::Value *DestPtr = ReturnValue.getValue();
+          bool DestIsVolatile = ReturnValue.isVolatile();
 
-        if (!DestPtr) {
-          DestPtr = CreateMemTemp(RetTy, "agg.tmp");
-          DestIsVolatile = false;
+          if (!DestPtr) {
+            DestPtr = CreateMemTemp(RetTy, "agg.tmp");
+            DestIsVolatile = false;
+          }
+          BuildAggStore(*this, CI, DestPtr, DestIsVolatile, false);
+          return RValue::getAggregate(DestPtr);
         }
-        BuildAggStore(*this, CI, DestPtr, DestIsVolatile, false);
-        return RValue::getAggregate(DestPtr);
-      }
-      case TEK_Scalar: {
-        // If the argument doesn't match, perform a bitcast to coerce it.  This
-        // can happen due to trivial type mismatches.
-        llvm::Value *V = CI;
-        if (V->getType() != RetIRTy)
-          V = Builder.CreateBitCast(V, RetIRTy);
-        return RValue::get(V);
+        case TEK_Scalar: {
+          // If the argument doesn't match, perform a bitcast to coerce it.  This
+          // can happen due to trivial type mismatches.
+          llvm::Value *V = CI;
+          if (V->getType() != RetIRTy)
+            V = Builder.CreateBitCast(V, RetIRTy);
+          return RValue::get(V);
+        }
+        }
+        llvm_unreachable("bad evaluation kind");
       }
+
+      llvm::Value *DestPtr = ReturnValue.getValue();
+      bool DestIsVolatile = ReturnValue.isVolatile();
+
+      if (!DestPtr) {
+        DestPtr = CreateMemTemp(RetTy, "coerce");
+        DestIsVolatile = false;
       }
-      llvm_unreachable("bad evaluation kind");
-    }
 
-    llvm::Value *DestPtr = ReturnValue.getValue();
-    bool DestIsVolatile = ReturnValue.isVolatile();
+      // If the value is offset in memory, apply the offset now.
+      llvm::Value *StorePtr = DestPtr;
+      if (unsigned Offs = RetAI.getDirectOffset()) {
+        StorePtr = Builder.CreateBitCast(StorePtr, Builder.getInt8PtrTy());
+        StorePtr = Builder.CreateConstGEP1_32(StorePtr, Offs);
+        StorePtr = Builder.CreateBitCast(StorePtr,
+                           llvm::PointerType::getUnqual(RetAI.getCoerceToType()));
+      }
+      CreateCoercedStore(CI, StorePtr, DestIsVolatile, *this);
 
-    if (!DestPtr) {
-      DestPtr = CreateMemTemp(RetTy, "coerce");
-      DestIsVolatile = false;
+      return convertTempToRValue(DestPtr, RetTy, SourceLocation());
     }
 
-    // If the value is offset in memory, apply the offset now.
-    llvm::Value *StorePtr = DestPtr;
-    if (unsigned Offs = RetAI.getDirectOffset()) {
-      StorePtr = Builder.CreateBitCast(StorePtr, Builder.getInt8PtrTy());
-      StorePtr = Builder.CreateConstGEP1_32(StorePtr, Offs);
-      StorePtr = Builder.CreateBitCast(StorePtr,
-                         llvm::PointerType::getUnqual(RetAI.getCoerceToType()));
+    case ABIArgInfo::Expand:
+      llvm_unreachable("Invalid ABI kind for return argument");
     }
-    CreateCoercedStore(CI, StorePtr, DestIsVolatile, *this);
 
-    return convertTempToRValue(DestPtr, RetTy, SourceLocation());
-  }
+    llvm_unreachable("Unhandled ABIArgInfo::Kind");
+  } ();
 
-  case ABIArgInfo::Expand:
-    llvm_unreachable("Invalid ABI kind for return argument");
+  if (Ret.isScalar() && TargetDecl) {
+    if (const auto *AA = TargetDecl->getAttr<AssumeAlignedAttr>()) {
+      llvm::Value *OffsetValue = nullptr;
+      if (const auto *Offset = AA->getOffset())
+        OffsetValue = EmitScalarExpr(Offset);
+
+      llvm::Value *Alignment = EmitScalarExpr(AA->getAlignment());
+      llvm::ConstantInt *AlignmentCI = cast<llvm::ConstantInt>(Alignment);
+      EmitAlignmentAssumption(Ret.getScalarVal(), AlignmentCI->getZExtValue(),
+                              OffsetValue);
+    }
   }
 
