CodeGen: Use a single SlotTracker in MachineFunction::print()

Expose enough of the IR-level `SlotTracker` so that
`MachineFunction::print()` can use a single one for printing
`BasicBlock`s.  Next step would be to lift this through a few more APIs
so that we can make other print methods faster.

Fixes PR23865, changing the runtime of `llc -print-machineinstrs` from
many minutes (killed after 3 minutes, but it wasn't very close) to
13 seconds for a 502185 line dump.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240842 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/ModuleSlotTracker.h

@@ -0,0 +1,63 @@
+//===-- llvm/IR/ModuleSlotTracker.h -----------------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_IR_MODULESLOTTRACKER_H
+#define LLVM_IR_MODULESLOTTRACKER_H
+
+#include <memory>
+
+namespace llvm {
+
+class Module;
+class Function;
+class SlotTracker;
+
+/// Manage lifetime of a slot tracker for printing IR.
+///
+/// Wrapper around the \a SlotTracker used internally by \a AsmWriter.  This
+/// class allows callers to share the cost of incorporating the metadata in a
+/// module or a function.
+///
+/// If the IR changes from underneath \a ModuleSlotTracker, strings like
+/// "<badref>" will be printed, or, worse, the wrong slots entirely.
+class ModuleSlotTracker {
+  /// Storage for a slot tracker.
+  std::unique_ptr<SlotTracker> MachineStorage;
+
+  const Module *M = nullptr;
+  const Function *F = nullptr;
+  SlotTracker *Machine = nullptr;
+
+public:
+  /// Wrap a preinitialized SlotTracker.
+  ModuleSlotTracker(SlotTracker &Machine, const Module *M,
+                    const Function *F = nullptr);
+
+  /// Construct a slot tracker from a module.
+  ///
+  /// If \a M is \c nullptr, uses a null slot tracker.
+  explicit ModuleSlotTracker(const Module *M);
+
+  /// Destructor to clean up storage.
+  ~ModuleSlotTracker();
+
+  SlotTracker *getMachine() const { return Machine; }
+  const Module *getModule() const { return M; }
+  const Function *getCurrentFunction() const { return F; }
+
+  /// Incorporate the given function.
+  ///
+  /// Purge the currently incorporated function and incorporate \c F.  If \c F
+  /// is currently incorporated, this is a no-op.
+  void incorporateFunction(const Function &F);
+};
+
+} // end namespace llvm
+
+#endif