Stub out a new LiveDebugVariables pass.

This analysis is going to run immediately after LiveIntervals. It will stay
alive during register allocation and keep track of user variables mentioned in
DBG_VALUE instructions.

When the register allocator is moving values between registers and the stack, it
is very hard to keep track of DBG_VALUE instructions. We usually get it wrong.
This analysis maintains a data structure that makes it easy to update DBG_VALUE
instructions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@120385 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/LiveDebugVariables.cpp

@@ -0,0 +1,50 @@
+//===- LiveDebugVariables.cpp - Tracking debug info variables -------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the LiveDebugVariables analysis.
+//
+// Remove all DBG_VALUE instructions referencing virtual registers and replace
+// them with a data structure tracking where live user variables are kept - in a
+// virtual register or in a stack slot.
+//
+// Allow the data structure to be updated during register allocation when values
+// are moved between registers and stack slots. Finally emit new DBG_VALUE
+// instructions after register allocation is complete.
+//
+//===----------------------------------------------------------------------===//
+
+#include "LiveDebugVariables.h"
+#include "llvm/CodeGen/LiveIntervalAnalysis.h"
+#include "llvm/CodeGen/MachineFunction.h"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/Target/TargetMachine.h"
+
+using namespace llvm;
+
+char LiveDebugVariables::ID = 0;
+
+INITIALIZE_PASS_BEGIN(LiveDebugVariables, "livedebugvars",
+                "Debug Variable Analysis", false, false)
+INITIALIZE_PASS_DEPENDENCY(LiveIntervals)
+INITIALIZE_PASS_END(LiveDebugVariables, "livedebugvars",
+                "Debug Variable Analysis", false, false)
+
+void LiveDebugVariables::getAnalysisUsage(AnalysisUsage &AU) const {
+  AU.addRequiredTransitive<LiveIntervals>();
+  AU.setPreservesAll();
+  MachineFunctionPass::getAnalysisUsage(AU);
+}
+
+LiveDebugVariables::LiveDebugVariables() : MachineFunctionPass(ID) {
+  initializeLiveDebugVariablesPass(*PassRegistry::getPassRegistry());
+}
+
+bool LiveDebugVariables::runOnMachineFunction(MachineFunction &mf) {
+  return false;
+}