Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -2772,6 +2772,10 @@
 def err_cannot_find_suitable_accessor : Error<
   "cannot find suitable %select{getter|setter}0 for property %1">;
 
+def warn_alloca : Warning<
+  "use of builtin function %0">,
+  InGroup<DiagGroup<"alloca">>, DefaultIgnore;
+
 def warn_alloca_align_alignof : Warning<
   "second argument to __builtin_alloca_with_align is supposed to be in bits">,
   InGroup<DiagGroup<"alloca-with-align-alignof">>;
Index: clang/lib/Sema/SemaChecking.cpp
===================================================================
--- clang/lib/Sema/SemaChecking.cpp
+++ clang/lib/Sema/SemaChecking.cpp
@@ -1169,6 +1169,10 @@
   case Builtin::BI__builtin_alloca_with_align:
     if (SemaBuiltinAllocaWithAlign(TheCall))
       return ExprError();
+    LLVM_FALLTHROUGH;
+  case Builtin::BI__builtin_alloca:
+    Diag(TheCall->getBeginLoc(), diag::warn_alloca)
+        << Context.BuiltinInfo.getName(BuiltinID);
     break;
   case Builtin::BI__assume:
   case Builtin::BI__builtin_assume:
Index: clang/test/Sema/warn-alloca.c
===================================================================
--- /dev/null
+++ clang/test/Sema/warn-alloca.c
@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -Walloca %s
+
+void test1(int a) {
+  __builtin_alloca(a); // expected-warning {{use of builtin function __builtin_alloca}}
+}
+
+void test2(int a) {
+  __builtin_alloca_with_align(a, 32); // expected-warning {{use of builtin function __builtin_alloca_with_align}}
+}
+