PPC: Implement base pointer and stack realignment

This builds on some frame-lowering code that has existed since 2005 (r24224)
but was disabled in 2008 (r48188) because it needed base pointer support to
function correctly. This implementation follows the strategy suggested by Dale
Johannesen in r48188 where the following comment was added:

  This does not currently work, because the delta between old and new stack
  pointers is added to offsets that reference incoming parameters after the
  prolog is generated, and the code that does that doesn't handle a variable
  delta.  You don't want to do that anyway; a better approach is to reserve
  another register that retains to the incoming stack pointer, and reference
  parameters relative to that.

And now we do exactly that. If we don't need a frame pointer, then we use r31
as a base pointer. If we do need a frame pointer, then we use r30 as a base
pointer. The base pointer retains the value of the stack pointer before it was
decremented in the prologue. We then use the base pointer to resolve all
negative frame indicies. The basic scheme follows that for base pointers in the
X86 backend.

We use a base pointer when we need to dynamically realign the incoming stack
pointer. This currently applies only to static objects (dynamic allocas with
large alignments, and base-pointer support in SjLj lowering will come in future
commits).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186478 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 70 insertions, 11 deletions

diff --git a/lib/Target/PowerPC/PPCRegisterInfo.cpp b/lib/Target/PowerPC/PPCRegisterInfo.cpp
index 8a0954c5bb..49de8da948 100644
--- a/lib/Target/PowerPC/PPCRegisterInfo.cpp
+++ b/lib/Target/PowerPC/PPCRegisterInfo.cpp
@@ -48,6 +48,14 @@
 
 using namespace llvm;
 
+static cl::opt<bool>
+EnableBasePointer("ppc-use-base-pointer", cl::Hidden, cl::init(true),
+         cl::desc("Enable use of a base pointer for complex stack frames"));
+
+static cl::opt<bool>
+AlwaysBasePointer("ppc-always-use-base-pointer", cl::Hidden, cl::init(false),
+         cl::desc("Force the use of a base pointer in every function"));
+
 PPCRegisterInfo::PPCRegisterInfo(const PPCSubtarget &ST)
   : PPCGenRegisterInfo(ST.isPPC64() ? PPC::LR8 : PPC::LR,
                        ST.isPPC64() ? 0 : 1,
@@ -170,18 +178,28 @@ BitVector PPCRegisterInfo::getReservedRegs(const MachineFunction &MF) const {
     Reserved.set(PPC::X1);
     Reserved.set(PPC::X13);
 
-    if (PPCFI->needsFP(MF))
+    if (PPCFI->needsFP(MF) || hasBasePointer(MF)) {
       Reserved.set(PPC::X31);
 
+      // If we need a base pointer, and we also have a frame pointer, then use
+      // r30 as the base pointer.
+      if (PPCFI->needsFP(MF) && hasBasePointer(MF))
+        Reserved.set(PPC::X30);
+    }
+
     // The 64-bit SVR4 ABI reserves r2 for the TOC pointer.
     if (Subtarget.isSVR4ABI()) {
       Reserved.set(PPC::X2);
     }
   }
 
-  if (PPCFI->needsFP(MF))
+  if (PPCFI->needsFP(MF) || hasBasePointer(MF)) {
     Reserved.set(PPC::R31);
 
+    if (PPCFI->needsFP(MF) && hasBasePointer(MF))
+      Reserved.set(PPC::R30);
+  }
+
   // Reserve Altivec registers when Altivec is unavailable.
   if (!Subtarget.hasAltivec())
     for (TargetRegisterClass::iterator I = PPC::VRRCRegClass.begin(),
@@ -524,7 +542,6 @@ PPCRegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator II,
   const TargetInstrInfo &TII = *MF.getTarget().getInstrInfo();
   // Get the frame info.
   MachineFrameInfo *MFI = MF.getFrameInfo();
-  const TargetFrameLowering *TFI = MF.getTarget().getFrameLowering();
   DebugLoc dl = MI.getDebugLoc();
 
   unsigned OffsetOperandNo = getOffsetONFromFION(MI, FIOperandNum);
@@ -562,12 +579,8 @@ PPCRegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator II,
   }
 
