Add an MRI::tracksLiveness() flag.

Late optimization passes like branch folding and tail duplication can
transform the machine code in a way that makes it expensive to keep the
register liveness information up to date. There is a fuzzy line between
register allocation and late scheduling where the liveness information
degrades.

The MRI::tracksLiveness() flag makes the line clear: While true,
liveness information is accurate, and can be used for register
scavenging. Once the flag is false, liveness information is not
accurate, and can only be used as a hint.

Late passes generally don't need the liveness information, but they will
sometimes use the register scavenger to help update it. The scavenger
enforces strict correctness, and we have to spend a lot of code to
update register liveness that may never be used.

llvm-svn: 153511

include/llvm/CodeGen/MachineRegisterInfo.h

@@ -32,6 +32,11 @@ class MachineRegisterInfo {
   /// registers have a single def.
   bool IsSSA;
 
+  /// TracksLiveness - True while register liveness is being tracked accurately.
+  /// Basic block live-in lists, kill flags, and implicit defs may not be
+  /// accurate when after this flag is cleared.
+  bool TracksLiveness;
+
   /// VRegInfo - Information we keep for each virtual register.
   ///
   /// Each element in this list contains the register class of the vreg and the
@@ -103,6 +108,23 @@ public:
   // leaveSSA - Indicates that the machine function is no longer in SSA form.
   void leaveSSA() { IsSSA = false; }
 
+  /// tracksLiveness - Returns true when tracking register liveness accurately.
+  ///
+  /// While this flag is true, register liveness information in basic block
+  /// live-in lists and machine instruction operands is accurate. This means it
+  /// can be used to change the code in ways that affect the values in
+  /// registers, for example by the register scavenger.
+  ///
+  /// When this flag is false, liveness is no longer reliable.
+  bool tracksLiveness() const { return TracksLiveness; }
+
+  /// invalidateLiveness - Indicates that register liveness is no longer being
+  /// tracked accurately.
+  ///
+  /// This should be called by late passes that invalidate the liveness
+  /// information.
+  void invalidateLiveness() { TracksLiveness = false; }
+
   //===--------------------------------------------------------------------===//
   // Register Info
   //===--------------------------------------------------------------------===//

lib/CodeGen/MachineRegisterInfo.cpp

@@ -18,7 +18,7 @@
 using namespace llvm;
 
 MachineRegisterInfo::MachineRegisterInfo(const TargetRegisterInfo &TRI)
-  : TRI(&TRI), IsSSA(true) {
+  : TRI(&TRI), IsSSA(true), TracksLiveness(true) {
   VRegInfo.reserve(256);
   RegAllocHints.reserve(256);
   UsedPhysRegs.resize(TRI.getNumRegs());

lib/CodeGen/RegisterScavenging.cpp

@@ -83,6 +83,11 @@ void RegScavenger::enterBasicBlock(MachineBasicBlock *mbb) {
   assert((NumPhysRegs == 0 || NumPhysRegs == TRI->getNumRegs()) &&
          "Target changed?");
 
+  // It is not possible to use the register scavenger after late optimization
+  // passes that don't preserve accurate liveness information.
+  assert(MRI->tracksLiveness() &&
+         "Cannot use register scavenger with inaccurate liveness");
+
   // Self-initialize.
   if (!MBB) {
     NumPhysRegs = TRI->getNumRegs();