Generalize the pass registration mechanism used by Polly to any third-party tool

There's quite a lot of references to Polly in the LLVM CMake codebase. However
the registration pattern used by Polly could be useful to other external
projects: thanks to that mechanism it would be possible to develop LLVM
extension without touching the LLVM code base.

This patch has two effects:

1. Remove all code specific to Polly in the llvm/clang codebase, replaicing it
   with a generic mechanism

2. Provide a generic mechanism to register compiler extensions.

A compiler extension is similar to a pass plugin, with the notable difference
that the compiler extension can be configured to be built dynamically (like
plugins) or statically (like regular passes).

As a result, people willing to add extra passes to clang/opt can do it using a
separate code repo, but still have their pass be linked in clang/opt as built-in
passes.

Differential Revision: https://reviews.llvm.org/D61446

3 files changed, 83 insertions, 0 deletions

diff --git a/llvm/examples/Bye/Bye.cpp b/llvm/examples/Bye/Bye.cpp
new file mode 100644
index 00000000000..6a2fea44d4c
--- /dev/null
+++ b/llvm/examples/Bye/Bye.cpp
@@ -0,0 +1,69 @@
+#include "llvm/IR/Function.h"
+#include "llvm/IR/LegacyPassManager.h"
+#include "llvm/Pass.h"
+#include "llvm/Passes/PassBuilder.h"
+#include "llvm/Passes/PassPlugin.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/Transforms/IPO/PassManagerBuilder.h"
+
+using namespace llvm;
+
+static cl::opt<bool> Wave("wave-goodbye", cl::init(false),
+                          cl::desc("wave good bye"));
+
+namespace {
+
+bool runBye(Function &F) {
+  if (Wave) {
+    errs() << "Bye: ";
+    errs().write_escaped(F.getName()) << '\n';
+  }
+  return false;
+}
+
+struct LegacyBye : public FunctionPass {
+  static char ID;
+  LegacyBye() : FunctionPass(ID) {}
+  bool runOnFunction(Function &F) override { return runBye(F); }
+};
+
+struct Bye : PassInfoMixin<Bye> {
+  PreservedAnalyses run(Function &F, FunctionAnalysisManager &) {
+    if (!runBye(F))
+      return PreservedAnalyses::all();
+    return PreservedAnalyses::none();
+  }
+};
+
+} // namespace
+
+char LegacyBye::ID = 0;
+
+static RegisterPass<LegacyBye> X("goodbye", "Good Bye World Pass",
+                                 false /* Only looks at CFG */,
+                                 false /* Analysis Pass */);
+
+/* Legacy PM Registration */
+static llvm::RegisterStandardPasses RegisterBye(
+    llvm::PassManagerBuilder::EP_EarlyAsPossible,
+    [](const llvm::PassManagerBuilder &Builder,
+       llvm::legacy::PassManagerBase &PM) { PM.add(new LegacyBye()); });
+
+/* New PM Registration */
+llvm::PassPluginLibraryInfo getByePluginInfo() {
+  return {LLVM_PLUGIN_API_VERSION, "Bye", LLVM_VERSION_STRING,
+          [](PassBuilder &PB) {
+            PB.registerVectorizerStartEPCallback(
+                [](llvm::FunctionPassManager &PM,
+                   llvm::PassBuilder::OptimizationLevel Level) {
+                  PM.addPass(Bye());
+                });
+          }};
+}
+
+#ifndef LLVM_BYE_LINK_INTO_TOOLS
+extern "C" LLVM_ATTRIBUTE_WEAK ::llvm::PassPluginLibraryInfo
+llvmGetPassPluginInfo() {
+  return getByePluginInfo();
+}
+#endif
diff --git a/llvm/examples/Bye/CMakeLists.txt b/llvm/examples/Bye/CMakeLists.txt
new file mode 100644
index 00000000000..9ce49fcc56e
--- /dev/null
+++ b/llvm/examples/Bye/CMakeLists.txt
@@ -0,0 +1,13 @@
+add_llvm_pass_plugin(Bye Bye.cpp)
+if (LLVM_LINK_LLVM_DYLIB)
+  target_link_libraries(Bye PUBLIC LLVM)
+else()
+  target_link_libraries(Bye
+    PUBLIC
+    LLVMSupport
+    LLVMCore
+    LLVMipo
+    LLVMPasses
+    )
+endif()
+
diff --git a/llvm/examples/CMakeLists.txt b/llvm/examples/CMakeLists.txt
index ad99d4c7e31..a464d52e28e 100644
--- a/llvm/examples/CMakeLists.txt
+++ b/llvm/examples/CMakeLists.txt
@@ -6,6 +6,7 @@ add_subdirectory(LLJITExamples)
 add_subdirectory(Kaleidoscope)
 add_subdirectory(ModuleMaker)
 add_subdirectory(SpeculativeJIT)
+add_subdirectory(Bye)
 
 if(LLVM_ENABLE_EH AND (NOT WIN32) AND (NOT "${LLVM_NATIVE_ARCH}" STREQUAL "ARM"))
     add_subdirectory(ExceptionDemo)