   // Replace the FrameIndex with base register with GPR1 (SP) or GPR31 (FP).
-
-  bool is64Bit = Subtarget.isPPC64();
-  MI.getOperand(FIOperandNum).ChangeToRegister(TFI->hasFP(MF) ?
-                                              (is64Bit ? PPC::X31 : PPC::R31) :
-                                                (is64Bit ? PPC::X1 : PPC::R1),
-                                              false);
+  MI.getOperand(FIOperandNum).ChangeToRegister(
+    FrameIndex < 0 ? getBaseRegister(MF) : getFrameRegister(MF), false);
 
   // Figure out if the offset in the instruction is shifted right two bits.
   bool isIXAddr = usesIXAddr(MI);
@@ -586,8 +599,10 @@ PPCRegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator II,
   // Naked functions have stack size 0, although getStackSize may not reflect that
   // because we didn't call all the pieces that compute it for naked functions.
   if (!MF.getFunction()->getAttributes().
-        hasAttribute(AttributeSet::FunctionIndex, Attribute::Naked))
-    Offset += MFI->getStackSize();
+        hasAttribute(AttributeSet::FunctionIndex, Attribute::Naked)) {
+    if (!(hasBasePointer(MF) && FrameIndex < 0))
+      Offset += MFI->getStackSize();
+  }
 
   // If we can, encode the offset directly into the instruction.  If this is a
   // normal PPC "ri" instruction, any 16-bit value can be safely encoded.  If
@@ -605,6 +620,7 @@ PPCRegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator II,
   // The offset doesn't fit into a single register, scavenge one to build the
   // offset in.
 
+  bool is64Bit = Subtarget.isPPC64();
   const TargetRegisterClass *G8RC = &PPC::G8RCRegClass;
   const TargetRegisterClass *GPRC = &PPC::GPRCRegClass;
   const TargetRegisterClass *RC = is64Bit ? G8RC : GPRC;
@@ -658,6 +674,49 @@ unsigned PPCRegisterInfo::getEHHandlerRegister() const {
   return !Subtarget.isPPC64() ? PPC::R4 : PPC::X4;
 }
 
+unsigned PPCRegisterInfo::getBaseRegister(const MachineFunction &MF) const {
+  const TargetFrameLowering *TFI = MF.getTarget().getFrameLowering();
+
+  if (!hasBasePointer(MF))
+    return getFrameRegister(MF);
+
+  if (!Subtarget.isPPC64())
+    return TFI->hasFP(MF) ? PPC::R30 : PPC::R31;
+  else
+    return TFI->hasFP(MF) ? PPC::X30 : PPC::X31;
+}
+
+bool PPCRegisterInfo::hasBasePointer(const MachineFunction &MF) const {
+  if (!EnableBasePointer)
+    return false;
+  if (AlwaysBasePointer)
+    return true;
+
+  // If we need to realign the stack, then the stack pointer can no longer
+  // serve as an offset into the caller's stack space. As a result, we need a
+  // base pointer.
+  return needsStackRealignment(MF);
+}
+
+bool PPCRegisterInfo::canRealignStack(const MachineFunction &MF) const {
+  if (!MF.getTarget().Options.RealignStack)
+    return false;
+
+  return true;
+}
+
+bool PPCRegisterInfo::needsStackRealignment(const MachineFunction &MF) const {
+  const MachineFrameInfo *MFI = MF.getFrameInfo();
+  const Function *F = MF.getFunction();
+  unsigned StackAlign = MF.getTarget().getFrameLowering()->getStackAlignment();
+  bool requiresRealignment =
+    ((MFI->getMaxAlignment() > StackAlign) ||
+     F->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
+                                     Attribute::StackAlignment));
+
+  return requiresRealignment && canRealignStack(MF);
+}
+
 /// Returns true if the instruction's frame index
 /// reference would be better served by a base register other than FP
 /// or SP. Used by LocalStackFrameAllocation to determine which frame index