-  llvm_unreachable("Unhandled ABIArgInfo::Kind");
+  return Ret;
 }
 
 /* VarArg handling */
Index: lib/Sema/SemaChecking.cpp
===================================================================
--- lib/Sema/SemaChecking.cpp
+++ lib/Sema/SemaChecking.cpp
@@ -777,7 +777,7 @@
     if (!NonNull->args_size()) {
       // Easy case: all pointer arguments are nonnull.
       for (const auto *Arg : Args)
-        if (S.isValidNonNullAttrType(Arg->getType()))
+        if (S.isValidPointerAttrType(Arg->getType()))
           CheckNonNullArgument(S, Arg, CallSiteLoc);
       return;
     }
Index: lib/Sema/SemaDeclAttr.cpp
===================================================================
--- lib/Sema/SemaDeclAttr.cpp
+++ lib/Sema/SemaDeclAttr.cpp
@@ -29,6 +29,7 @@
 #include "clang/Sema/Lookup.h"
 #include "clang/Sema/Scope.h"
 #include "llvm/ADT/StringExtras.h"
+#include "llvm/Support/MathExtras.h"
 using namespace clang;
 using namespace sema;
 
@@ -1125,11 +1126,16 @@
                                     Attr.getAttributeSpellingListIndex()));
 }
 
-bool Sema::isValidNonNullAttrType(QualType T) {
-  T = T.getNonReferenceType();
+bool Sema::isValidPointerAttrType(QualType T, bool RefOkay) {
+  if (RefOkay) {
+    if (T->isReferenceType())
+      return true;
+  } else {
+    T = T.getNonReferenceType();
+  }
 
-  // The nonnull attribute can be applied to a transparent union that
-  // contains a pointer type.
+  // The nonnull attribute, and other similar attributes, can be applied to a
+  // transparent union that contains a pointer type.
   if (const RecordType *UT = T->getAsUnionType()) {
     if (UT && UT->getDecl()->hasAttr<TransparentUnionAttr>()) {
       RecordDecl *UD = UT->getDecl();
@@ -1146,13 +1152,13 @@
 
 static bool attrNonNullArgCheck(Sema &S, QualType T, const AttributeList &Attr,
                                 SourceRange AttrParmRange,
-                                SourceRange NonNullTypeRange,
+                                SourceRange TypeRange,
                                 bool isReturnValue = false) {
-  if (!S.isValidNonNullAttrType(T)) {
+  if (!S.isValidPointerAttrType(T)) {
     S.Diag(Attr.getLoc(), isReturnValue
                               ? diag::warn_attribute_return_pointers_only
                               : diag::warn_attribute_pointers_only)
-        << Attr.getName() << AttrParmRange << NonNullTypeRange;
+        << Attr.getName() << AttrParmRange << TypeRange;
     return false;
   }
   return true;
@@ -1186,7 +1192,7 @@
     for (unsigned I = 0, E = getFunctionOrMethodNumParams(D);
          I != E && !AnyPointers; ++I) {
       QualType T = getFunctionOrMethodParamType(D, I);
-      if (T->isDependentType() || S.isValidNonNullAttrType(T))
+      if (T->isDependentType() || S.isValidPointerAttrType(T))
         AnyPointers = true;
     }
 
@@ -1237,6 +1243,65 @@
                                Attr.getAttributeSpellingListIndex()));
 }
 
