Introduce a BarrierNoop pass, a hack designed to allow *some* control

over the implicitly-formed-and-nesting CGSCC pass manager and function
pass managers, especially when using them on the opt commandline or
using extension points in the module builder. The '-barrier' opt flag
(or the pass itself) will create a no-op module pass in the pipeline,
resetting the pass manager stack, and allowing the creation of a new
pipeline of function passes or CGSCC passes to be created that is
independent from any previous pipelines.

For example, this can be used to test running two CGSCC passes in
independent CGSCC pass managers as opposed to in the same CGSCC pass
manager. It also allows us to introduce a further hack into the
PassManagerBuilder to separate the O0 pipeline extension passes from the
always-inliner's CGSCC pass manager, which they likely do not want to
participate in... At the very least none of the Sanitizer passes want
this behavior.

This fixes a bug with ASan at O0 currently, and I'll commit the ASan
test which covers this pass. I'm happy to add a test case that this pass
exists and works, but not sure how much time folks would like me to
spend adding test cases for the details of its behavior of partition
pass managers.... The whole thing is just vile, and mostly intended to
unblock ASan, so I'm hoping to rip this all out in a brave new pass
manager world.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@166172 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/InitializePasses.h

@@ -66,6 +66,7 @@ void initializeAliasDebuggerPass(PassRegistry&);
 void initializeAliasSetPrinterPass(PassRegistry&);
 void initializeAlwaysInlinerPass(PassRegistry&);
 void initializeArgPromotionPass(PassRegistry&);
+void initializeBarrierNoopPass(PassRegistry&);
 void initializeBasicAliasAnalysisPass(PassRegistry&);
 void initializeBasicCallGraphPass(PassRegistry&);
 void initializeBlockExtractorPassPass(PassRegistry&);

include/llvm/Transforms/IPO.h

@@ -198,6 +198,11 @@ ModulePass *createPartialInliningPass();
 //
 ModulePass *createMetaRenamerPass();
 
+//===----------------------------------------------------------------------===//
+/// createBarrierNoopPass - This pass is purely a module pass barrier in a pass
+/// manager.
+ModulePass *createBarrierNoopPass();
+
 } // End llvm namespace
 
 #endif

lib/Transforms/IPO/BarrierNoopPass.cpp

@@ -0,0 +1,47 @@
+//===- BarrierNoopPass.cpp - A barrier pass for the pass manager ----------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// NOTE: DO NOT USE THIS IF AVOIDABLE
+//
+// This pass is a nonce pass intended to allow manipulation of the implicitly
+// nesting pass manager. For example, it can be used to cause a CGSCC pass
+// manager to be closed prior to running a new collection of function passes.
+//
+// FIXME: This is a huge HACK. This should be removed when the pass manager's
+// nesting is made explicit instead of implicit.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Pass.h"
+#include "llvm/Transforms/IPO.h"
+using namespace llvm;
+
+namespace {
+/// \brief A nonce module pass used to place a barrier in a pass manager.
+///
+/// There is no mechanism for ending a CGSCC pass manager once one is started.
+/// This prevents extension points from having clear deterministic ordering
+/// when they are phrased as non-module passes.
+class BarrierNoop : public ModulePass {
+public:
+  static char ID; // Pass identification.
+
+  BarrierNoop() : ModulePass(ID) {
+    initializeBarrierNoopPass(*PassRegistry::getPassRegistry());
+  }
+
+  bool runOnModule(Module &M) { return false; }
+};
+}
+
+ModulePass *llvm::createBarrierNoopPass() { return new BarrierNoop(); }
+
+char BarrierNoop::ID = 0;
+INITIALIZE_PASS(BarrierNoop, "barrier", "A No-Op Barrier Pass",
+                false, false)

lib/Transforms/IPO/CMakeLists.txt

@@ -1,5 +1,6 @@
 add_llvm_library(LLVMipo
   ArgumentPromotion.cpp
+  BarrierNoopPass.cpp
   ConstantMerge.cpp
   DeadArgumentElimination.cpp
   ExtractGV.cpp

lib/Transforms/IPO/PassManagerBuilder.cpp

@@ -119,6 +119,14 @@ void PassManagerBuilder::populateModulePassManager(PassManagerBase &MPM) {
       MPM.add(Inliner);
       Inliner = 0;
     }
+
+    // FIXME: This is a HACK! The inliner pass above implicitly creates a CGSCC
+    // pass manager, but we don't want to add extensions into that pass manager.
+    // To prevent this we must insert a no-op module pass to reset the pass
+    // manager to get the same behavior as EP_OptimizerLast in non-O0 builds.
+    if (!GlobalExtensions->empty() || !Extensions.empty())
+      MPM.add(createBarrierNoopPass());
+
     addExtensionsToPM(EP_EnabledOnOptLevel0, MPM);
     return;
   }