Freeze reserved registers before starting register allocation.

The register allocators don't currently support adding reserved
registers while they are running.  Extend the MRI API to keep track of
the set of reserved registers when register allocation started.

Target hooks like hasFP() and needsStackRealignment() can look at this
set to avoid reserving more registers during register allocation.

llvm-svn: 147577

docs/ReleaseNotes.html

@@ -351,6 +351,10 @@ Release Notes</a>.</h1>
   represent combinations of constraints from instructions and sub-registers.
   The synthetic register classes inherit most of their properties form their
   closest user-defined super-class.</li>
+  <li><code>MachineRegisterInfo</code> now allows the reserved registers to be
+  frozen when register allocation starts.  Target hooks should use the
+  <code>MRI->canReserveReg(FramePtr)</code> method to avoid accidentally
+  disabling frame pointer elimination during register allocation.</li>
 </ul>
 </div>
 

include/llvm/CodeGen/MachineRegisterInfo.h

@@ -57,7 +57,13 @@ class MachineRegisterInfo {
   /// so that the code generator knows which callee save registers to save and
   /// for other target specific uses.
   BitVector UsedPhysRegs;
-  
+
+  /// ReservedRegs - This is a bit vector of reserved registers.  The target
+  /// may change its mind about which registers should be reserved.  This
+  /// vector is the frozen set of reserved registers when register allocation
+  /// started.
+  BitVector ReservedRegs;
+
   /// LiveIns/LiveOuts - Keep track of the physical registers that are
   /// livein/liveout of the function.  Live in values are typically arguments in
   /// registers, live out values are typically return values in registers.
@@ -309,6 +315,37 @@ public:
   /// subregisters. That means that if R is used, so are all subregisters.
   void closePhysRegsUsed(const TargetRegisterInfo&);
 
+
+  //===--------------------------------------------------------------------===//
+  // Reserved Register Info
+  //===--------------------------------------------------------------------===//
+  //
+  // The set of reserved registers must be invariant during register
+  // allocation.  For example, the target cannot suddenly decide it needs a
+  // frame pointer when the register allocator has already used the frame
+  // pointer register for something else.
+  //
+  // These methods can be used by target hooks like hasFP() to avoid changing
+  // the reserved register set during register allocation.
+
+  /// freezeReservedRegs - Called by the register allocator to freeze the set
+  /// of reserved registers before allocation begins.
+  void freezeReservedRegs(const MachineFunction&);
+
+  /// reservedRegsFrozen - Returns true after freezeReservedRegs() was called
+  /// to ensure the set of reserved registers stays constant.
+  bool reservedRegsFrozen() const {
+    return !ReservedRegs.empty();
+  }
+
+  /// canReserveReg - Returns true if PhysReg can be used as a reserved
+  /// register.  Any register can be reserved before freezeReservedRegs() is
+  /// called.
+  bool canReserveReg(unsigned PhysReg) const {
+    return !reservedRegsFrozen() || ReservedRegs.test(PhysReg);
+  }
+
+
   //===--------------------------------------------------------------------===//
   // LiveIn/LiveOut Management
   //===--------------------------------------------------------------------===//

lib/CodeGen/MachineRegisterInfo.cpp

@@ -259,3 +259,7 @@ void MachineRegisterInfo::dumpUses(unsigned Reg) const {
     I.getOperand().getParent()->dump();
 }
 #endif
+
+void MachineRegisterInfo::freezeReservedRegs(const MachineFunction &MF) {
+  ReservedRegs = TRI->getReservedRegs(MF);
+}

lib/CodeGen/RegAllocBasic.cpp

@@ -233,6 +233,7 @@ void RegAllocBase::init(VirtRegMap &vrm, LiveIntervals &lis) {
   MRI = &vrm.getRegInfo();
   VRM = &vrm;
   LIS = &lis;
+  MRI->freezeReservedRegs(vrm.getMachineFunction());
   RegClassInfo.runOnMachineFunction(vrm.getMachineFunction());
 
   const unsigned NumRegs = TRI->getNumRegs();

lib/CodeGen/RegAllocFast.cpp

@@ -1047,6 +1047,7 @@ bool RAFast::runOnMachineFunction(MachineFunction &Fn) {
   TM = &Fn.getTarget();
   TRI = TM->getRegisterInfo();
   TII = TM->getInstrInfo();
+  MRI->freezeReservedRegs(Fn);
   RegClassInfo.runOnMachineFunction(Fn);
   UsedInInstr.resize(TRI->getNumRegs());
 

lib/CodeGen/RegAllocPBQP.cpp

@@ -619,6 +619,7 @@ bool RegAllocPBQP::runOnMachineFunction(MachineFunction &MF) {
   vrm = &getAnalysis<VirtRegMap>();
   spiller.reset(createInlineSpiller(*this, MF, *vrm));
 
+  mri->freezeReservedRegs(MF);
 
   DEBUG(dbgs() << "PBQP Register Allocating for " << mf->getFunction()->getName() << "\n");