[LTO] Add configuartion option to use default optimization pipeline

This patch adds a configuration option to simply use the default pass
pipeline in favor of the LTO-specific one. We observed some severe
performance penalties when uding device-side LTO for OpenMP offloading
applications caused by the LTO-pass pipeline. This is primarily because
OpenMP uses an LLVM bitcode library to implement a GPU runtime library.
In a standard compilation we link this bitcode library into each source
file and optimize it with the default pipeline. When performing LTO we
link it late with all the files, but the bitcode library never has the
regular optimization pipeline applied to it so we miss a few
optimizations just using the LTO pipeline to optimize it.

I'm not committed to this solution, but it's the easiest method to solve
this performance regression when using LTO without changing the
optimizatin pipeline for other users.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D122133

clang/tools/clang-linker-wrapper/ClangLinkerWrapper.cpp

@@ -860,6 +860,7 @@ std::unique_ptr<lto::LTO> createLTO(
   // TODO: Handle index-only thin-LTO
   Backend = lto::createInProcessThinBackend(
       llvm::heavyweight_hardware_concurrency(1));
+  Conf.UseDefaultPipeline = true;
 
   Conf.CPU = Arch.str();
   Conf.Options = codegen::InitTargetOptionsFromCodeGenFlags(TheTriple);

llvm/include/llvm/LTO/Config.h

@@ -60,6 +60,9 @@ struct Config {
   /// Use the new pass manager
   bool UseNewPM = LLVM_ENABLE_NEW_PASS_MANAGER;
 
+  /// Use the standard optimization pipeline.
+  bool UseDefaultPipeline = false;
+
   /// Flag to indicate that the optimizer should not assume builtins are present
   /// on the target.
   bool Freestanding = false;

llvm/lib/LTO/LTOBackend.cpp

@@ -298,6 +298,8 @@ static void runNewPMPasses(const Config &Conf, Module &Mod, TargetMachine *TM,
       report_fatal_error(Twine("unable to parse pass pipeline description '") +
                          Conf.OptPipeline + "': " + toString(std::move(Err)));
     }
+  } else if (Conf.UseDefaultPipeline) {
+    MPM.addPass(PB.buildPerModuleDefaultPipeline(OL));
   } else if (IsThinLTO) {
     MPM.addPass(PB.buildThinLTODefaultPipeline(OL, ImportSummary));
   } else {