+static void handleAssumeAlignedAttr(Sema &S, Decl *D,
+                                    const AttributeList &Attr) {
+  Expr *E = Attr.getArgAsExpr(0),
+       *OE = Attr.getNumArgs() > 1 ? Attr.getArgAsExpr(1) : nullptr;
+  S.AddAssumeAlignedAttr(Attr.getRange(), D, E, OE,
+                         Attr.getAttributeSpellingListIndex());
+}
+
+void Sema::AddAssumeAlignedAttr(SourceRange AttrRange, Decl *D, Expr *E,
+                                Expr *OE, unsigned SpellingListIndex) {
+  QualType ResultType = getFunctionOrMethodResultType(D);
+  SourceRange SR = getFunctionOrMethodResultSourceRange(D);
+
+  AssumeAlignedAttr TmpAttr(AttrRange, Context, E, OE, SpellingListIndex);
+  SourceLocation AttrLoc = AttrRange.getBegin();
+
+  if (!isValidPointerAttrType(ResultType, /* RefOkay */ true)) {
+    Diag(AttrLoc, diag::warn_attribute_return_pointers_refs_only)
+      << &TmpAttr << AttrRange << SR;
+    return;
+  }
+
+  if (!E->isValueDependent()) {
+    llvm::APSInt I(64);
+    if (!E->isIntegerConstantExpr(I, Context)) {
+      if (OE)
+        Diag(AttrLoc, diag::err_attribute_argument_n_type)
+          << &TmpAttr << 1 << AANT_ArgumentIntegerConstant
+          << E->getSourceRange();
+      else
+        Diag(AttrLoc, diag::err_attribute_argument_type)
+          << &TmpAttr << AANT_ArgumentIntegerConstant
+          << E->getSourceRange();
+      return;
+    }
+
+    if (!I.isPowerOf2()) {
+      Diag(AttrLoc, diag::err_alignment_not_power_of_two)
+        << E->getSourceRange();
+      return;
+    }
+  }
+
+  if (OE) {
+    if (!OE->isValueDependent()) {
+      llvm::APSInt I(64);
+      if (!OE->isIntegerConstantExpr(I, Context)) {
+        Diag(AttrLoc, diag::err_attribute_argument_n_type)
+          << &TmpAttr << 2 << AANT_ArgumentIntegerConstant
+          << OE->getSourceRange();
+        return;
+      }
+    }
+  }
+
+  D->addAttr(::new (Context)
+            AssumeAlignedAttr(AttrRange, Context, E, OE, SpellingListIndex));
+}
+
 static void handleOwnershipAttr(Sema &S, Decl *D, const AttributeList &AL) {
   // This attribute must be applied to a function declaration. The first
   // argument to the attribute must be an identifier, the name of the resource,
@@ -4212,6 +4277,9 @@
   case AttributeList::AT_ReturnsNonNull:
     handleReturnsNonNullAttr(S, D, Attr);
     break;
+  case AttributeList::AT_AssumeAligned:
+    handleAssumeAlignedAttr(S, D, Attr);
+    break;
   case AttributeList::AT_Overloadable:
     handleSimpleAttribute<OverloadableAttr>(S, D, Attr);
     break;
Index: lib/Sema/SemaTemplateInstantiateDecl.cpp
===================================================================
--- lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -129,6 +129,29 @@
   }
 }
 
+static void instantiateDependentAssumeAlignedAttr(
+    Sema &S, const MultiLevelTemplateArgumentList &TemplateArgs,
+    const AssumeAlignedAttr *Aligned, Decl *New) {
+  // The alignment expression is a constant expression.
+  EnterExpressionEvaluationContext Unevaluated(S, Sema::ConstantEvaluated);
+
+  Expr *E, *OE = nullptr;
+  ExprResult Result = S.SubstExpr(Aligned->getAlignment(), TemplateArgs);
+  if (Result.isInvalid())
+    return;
+  E = Result.getAs<Expr>();
+
+  if (Aligned->getOffset()) {
+    Result = S.SubstExpr(Aligned->getOffset(), TemplateArgs);
+    if (Result.isInvalid())
+      return;
+    OE = Result.getAs<Expr>();
+  }
+
+  S.AddAssumeAlignedAttr(Aligned->getLocation(), New, E, OE,
+                         Aligned->getSpellingListIndex());
+}
+
 static void instantiateDependentEnableIfAttr(
     Sema &S, const MultiLevelTemplateArgumentList &TemplateArgs,
     const EnableIfAttr *A, const Decl *Tmpl, Decl *New) {
@@ -176,6 +199,12 @@
       continue;
     }
 
+    const AssumeAlignedAttr *AssumeAligned = dyn_cast<AssumeAlignedAttr>(TmplAttr);
+    if (AssumeAligned) {
+      instantiateDependentAssumeAlignedAttr(*this, TemplateArgs, AssumeAligned, New);
+      continue;
+    }
+
     const EnableIfAttr *EnableIf = dyn_cast<EnableIfAttr>(TmplAttr);
     if (EnableIf && EnableIf->getCond()->isValueDependent()) {
       instantiateDependentEnableIfAttr(*this, TemplateArgs, EnableIf, Tmpl,
Index: test/CodeGen/builtin-assume-aligned.c
===================================================================
--- test/CodeGen/builtin-assume-aligned.c
+++ test/CodeGen/builtin-assume-aligned.c
@@ -42,3 +42,26 @@
   return a[0];
 }
 
