[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, 4 insertions, 0 deletions

diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index 4d924e072f5..04c401ff880 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -5979,6 +5979,10 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
     // fails, so that the main compilation's fallback to cl.exe runs.
     C.addCommand(std::make_unique<ForceSuccessCommand>(JA, *this, Exec,
                                                         CmdArgs, Inputs));
+  } else if (D.CC1Main && !D.CCGenDiagnostics) {
+    // Invoke the CC1 directly in this process
+    C.addCommand(
+        std::make_unique<CC1Command>(JA, *this, Exec, CmdArgs, Inputs));
   } else {
     C.addCommand(std::make_unique<Command>(JA, *this, Exec, CmdArgs, Inputs));
   }