[Clang][Driver] Re-use the calling process instead of creating a new process for the cc1 invocation

With this patch, the clang tool will now call the -cc1 invocation directly inside the same process. Previously, the -cc1 invocation was creating, and waiting for, a new process.
This patch therefore reduces the number of created processes during a build, thus it reduces build times on platforms where process creation can be costly (Windows) and/or impacted by a antivirus.
It also makes debugging a bit easier, as there's no need to attach to the secondary -cc1 process anymore, breakpoints will be hit inside the same process.

Crashes or signaling inside the -cc1 invocation will have the same side-effect as before, and will be reported through the same means.

This behavior can be controlled at compile-time through the CLANG_SPAWN_CC1 cmake flag, which defaults to OFF. Setting it to ON will revert to the previous behavior, where any -cc1 invocation will create/fork a secondary process.
At run-time, it is also possible to tweak the CLANG_SPAWN_CC1 environment variable. Setting it and will override the compile-time setting. A value of 0 calls -cc1 inside the calling process; a value of 1 will create a secondary process, as before.

Differential Revision: https://reviews.llvm.org/D69825

1 files changed, 37 insertions, 6 deletions

diff --git a/clang/tools/driver/driver.cpp b/clang/tools/driver/driver.cpp
index f1600490017..4cdf8015b1b 100644
--- a/clang/tools/driver/driver.cpp
+++ b/clang/tools/driver/driver.cpp
@@ -14,6 +14,7 @@
 #include "clang/Driver/Driver.h"
 #include "clang/Basic/DiagnosticOptions.h"
 #include "clang/Basic/Stack.h"
+#include "clang/Config/config.h"
 #include "clang/Driver/Compilation.h"
 #include "clang/Driver/DriverDiagnostic.h"
 #include "clang/Driver/Options.h"
@@ -30,6 +31,7 @@
 #include "llvm/Option/OptTable.h"
 #include "llvm/Option/Option.h"
 #include "llvm/Support/CommandLine.h"
+#include "llvm/Support/CrashRecoveryContext.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Host.h"
@@ -239,6 +241,8 @@ static void getCLEnvVarOptions(std::string &EnvValue, llvm::StringSaver &Saver,
       *NumberSignPtr = '=';
 }
 
+static int ExecuteCC1Tool(ArrayRef<const char *> argv);
+
 static void SetBackdoorDriverOutputsFromEnvVars(Driver &TheDriver) {
   // Handle CC_PRINT_OPTIONS and CC_PRINT_OPTIONS_FILE.
   TheDriver.CCPrintOptions = !!::getenv("CC_PRINT_OPTIONS");
@@ -254,6 +258,27 @@ static void SetBackdoorDriverOutputsFromEnvVars(Driver &TheDriver) {
   TheDriver.CCLogDiagnostics = !!::getenv("CC_LOG_DIAGNOSTICS");
   if (TheDriver.CCLogDiagnostics)
     TheDriver.CCLogDiagnosticsFilename = ::getenv("CC_LOG_DIAGNOSTICS_FILE");
+
+  // Whether the cc1 tool should be called inside the current process, or if we
+  // should spawn a new clang process (old behavior).
+  // Not having an additional process saves some execution time of Windows,
+  // and makes debugging easier.
+  bool UseNewCC1Process = CLANG_SPAWN_CC1;
+
+  StringRef SpawnCC1Str = ::getenv("CLANG_SPAWN_CC1");
+  if (!SpawnCC1Str.empty()) {
+    if (SpawnCC1Str != "0" && SpawnCC1Str != "1") {
+      llvm::errs() << "error: the value of the environment variable "
+                      "CLANG_SPAWN_CC1 must be either 0 or 1.\n";
+      ::exit(1);
+    }
+    UseNewCC1Process = SpawnCC1Str[0] - '0';
+  }
+  if (!UseNewCC1Process) {
+    TheDriver.CC1Main = &ExecuteCC1Tool;
+    // Ensure the CC1Command actually catches cc1 crashes
+    llvm::CrashRecoveryContext::Enable();
+  }
 }
 
 static void FixupDiagPrefixExeName(TextDiagnosticPrinter *DiagClient,
@@ -303,13 +328,19 @@ static void SetInstallDir(SmallVectorImpl<const char *> &argv,
     TheDriver.setInstalledDir(InstalledPathParent);
 }
 
-static int ExecuteCC1Tool(ArrayRef<const char *> argv, StringRef Tool) {
+static int ExecuteCC1Tool(ArrayRef<const char *> argv) {
+  // If we call the cc1 tool from the clangDriver library (through
+  // Driver::CC1Main), we need to cleanup the options usage count. The options
+  // are currently global, and they might have been used previously by the
+  // driver.
+  llvm::cl::ResetAllOptionOccurrences();
+  StringRef Tool = argv[1];
   void *GetExecutablePathVP = (void *)(intptr_t) GetExecutablePath;
-  if (Tool == "")
+  if (Tool == "-cc1")
     return cc1_main(argv.slice(2), argv[0], GetExecutablePathVP);
-  if (Tool == "as")
+  if (Tool == "-cc1as")
     return cc1as_main(argv.slice(2), argv[0], GetExecutablePathVP);
-  if (Tool == "gen-reproducer")
+  if (Tool == "-cc1gen-reproducer")
     return cc1gen_reproducer_main(argv.slice(2), argv[0], GetExecutablePathVP);
 
   // Reject unknown tools.
@@ -379,7 +410,7 @@ int main(int argc_, const char **argv_) {
       auto newEnd = std::remove(argv.begin(), argv.end(), nullptr);
       argv.resize(newEnd - argv.begin());
     }
-    return ExecuteCC1Tool(argv, argv[1] + 4);
+    return ExecuteCC1Tool(argv);
   }
 
   bool CanonicalPrefixes = true;
@@ -503,7 +534,7 @@ int main(int argc_, const char **argv_) {
 
 #ifdef _WIN32
   // Exit status should not be negative on Win32, unless abnormal termination.
-  // Once abnormal termiation was caught, negative status should not be
+  // Once abnormal termination was caught, negative status should not be
   // propagated.
   if (Res < 0)
     Res = 1;