+int *m1() __attribute__((assume_aligned(64)));
+
+// CHECK-LABEL: @test5
+int test5() {
+  return *m1();
+// CHECK: %ptrint = ptrtoint
+// CHECK: %maskedptr = and i64 %ptrint, 63
+// CHECK: %maskcond = icmp eq i64 %maskedptr, 0
+// CHECK: call void @llvm.assume(i1 %maskcond)
+}
+
+int *m2() __attribute__((assume_aligned(64, 12)));
+
+// CHECK-LABEL: @test6
+int test6() {
+  return *m2();
+// CHECK: %ptrint = ptrtoint
+// CHECK: %offsetptr = sub i64 %ptrint, 12
+// CHECK: %maskedptr = and i64 %offsetptr, 63
+// CHECK: %maskcond = icmp eq i64 %maskedptr, 0
+// CHECK: call void @llvm.assume(i1 %maskcond)
+}
+
Index: test/Sema/builtin-assume-aligned.c
===================================================================
--- test/Sema/builtin-assume-aligned.c
+++ test/Sema/builtin-assume-aligned.c
@@ -43,3 +43,18 @@
   return a[0];
 }
 
+void test_void_assume_aligned(void) __attribute__((assume_aligned(32))); // expected-warning {{'assume_aligned' attribute only applies to return values that are pointers}}
+int test_int_assume_aligned(void) __attribute__((assume_aligned(16))); // expected-warning {{'assume_aligned' attribute only applies to return values that are pointers}}
+void *test_ptr_assume_aligned(void) __attribute__((assume_aligned(64))); // no-warning
+
+int j __attribute__((assume_aligned(8))); // expected-warning {{'assume_aligned' attribute only applies to functions and methods}}
+void *test_no_fn_proto() __attribute__((assume_aligned(32))); // no-warning
+void *test_with_fn_proto(void) __attribute__((assume_aligned(128))); // no-warning
+
+void *test_no_fn_proto() __attribute__((assume_aligned(31))); // expected-error {{requested alignment is not a power of 2}}
+void *test_no_fn_proto() __attribute__((assume_aligned(32, 73))); // no-warning
+
+void *test_no_fn_proto() __attribute__((assume_aligned)); // expected-error {{'assume_aligned' attribute takes at least 1 argument}}
+void *test_no_fn_proto() __attribute__((assume_aligned())); // expected-error {{'assume_aligned' attribute takes at least 1 argument}}
+void *test_no_fn_proto() __attribute__((assume_aligned(32, 45, 37))); // expected-error {{'assume_aligned' attribute takes no more than 2 arguments}}
+
Index: test/SemaCXX/builtin-assume-aligned-tmpl.cpp
===================================================================
--- test/SemaCXX/builtin-assume-aligned-tmpl.cpp
+++ test/SemaCXX/builtin-assume-aligned-tmpl.cpp
@@ -20,3 +20,48 @@
   return test10(a, 42); // expected-note {{in instantiation of function template specialization 'test10<int>' requested here}}
 }
 
+template <int q>
+void *atest() __attribute__((assume_aligned(q))); // expected-error {{requested alignment is not a power of 2}}
+
+template <int q, int o>
+void *atest2() __attribute__((assume_aligned(q, o))); // expected-error {{requested alignment is not a power of 2}}
+
+void test20() {
+  atest<31>(); // expected-note {{in instantiation of function template specialization 'atest<31>' requested here}}
+  atest<32>();
+
+  atest2<31, 5>(); // expected-note {{in instantiation of function template specialization 'atest2<31, 5>' requested here}}
+  atest2<32, 4>();
+}
+
+// expected-error@+1 {{invalid application of 'sizeof' to a function type}}
+template<typename T> __attribute__((assume_aligned(sizeof(int(T()))))) T *f();
+void test21() {
+  void *p = f<void>(); // expected-note {{in instantiation of function template specialization 'f<void>' requested here}}
+}
+
+// expected-error@+1 {{functional-style cast from 'void' to 'int' is not allowed}}
+template<typename T> __attribute__((assume_aligned(sizeof((int(T())))))) T *g();
+void test23() {
+  void *p = g<void>(); // expected-note {{in instantiation of function template specialization 'g<void>' requested here}}
+}
+
+template <typename T, int o>
+T *atest3() __attribute__((assume_aligned(31, o))); // expected-error {{requested alignment is not a power of 2}}
+
+template <typename T, int o>
+T *atest4() __attribute__((assume_aligned(32, o)));
+
+void test22() {
+  atest3<int, 5>();
+  atest4<int, 5>();
+}
+
+// expected-warning@+1 {{'assume_aligned' attribute only applies to functions and methods}}
+class __attribute__((assume_aligned(32))) x {
+  int y;
+};
+
+// expected-warning@+1 {{'assume_aligned' attribute only applies to return values that are pointers or references}}
+x foo() __attribute__((assume_aligned(32)));
+