Add support for dynamic stack realignment in the presence of dynamic allocas on

X86.  Basically, this is a reapplication of r158087 with a few fixes.

Specifically, (1) the stack pointer is restored from the base pointer before
popping callee-saved registers and (2) in obscure cases (see comments in patch)
we must cache the value of the original stack adjustment in the prologue and
apply it in the epilogue.

rdar://11496434


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@160002 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/MachineFrameInfo.h

@@ -215,6 +215,10 @@ class MachineFrameInfo {
   /// just allocate them normally.
   bool UseLocalStackAllocationBlock;
 
+  /// After the stack pointer has been restore from the base pointer we
+  /// use a cached adjusment.  Currently only used for x86.
+  int64_t BPAdj;
+
 public:
     explicit MachineFrameInfo(const TargetFrameLowering &tfi) : TFI(tfi) {
     StackSize = NumFixedObjects = OffsetAdjustment = MaxAlignment = 0;
@@ -230,6 +234,7 @@ public:
     LocalFrameSize = 0;
     LocalFrameMaxAlign = 0;
     UseLocalStackAllocationBlock = false;
+    BPAdj = 0;
   }
 
   /// hasStackObjects - Return true if there are any stack objects in this
@@ -538,6 +543,16 @@ public:
 
   void setCalleeSavedInfoValid(bool v) { CSIValid = v; }
 
+  /// setBasePtrStackAdjustment - If we're restoring the stack pointer from the
+  /// base pointer, due to dynamic stack realignment + VLAs, we cache the
+  /// number of bytes initially allocated for the stack frame.  In obscure
+  /// cases (e.g., tail calls with byval argument and no stack protector), the
+  /// stack gets adjusted outside of the prolog, but these shouldn't be 
+  /// considered when restoring from the base pointer.  Currently, this is only
+  /// needed for x86.
+  void setBasePtrStackAdjustment(int64_t adj) { BPAdj = adj; }
+  int64_t getBasePtrStackAdjustment() const { return BPAdj; }
+
   /// getPristineRegs - Return a set of physical registers that are pristine on
   /// entry to the MBB.
   ///

lib/Target/X86/X86FrameLowering.cpp

@@ -650,6 +650,7 @@ void X86FrameLowering::emitPrologue(MachineFunction &MF) const {
   unsigned SlotSize = RegInfo->getSlotSize();
   unsigned FramePtr = RegInfo->getFrameRegister(MF);
   unsigned StackPtr = RegInfo->getStackRegister();
+  unsigned BasePtr = RegInfo->getBaseRegister();
   DebugLoc DL;
 
   // If we're forcing a stack realignment we can't rely on just the frame
@@ -913,6 +914,20 @@ void X86FrameLowering::emitPrologue(MachineFunction &MF) const {
     emitSPUpdate(MBB, MBBI, StackPtr, -(int64_t)NumBytes, Is64Bit,
                  UseLEA, TII, *RegInfo);
 
+  // If we need a base pointer, set it up here. It's whatever the value
+  // of the stack pointer is at this point. Any variable size objects
+  // will be allocated after this, so we can still use the base pointer
+  // to reference locals.
+  if (RegInfo->hasBasePointer(MF)) {
+    // Update the frame pointer with the current stack pointer.
+    unsigned Opc = Is64Bit ? X86::MOV64rr : X86::MOV32rr;
+    BuildMI(MBB, MBBI, DL, TII.get(Opc), BasePtr)
+      .addReg(StackPtr)
+      .setMIFlag(MachineInstr::FrameSetup);
+
+    MFI->setBasePtrStackAdjustment(NumBytes);
+  }
+
   if (( (!HasFP && NumBytes) || PushedRegs) && needsFrameMoves) {
     // Mark end of stack pointer adjustment.
     MCSymbol *Label = MMI.getContext().CreateTempSymbol();
@@ -960,6 +975,7 @@ void X86FrameLowering::emitEpilogue(MachineFunction &MF,
   unsigned SlotSize = RegInfo->getSlotSize();
   unsigned FramePtr = RegInfo->getFrameRegister(MF);
   unsigned StackPtr = RegInfo->getStackRegister();
+  unsigned BasePtr = RegInfo->getBaseRegister();
 
   switch (RetOpcode) {
   default:
@@ -1029,6 +1045,15 @@ void X86FrameLowering::emitEpilogue(MachineFunction &MF,
   if (NumBytes || MFI->hasVarSizedObjects())
     mergeSPUpdatesUp(MBB, MBBI, StackPtr, &NumBytes);
 
