Index: lib/CodeGen/CGBuiltin.cpp
===================================================================
--- lib/CodeGen/CGBuiltin.cpp
+++ lib/CodeGen/CGBuiltin.cpp
@@ -44,6 +44,22 @@
   return std::min(High, std::max(Low, Value));
 }
 
+static void initializeAlloca(CodeGenFunction &CGF, AllocaInst *AI, Value *Size, unsigned AlignmentInBytes) {
+  ConstantInt *Byte;
+  switch (CGF.getLangOpts().getTrivialAutoVarInit()) {
+  case LangOptions::TrivialAutoVarInitKind::Uninitialized:
+    // Nothing to initialize.
+    return;
+  case LangOptions::TrivialAutoVarInitKind::Zero:
+    Byte = CGF.Builder.getInt8(0);
+    break;
+  case LangOptions::TrivialAutoVarInitKind::Pattern:
+    Byte = CGF.Builder.getInt8(0xAA);
+    break;
+  }
+  CGF.Builder.CreateMemSet(AI, Byte, Size, AlignmentInBytes);
+}
+
 /// getBuiltinLibFunction - Given a builtin id for a function like
 /// "__builtin_fabsf", return a Function* for "fabsf".
 llvm::Constant *CodeGenModule::getBuiltinLibFunction(const FunctionDecl *FD,
@@ -2259,6 +2275,7 @@
             .getQuantity();
     AllocaInst *AI = Builder.CreateAlloca(Builder.getInt8Ty(), Size);
     AI->setAlignment(SuitableAlignmentInBytes);
+    initializeAlloca(*this, AI, Size, SuitableAlignmentInBytes);
     return RValue::get(AI);
   }
 
@@ -2271,6 +2288,7 @@
         CGM.getContext().toCharUnitsFromBits(AlignmentInBits).getQuantity();
     AllocaInst *AI = Builder.CreateAlloca(Builder.getInt8Ty(), Size);
     AI->setAlignment(AlignmentInBytes);
+    initializeAlloca(*this, AI, Size, AlignmentInBytes);
     return RValue::get(AI);
   }
 
Index: test/CodeGenCXX/trivial-auto-var-init.cpp
===================================================================
--- test/CodeGenCXX/trivial-auto-var-init.cpp
+++ test/CodeGenCXX/trivial-auto-var-init.cpp
@@ -173,6 +173,34 @@
   used(ptr);
 }
 
+// UNINIT-LABEL:  test_alloca(
+// ZERO-LABEL:    test_alloca(
+// ZERO:          %[[SIZE:[a-z0-9]+]] = sext i32 %{{.*}} to i64
+// ZERO-NEXT:     %[[ALLOCA:[a-z0-9]+]] = alloca i8, i64 %[[SIZE]], align [[ALIGN:[0-9]+]]
+// ZERO-NEXT:     call void @llvm.memset{{.*}}(i8* align [[ALIGN]] %[[ALLOCA]], i8 0, i64 %[[SIZE]], i1 false)
+// PATTERN-LABEL: test_alloca(
+// PATTERN:       %[[SIZE:[a-z0-9]+]] = sext i32 %{{.*}} to i64
+// PATTERN-NEXT:  %[[ALLOCA:[a-z0-9]+]] = alloca i8, i64 %[[SIZE]], align [[ALIGN:[0-9]+]]
+// PATTERN-NEXT:  call void @llvm.memset{{.*}}(i8* align [[ALIGN]] %[[ALLOCA]], i8 -86, i64 %[[SIZE]], i1 false)
+void test_alloca(int size) {
+  void *ptr = __builtin_alloca(size);
+  used(ptr);
+}
+
+// UNINIT-LABEL:  test_alloca_with_align(
+// ZERO-LABEL:    test_alloca_with_align(
+// ZERO:          %[[SIZE:[a-z0-9]+]] = sext i32 %{{.*}} to i64
+// ZERO-NEXT:     %[[ALLOCA:[a-z0-9]+]] = alloca i8, i64 %[[SIZE]], align 128
+// ZERO-NEXT:     call void @llvm.memset{{.*}}(i8* align 128 %[[ALLOCA]], i8 0, i64 %[[SIZE]], i1 false)
+// PATTERN-LABEL: test_alloca_with_align(
+// PATTERN:       %[[SIZE:[a-z0-9]+]] = sext i32 %{{.*}} to i64
+// PATTERN-NEXT:  %[[ALLOCA:[a-z0-9]+]] = alloca i8, i64 %[[SIZE]], align 128
+// PATTERN-NEXT:  call void @llvm.memset{{.*}}(i8* align 128 %[[ALLOCA]], i8 -86, i64 %[[SIZE]], i1 false)
+void test_alloca_with_align(int size) {
+  void *ptr = __builtin_alloca_with_align(size, 1024);
+  used(ptr);
+}
+
 // UNINIT-LABEL:  test_struct_vla(
 // ZERO-LABEL:    test_struct_vla(
 // ZERO:  %[[SIZE:[0-9]+]] = mul nuw i64 %{{.*}}, 16