1 files changed, 50 insertions, 10 deletions

diff --git a/lib/Target/X86/X86RegisterInfo.cpp b/lib/Target/X86/X86RegisterInfo.cpp
index b22a086b24..3b727881c7 100644
--- a/lib/Target/X86/X86RegisterInfo.cpp
+++ b/lib/Target/X86/X86RegisterInfo.cpp
@@ -50,6 +50,10 @@ ForceStackAlign("force-align-stack",
                            " needed for the function."),
                  cl::init(false), cl::Hidden);
 
+cl::opt<bool>
+EnableBasePointer("x86-use-base-pointer", cl::Hidden, cl::init(true),
+          cl::desc("Enable use of a base pointer for complex stack frames"));
+
 X86RegisterInfo::X86RegisterInfo(X86TargetMachine &tm,
                                  const TargetInstrInfo &tii)
   : X86GenRegisterInfo(tm.getSubtarget<X86Subtarget>().is64Bit()
@@ -68,10 +72,12 @@ X86RegisterInfo::X86RegisterInfo(X86TargetMachine &tm,
     SlotSize = 8;
     StackPtr = X86::RSP;
     FramePtr = X86::RBP;
+    BasePtr = X86::RBX;
   } else {
     SlotSize = 4;
     StackPtr = X86::ESP;
     FramePtr = X86::EBP;
+    BasePtr = X86::EBX;
   }
 }
 
@@ -290,6 +296,20 @@ BitVector X86RegisterInfo::getReservedRegs(const MachineFunction &MF) const {
       Reserved.set(*I);
   }
 
+  // Set the base-pointer register and its aliases as reserved if needed.
+  if (hasBasePointer(MF)) {
+    CallingConv::ID CC = MF.getFunction()->getCallingConv();
+    const uint32_t* RegMask = getCallPreservedMask(CC);
+    if (MachineOperand::clobbersPhysReg(RegMask, getBaseRegister()))
+      report_fatal_error(
+        "Stack realignment in presence of dynamic allocas is not supported with"
+        "this calling convention.");
+
+    Reserved.set(getBaseRegister());
+    for (MCSubRegIterator I(getBaseRegister(), this); I.isValid(); ++I)
+      Reserved.set(*I);
+  }
+
   // Mark the segment registers as reserved.
   Reserved.set(X86::CS);
   Reserved.set(X86::SS);
@@ -340,10 +360,35 @@ BitVector X86RegisterInfo::getReservedRegs(const MachineFunction &MF) const {
 // Stack Frame Processing methods
 //===----------------------------------------------------------------------===//
 
+bool X86RegisterInfo::hasBasePointer(const MachineFunction &MF) const {
+   const MachineFrameInfo *MFI = MF.getFrameInfo();
+
+   if (!EnableBasePointer)
+     return false;
+
+   // When we need stack realignment and there are dynamic allocas, we can't 
+   // reference off of the stack pointer, so we reserve a base pointer.
+   if (needsStackRealignment(MF) && MFI->hasVarSizedObjects())
+     return true;
+
+   return false;
+}
+
 bool X86RegisterInfo::canRealignStack(const MachineFunction &MF) const {
   const MachineFrameInfo *MFI = MF.getFrameInfo();
-  return (MF.getTarget().Options.RealignStack &&
-          !MFI->hasVarSizedObjects());
+  const MachineRegisterInfo *MRI = &MF.getRegInfo();
+  if (!MF.getTarget().Options.RealignStack)
+    return false;
+
+  // Stack realignment requires a frame pointer.  If we already started
+  // register allocation with frame pointer elimination, it is too late now.
+  if (!MRI->canReserveReg(FramePtr))
+    return false;
+
+  // If base pointer is necessary.  Check that it isn't too late to reserve it.
+  if (MFI->hasVarSizedObjects())
+    return MRI->canReserveReg(BasePtr);
+  return true;
 }
 
 bool X86RegisterInfo::needsStackRealignment(const MachineFunction &MF) const {
@@ -353,13 +398,6 @@ bool X86RegisterInfo::needsStackRealignment(const MachineFunction &MF) const {
   bool requiresRealignment = ((MFI->getMaxAlignment() > StackAlign) ||
                                F->hasFnAttr(Attribute::StackAlignment));
 
-  // FIXME: Currently we don't support stack realignment for functions with
-  //        variable-sized allocas.
-  // FIXME: It's more complicated than this...
-  if (0 && requiresRealignment && MFI->hasVarSizedObjects())
-    report_fatal_error(
-      "Stack realignment in presence of dynamic allocas is not supported");
-
   // If we've requested that we force align the stack do so now.
   if (ForceStackAlign)
     return canRealignStack(MF);
@@ -499,7 +537,9 @@ X86RegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator II,
 
   unsigned Opc = MI.getOpcode();
   bool AfterFPPop = Opc == X86::TAILJMPm64 || Opc == X86::TAILJMPm;
-  if (needsStackRealignment(MF))
+  if (hasBasePointer(MF))
+    BasePtr = getBaseRegister();
+  else if (needsStackRealignment(MF))
     BasePtr = (FrameIndex < 0 ? FramePtr : StackPtr);
   else if (AfterFPPop)
     BasePtr = StackPtr;