+  // Restore the SP from the BP, if necessary.
+  if (RegInfo->hasBasePointer(MF)) {
+    BuildMI(MBB, MBBI, DL, TII.get(Is64Bit ? X86::MOV64rr : X86::MOV32rr),
+            StackPtr).addReg(BasePtr);
+
+    // When restoring from the BP we must use a cached SP adjustment.
+    NumBytes = MFI->getBasePtrStackAdjustment();
+  }
+
   // If dynamic alloca is used, then reset esp to point to the last callee-saved
   // slot before popping them off! Same applies for the case, when stack was
   // realigned.
@@ -1147,7 +1172,16 @@ int X86FrameLowering::getFrameIndexOffset(const MachineFunction &MF, int FI) con
   int Offset = MFI->getObjectOffset(FI) - getOffsetOfLocalArea();
   uint64_t StackSize = MFI->getStackSize();
 
-  if (RegInfo->needsStackRealignment(MF)) {
+  if (RegInfo->hasBasePointer(MF)) {
+    assert (hasFP(MF) && "VLAs and dynamic stack realign, but no FP?!");
+    if (FI < 0) {
+      // Skip the saved EBP.
+      return Offset + RegInfo->getSlotSize();
+    } else {
+      assert((-(Offset + StackSize)) % MFI->getObjectAlignment(FI) == 0);
+      return Offset + StackSize;
+    }
+  } else if (RegInfo->needsStackRealignment(MF)) {
     if (FI < 0) {
       // Skip the saved EBP.
       return Offset + RegInfo->getSlotSize();
@@ -1178,9 +1212,14 @@ int X86FrameLowering::getFrameIndexReference(const MachineFunction &MF, int FI,
   const X86RegisterInfo *RegInfo =
       static_cast<const X86RegisterInfo*>(MF.getTarget().getRegisterInfo());
   // We can't calculate offset from frame pointer if the stack is realigned,
-  // so enforce usage of stack pointer.
-  FrameReg = (RegInfo->needsStackRealignment(MF)) ? 
-    RegInfo->getStackRegister() : RegInfo->getFrameRegister(MF);
+  // so enforce usage of stack/base pointer.  The base pointer is used when we
+  // have dynamic allocas in addition to dynamic realignment.
+  if (RegInfo->hasBasePointer(MF))
+    FrameReg = RegInfo->getBaseRegister();
+  else if (RegInfo->needsStackRealignment(MF))
+    FrameReg = RegInfo->getStackRegister();
+  else
+    FrameReg = RegInfo->getFrameRegister(MF);
   return getFrameIndexOffset(MF, FI);
 }
 
@@ -1317,6 +1356,10 @@ X86FrameLowering::processFunctionBeforeCalleeSavedScan(MachineFunction &MF,
            "Slot for EBP register must be last in order to be found!");
     (void)FrameIdx;
   }
+
+  // Spill the BasePtr if it's used.
+  if (RegInfo->hasBasePointer(MF))
+    MF.getRegInfo().setPhysRegUsed(RegInfo->getBaseRegister());
 }
 
 static bool

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 +360,36 @@ 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 a 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 +399,6 @@ 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 +538,9 @@ X86RegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator II,
 
   unsigned Opc = MI.getOpcode();
   bool AfterFPPop = Opc == X86::TAILJMPm64 || Opc == X86::TAILJMPm;
-  if (needsStackRealignment(MF))
+  if (hasBasePointer(MF))
+    BasePtr = (FrameIndex < 0 ? FramePtr : getBaseRegister());
+  else if (needsStackRealignment(MF))
     BasePtr = (FrameIndex < 0 ? FramePtr : StackPtr);
   else if (AfterFPPop)
     BasePtr = StackPtr;

lib/Target/X86/X86RegisterInfo.h

@@ -50,6 +50,11 @@ private:
   ///
   unsigned FramePtr;
 
