Index: include/clang/Driver/Options.td
===================================================================
--- include/clang/Driver/Options.td
+++ include/clang/Driver/Options.td
@@ -380,6 +380,8 @@
     HelpText<"Add missing headers to depfile">;
 def MJ : JoinedOrSeparate<["-"], "MJ">, Group<M_Group>,
     HelpText<"Write a compilation database entry per input">;
+def MJJ : JoinedOrSeparate<["-"], "MJJ">, Group<M_Group>,
+    HelpText<"Write or append to a compilation database entry per input">;
 def MP : Flag<["-"], "MP">, Group<M_Group>, Flags<[CC1Option]>,
     HelpText<"Create phony target for each dependency (other than main file)">;
 def MQ : JoinedOrSeparate<["-"], "MQ">, Group<M_Group>, Flags<[CC1Option]>,
Index: lib/Driver/ToolChains/Clang.h
===================================================================
--- lib/Driver/ToolChains/Clang.h
+++ lib/Driver/ToolChains/Clang.h
@@ -94,7 +94,8 @@
   void DumpCompilationDatabase(Compilation &C, StringRef Filename,
                                StringRef Target,
                                const InputInfo &Output, const InputInfo &Input,
-                               const llvm::opt::ArgList &Args) const;
+                               const llvm::opt::ArgList &Args,
+                               bool Append) const;
 
 public:
   Clang(const ToolChain &TC);
Index: lib/Driver/ToolChains/Clang.cpp
===================================================================
--- lib/Driver/ToolChains/Clang.cpp
+++ lib/Driver/ToolChains/Clang.cpp
@@ -1909,7 +1909,8 @@
 
 void Clang::DumpCompilationDatabase(Compilation &C, StringRef Filename,
                                     StringRef Target, const InputInfo &Output,
-                                    const InputInfo &Input, const ArgList &Args) const {
+                                    const InputInfo &Input, const ArgList &Args,
+                                    bool Append) const {
   // If this is a dry run, do not create the compilation database file.
   if (C.getArgs().hasArg(options::OPT__HASH_HASH_HASH))
     return;
@@ -1919,7 +1920,11 @@
 
   if (!CompilationDatabase) {
     std::error_code EC;
-    auto File = llvm::make_unique<llvm::raw_fd_ostream>(Filename, EC, llvm::sys::fs::F_Text);
+    llvm::sys::fs::OpenFlags Flags = llvm::sys::fs::F_Text;
+    if (Append) {
+      Flags |= llvm::sys::fs::OF_Append;
+    }
+    auto File = llvm::make_unique<llvm::raw_fd_ostream>(Filename, EC, Flags);
     if (EC) {
       D.Diag(clang::diag::err_drv_compilationdatabase) << Filename
                                                        << EC.message();
@@ -3365,8 +3370,12 @@
   CmdArgs.push_back("-triple");
   CmdArgs.push_back(Args.MakeArgString(TripleStr));
 
-  if (const Arg *MJ = Args.getLastArg(options::OPT_MJ)) {
-    DumpCompilationDatabase(C, MJ->getValue(), TripleStr, Output, Input, Args);
+  Arg *MJ = nullptr;
+  if ((MJ = Args.getLastArg(options::OPT_MJ)) ||
+      (MJ = Args.getLastArg(options::OPT_MJJ))) {
+    bool Append = MJ->getOption().matches(options::OPT_MJJ);
+    DumpCompilationDatabase(C, MJ->getValue(), TripleStr, Output, Input, Args,
+                            Append);
     Args.ClaimAllArgs(options::OPT_MJ);
   }
 
Index: test/Driver/compilation_database_append.c
===================================================================
--- /dev/null
+++ test/Driver/compilation_database_append.c
@@ -0,0 +1,16 @@
+// RUN: mkdir -p %t && cd %t
+// RUN: %clang -MD -MP --sysroot=somewhere -c -x c %s -xc++ %s -Wall -MJJ - -no-canonical-prefixes 2>&1 | FileCheck %s
+// RUN: %clang -c -x c %s -MJJ json
+// RUN: %clang -E -x c %s -MJJ json
+// RUN: cat json | FileCheck --check-prefix=DOUBLE %s
+// RUN: not %clang -c -x c %s -MJJ %s/non-existant -no-canonical-prefixes 2>&1 | FileCheck --check-prefix=ERROR %s
+
+// CHECK: { "directory": "{{.*}}",  "file": "[[SRC:[^"]+[/|\\]compilation_database_append.c]]", "output": "compilation_database_append.o", "arguments": ["{{[^"]*}}clang{{[^"]*}}", "-xc", "[[SRC]]", "--sysroot=somewhere", "-c", "-Wall",{{.*}} "--target={{[^"]+}}"]},
+// CHECK: { "directory": "{{.*}}",  "file": "[[SRC:[^"]+[/|\\]compilation_database_append.c]]", "output": "compilation_database_append.o", "arguments": ["{{[^"]*}}clang{{[^"]*}}", "-xc++", "[[SRC]]", "--sysroot=somewhere", "-c", "-Wall",{{.*}} "--target={{[^"]+}}"]},
+// DOUBLE: { "directory": "{{.*}}",  "file": "[[SRC:[^"]+[/|\\]compilation_database_append.c]]", "output": "compilation_database_append.o", "arguments": ["{{[^"]*}}clang{{[^"]*}}", "-xc", "[[SRC]]", "-c",{{.*}} "--target={{[^"]+}}"]},
+// DOUBLE-NEXT: { "directory": "{{.*}}",  "file": "[[SRC:[^"]+[/|\\]compilation_database_append.c]]", "output": "-", "arguments": ["{{[^"]*}}clang{{[^"]*}}", "-xc", "[[SRC]]", "-E",{{.*}} "--target={{[^"]+}}"]},
+// ERROR: error: compilation database '{{.*}}/non-existant' could not be opened:
+
+int main(void) {
+  return 0;
+}