Index: clang/include/clang/Driver/Compilation.h
===================================================================
--- clang/include/clang/Driver/Compilation.h
+++ clang/include/clang/Driver/Compilation.h
@@ -124,12 +124,20 @@
   /// Whether to keep temporary files regardless of -save-temps.
   bool ForceKeepTempFiles = false;
 
+  // XXX comment
+  // XXX pass Driver Diags object?
+  int (*CC1Call)(const llvm::opt::ArgStringList &CC1Args) = nullptr;
+
 public:
   Compilation(const Driver &D, const ToolChain &DefaultToolChain,
               llvm::opt::InputArgList *Args,
               llvm::opt::DerivedArgList *TranslatedArgs, bool ContainsError);
   ~Compilation();
 
+  void setCC1Call(int (*CC1CallFn)(const llvm::opt::ArgStringList &CC1Args)) {
+    CC1Call = CC1CallFn;
+  }
+
   const Driver &getDriver() const { return TheDriver; }
 
   const ToolChain &getDefaultToolChain() const { return DefaultToolChain; }
Index: clang/lib/Driver/Compilation.cpp
===================================================================
--- clang/lib/Driver/Compilation.cpp
+++ clang/lib/Driver/Compilation.cpp
@@ -150,6 +150,21 @@
 
 int Compilation::ExecuteCommand(const Command &C,
                                 const Command *&FailingCommand) const {
+
+  // XXX do this checking in ExecuteJobs()
+  // XXX maybe don't do this in the first caller of ExecuteJobs()
+  // (the one that makes stuff crash detection)
+  // XXX only do this if the command is actually a clang -cc1 command too
+  // XXX or cc1as... (P4)
+  // XXX share code for this conditional
+  // XXX could do this with more than 1 job too as long as they're all cc1
+  // commands. could even do this just for the cc1 commands even if there are
+  // other commands. this gives a kind of jumbo behavior...
+  // XXX if doing more than one in-process cc1, maybe reuse SourceManager etc?
+  // v2...
+  // XXX only if Redirects is empty too
+  bool CanExec = Jobs.size() == 1 && isa<driver::Command>(*Jobs.begin());
+
   if ((getDriver().CCPrintOptions ||
        getArgs().hasArg(options::OPT_v)) && !getDriver().CCGenDiagnostics) {
     raw_ostream *OS = &llvm::errs();
@@ -174,15 +189,43 @@
     if (getDriver().CCPrintOptions)
       *OS << "[Logging clang options]";
 
+    // XXX want "exec " prefix here too probably
+    if (CanExec)
+      *OS << "exec ";
     C.Print(*OS, "\n", /*Quote=*/getDriver().CCPrintOptions);
   }
 
-  std::string Error;
   bool ExecutionFailed;
-  int Res = C.Execute(Redirects, &Error, &ExecutionFailed);
-  if (!Error.empty()) {
-    assert(Res && "Error string set with 0 result code!");
-    getDriver().Diag(diag::err_drv_command_failure) << Error;
+  int Res;
+
+  if (!CanExec) {
+    std::string Error;
+    Res = C.Execute(Redirects, &Error, &ExecutionFailed);
+    if (!Error.empty()) {
+      assert(Res && "Error string set with 0 result code!");
+      getDriver().Diag(diag::err_drv_command_failure) << Error;
+    }
+  } else {
+    // XXX Driver must not depend on Frontend, hence need a delegate
+    assert(CC1Call);
+    // XXX need ensureStackAddressSpace(), ensureSufficientStack
+    // from cc1_main() when doing in-process stuff
+    // XXX also the timetrace bits
+    // XXX also why is --print-supported-cpus a cc1 option and
+    // implemented so weirdly?
+    // XXX and the llvm error handler, jeez
+    // XXX call ExecuteCompilerInvocation
+    // XXX make sure -disable-free is honored (and do it for driver
+    // too?)
+    // XXX this breaks the crash handling (compare
+    // `#pragma clang __debug parser_crash` with and without exec). Maybe
+    // in the signal handler for crashes, exec driver again with -fno-exec
+    // so that that (hopefully) produces a crash report?
+    const driver::Command &Cmd = cast<driver::Command>(*Jobs.begin());
+    assert(&Cmd == &C);
+    const ArgStringList &CCArgs = Cmd.getArguments();
+    Res = CC1Call(CCArgs);
+    ExecutionFailed = Res != 0;
   }
 
   if (Res)
Index: clang/lib/Driver/Driver.cpp
===================================================================
--- clang/lib/Driver/Driver.cpp
+++ clang/lib/Driver/Driver.cpp
@@ -1450,6 +1450,7 @@
     Compilation &C,
     SmallVectorImpl<std::pair<int, const Command *>> &FailingCommands) {
   // Just print if -### was present.
+  // XXX do same change at other OPT__HASH_HASH_HASH handling sites
   if (C.getArgs().hasArg(options::OPT__HASH_HASH_HASH)) {
     C.getJobs().Print(llvm::errs(), "\n", true);
     return 0;
Index: clang/lib/Driver/Job.cpp
===================================================================
--- clang/lib/Driver/Job.cpp
+++ clang/lib/Driver/Job.cpp
@@ -434,6 +434,10 @@
 
 void JobList::Print(raw_ostream &OS, const char *Terminator, bool Quote,
                     CrashReportInfo *CrashInfo) const {
+  // XXX share code for this conditional
+  bool CanExec = Jobs.size() == 1 && isa<driver::Command>(*begin());
+  if (CanExec)
+    OS << "exec ";
   for (const auto &Job : *this)
     Job.Print(OS, Terminator, Quote, CrashInfo);
 }
Index: clang/tools/driver/driver.cpp
===================================================================
--- clang/tools/driver/driver.cpp
+++ clang/tools/driver/driver.cpp
@@ -318,6 +318,20 @@
   return 1;
 }
 
+static int cc1call(const ArgStringList& CC1Args) {
+  void *GetExecutablePathVP = (void *)(intptr_t) GetExecutablePath;
+
+  ArrayRef<const char *> argv = CC1Args;
+  //for (const char *A : argv) {
+    //fprintf(stderr, "%s ", A);
+  //}
+
+  // XXX argv[0] is "-cc1"
+  // XXX also not clear if we want to call cc1_main here or something more
+  // granular
+  return cc1_main(argv.slice(1), argv[0], GetExecutablePathVP);
+}
+
 int main(int argc_, const char **argv_) {
   noteBottomOfStack();
   llvm::InitLLVM X(argc_, argv_);
@@ -459,6 +473,12 @@
   int Res = 1;
   if (C && !C->containsError()) {
     SmallVector<std::pair<int, const Command *>, 4> FailingCommands;
+
+    // XXX should ExecuteCompilation() even return if this is set?
+    // Probably yes, for TimerGroup flushing and stuff?
+    // XXX which function do i want to pass?
+    C->setCC1Call(cc1call);
+
     Res = TheDriver.ExecuteCompilation(*C, FailingCommands);
 
     // Force a crash to test the diagnostics.