+  /// BasePtr - X86 physical register used as a base ptr in complex stack
+  /// frames. I.e., when we need a 3rd base, not just SP and FP, due to
+  /// variable size stack objects.
+  unsigned BasePtr;
+
 public:
   X86RegisterInfo(X86TargetMachine &tm, const TargetInstrInfo &tii);
 
@@ -106,6 +111,8 @@ public:
   /// register scavenger to determine what registers are free.
   BitVector getReservedRegs(const MachineFunction &MF) const;
 
+  bool hasBasePointer(const MachineFunction &MF) const;
+
   bool canRealignStack(const MachineFunction &MF) const;
 
   bool needsStackRealignment(const MachineFunction &MF) const;
@@ -123,6 +130,7 @@ public:
   // Debug information queries.
   unsigned getFrameRegister(const MachineFunction &MF) const;
   unsigned getStackRegister() const { return StackPtr; }
+  unsigned getBaseRegister() const { return BasePtr; }
   // FIXME: Move to FrameInfok
   unsigned getSlotSize() const { return SlotSize; }
 

test/CodeGen/X86/alloca-align-rounding-32.ll

@@ -15,5 +15,6 @@ define void @foo2(i32 %h) {
   call void @bar(<2 x i64>* %p)
   ret void
 ; CHECK: foo2
+; CHECK: andl $-32, %esp
 ; CHECK: andl $-32, %eax
 }

test/CodeGen/X86/alloca-align-rounding.ll

@@ -15,5 +15,6 @@ define void @foo2(i64 %h) {
   call void @bar(<2 x i64>* %p)
   ret void
 ; CHECK: foo2
+; CHECK: andq $-32, %rsp
 ; CHECK: andq $-32, %rax
 }

test/CodeGen/X86/force-align-stack-alloca.ll

@@ -17,10 +17,15 @@ entry:
 
 define i64 @g(i32 %i) nounwind {
 ; CHECK: g:
-; CHECK:      pushl
+; CHECK:      pushl  %ebp
 ; CHECK-NEXT: movl   %esp, %ebp
+; CHECK-NEXT: andl   $-32, %esp
 ; CHECK-NEXT: pushl
-; CHECK-NEXT: subl   $20, %esp
+; CHECK-NEXT: pushl
+; CHECK-NEXT: subl   $24, %esp
+;
+; Now setup the base pointer (%ebx).
+; CHECK-NEXT: movl   %esp, %ebx
 ; CHECK-NOT:         {{[^ ,]*}}, %esp
 ;
 ; The next adjustment of the stack is due to the alloca.
@@ -41,12 +46,18 @@ define i64 @g(i32 %i) nounwind {
 ; CHECK-NEXT: addl   $32, %esp
 ; CHECK-NOT:         {{[^ ,]*}}, %esp
 ;
-; Finally we nede to restore %esp from %ebp, the alloca prevents us from
-; restoring it directly.
+; Restore %esp from %ebx (base pointer) so we can pop the callee-saved
+; registers.  This is the state prior to the allocation of VLAs.
 ; CHECK-NOT:  popl
-; CHECK:      leal   -4(%ebp), %esp
+; CHECK:      movl   %ebx, %esp
+; CHECK-NEXT: addl   $24, %esp
 ; CHECK-NEXT: popl
 ; CHECK-NEXT: popl
+;
+; Finally we need to restore %esp from %ebp due to dynamic stack
+; realignment.
+; CHECK-NEXT: movl   %ebp, %esp
+; CHECK-NEXT: popl   %ebp
 ; CHECK-NEXT: ret
 
 entry: