diff --git a/.github/workflows/llvm.yml b/.github/workflows/llvm.yml
new file mode 100644
index 0000000000000..e5541162064e0
--- /dev/null
+++ b/.github/workflows/llvm.yml
@@ -0,0 +1,34 @@
+name: CI
+
+on: [push]
+
+jobs:
+  build_llvm:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os:
+          - ubuntu-latest
+          - windows-latest
+          - macOS-latest
+        cmake_args:
+          - "-DLLVM_ENABLE_PROJECTS='clang'"
+    steps:
+    - name: Setup Windows
+      if: startsWith(matrix.os, 'windows')
+      uses: tstellar/actions/setup-windows@master
+      with:
+        arch: amd64
+    - uses: actions/checkout@v1
+      with:
+        fetch-depth: 1
+    - name: Install Ninja
+      uses: tstellar/actions/install-ninja@master
+      with:
+        os: ${{ runner.os }}
+    - name: Test LLVM
+      uses: tstellar/actions/build-test-llvm-project@master
+      with:
+        cmake_args: -G Ninja -DCMAKE_BUILD_TYPE=Release ${{ matrix.cmake_args }}
+        os: ${{ runner.os }}
diff --git a/clang/include/clang/AST/Decl.h b/clang/include/clang/AST/Decl.h
index f4913540bab4d..b6d803cf3e768 100644
--- a/clang/include/clang/AST/Decl.h
+++ b/clang/include/clang/AST/Decl.h
@@ -2272,6 +2272,9 @@ class FunctionDecl : public DeclaratorDecl,
   /// true through IsAligned.
   bool isReplaceableGlobalAllocationFunction(bool *IsAligned = nullptr) const;
 
+  /// Determine if this function provides an inline implementation of a builtin.
+  bool isInlineBuiltinDeclaration() const;
+
   /// Determine whether this is a destroying operator delete.
   bool isDestroyingOperatorDelete() const;
 
diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
index a3ba6ebbd0fb6..6ccf602579878 100644
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -3003,6 +3003,14 @@ bool FunctionDecl::isReplaceableGlobalAllocationFunction(bool *IsAligned) const
   return Params == FPT->getNumParams();
 }
 
+bool FunctionDecl::isInlineBuiltinDeclaration() const {
+  if (!getBuiltinID())
+    return false;
+
+  const FunctionDecl *Definition;
+  return hasBody(Definition) && Definition->isInlineSpecified();
+}
+
 bool FunctionDecl::isDestroyingOperatorDelete() const {
   // C++ P0722:
   //   Within a class C, a single object deallocation function with signature
diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
index cd42faefab00b..76b89cc44ef1a 100644
--- a/clang/lib/CodeGen/CGExpr.cpp
+++ b/clang/lib/CodeGen/CGExpr.cpp
@@ -4596,8 +4596,15 @@ RValue CodeGenFunction::EmitSimpleCallExpr(const CallExpr *E,
 }
 
 static CGCallee EmitDirectCallee(CodeGenFunction &CGF, const FunctionDecl *FD) {
+
   if (auto builtinID = FD->getBuiltinID()) {
-    return CGCallee::forBuiltin(builtinID, FD);
+    // Replaceable builtin provide their own implementation of a builtin. Unless
+    // we are in the builtin implementation itself, don't call the actual
+    // builtin. If we are in the builtin implementation, avoid trivial infinite
+    // recursion.
+    if (!FD->isInlineBuiltinDeclaration() ||
+        CGF.CurFn->getName() == FD->getName())
+      return CGCallee::forBuiltin(builtinID, FD);
   }
 
   llvm::Constant *calleePtr = EmitFunctionDeclPointer(CGF.CGM, FD);
diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 4959b80faec7a..6ddbb3903e754 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -1831,6 +1831,11 @@ void CodeGenModule::SetFunctionAttributes(GlobalDecl GD, llvm::Function *F,
   else if (const auto *SA = FD->getAttr<SectionAttr>())
      F->setSection(SA->getName());
 
+  if (FD->isInlineBuiltinDeclaration()) {
+    F->addAttribute(llvm::AttributeList::FunctionIndex,
+                    llvm::Attribute::NoBuiltin);
+  }
+
   if (FD->isReplaceableGlobalAllocationFunction()) {
     // A replaceable global allocation function does not act like a builtin by
     // default, only if it is invoked by a new-expression or delete-expression.
diff --git a/clang/test/CodeGen/memcpy-nobuiltin.c b/clang/test/CodeGen/memcpy-nobuiltin.c
new file mode 100644
index 0000000000000..fb51d87413a16
--- /dev/null
+++ b/clang/test/CodeGen/memcpy-nobuiltin.c
@@ -0,0 +1,15 @@
+// RUN: %clang_cc1 -verify -S -emit-llvm -o- %s -isystem %S -DWITH_DECL | FileCheck --check-prefix=CHECK-WITH-DECL %s
+// RUN: %clang_cc1 -verify -S -emit-llvm -o- %s -isystem %S -UWITH_DECL | FileCheck --check-prefix=CHECK-NO-DECL %s
+// RUN: %clang_cc1 -verify -S -emit-llvm -o- %s -isystem %S -DWITH_SELF_REFERENCE_DECL | FileCheck --check-prefix=CHECK-SELF-REF-DECL %s
+//
+// CHECK-WITH-DECL-NOT: @llvm.memcpy
+// CHECK-NO-DECL: @llvm.memcpy
+// CHECK-SELF-REF-DECL: @llvm.memcpy
+//
+#include <memcpy-nobuiltin.inc>
+void test(void *dest, void const *from, size_t n) {
+  memcpy(dest, from, n);
+
+  static char buffer[1];
+  memcpy(buffer, from, 2); // expected-warning {{'memcpy' will always overflow; destination buffer has size 1, but size argument is 2}}
+}
diff --git a/clang/test/CodeGen/memcpy-nobuiltin.inc b/clang/test/CodeGen/memcpy-nobuiltin.inc
new file mode 100644
index 0000000000000..25eab0a9ffd02
--- /dev/null
+++ b/clang/test/CodeGen/memcpy-nobuiltin.inc
@@ -0,0 +1,19 @@
+#include <stddef.h>
+extern void *memcpy(void *dest, void const *from, size_t n);
+
+#ifdef WITH_DECL
+inline void *memcpy(void *dest, void const *from, size_t n) {
+  char const *ifrom = from;
+  char *idest = dest;
+  while (n--)
+    *idest++ = *ifrom++;
+  return dest;
+}
+#endif
+#ifdef WITH_SELF_REFERENCE_DECL
+inline void *memcpy(void *dest, void const *from, size_t n) {
+  if (n != 0)
+    memcpy(dest, from, n);
+  return dest;
+}
+#endif