[SystemZ] Support llvm.frameaddress/llvm.returnaddress intrinsics

[bryanpkc]

Summary

Enable the SystemZ back-end to lower FRAMEADDR and RETURNADDR, which previously would cause the back-end to crash. Currently, only a frame count of zero is supported.

[uweigand]

This implementation of -mattr=backchain is not quite compatible to GCC's -mbackchain flag: with GCC, if a function does not need any stack frame, -mbackchain by itself does *not* force allocation of a frame, as your patch does. -mbackchain only says that *if* a frame is allocated, then the frame's back chain slot is always kept valid. In addition, GCC keeps the back chain slot valid across alloca calls and other dynamic stack allocations, which your current implementation does not. (This latter is actually a bug, not just an incompatibility.)

(Note that if a function calls __builtin_frame_address or __builtin_return_address with a nonzero frame count, then GCC does force allocation of a stack frame. This in in turn is independent of -mbackchain, however.)

Now, if we actually implement -mattr=backchain, we should do it in a way compatible with GCC, and also enable it in clang via a -mbackchain command line option. However, I'm somewhat sceptical that this is really useful. (Note that for __builtin_frame_address and __builtin_return_address to be actually safe with larger count values, all of the program needs to be built with` -mbackchain` --- but this is not the case for any system-installed libraries on any current Linux distribution ...)

[bryanpkc]

Hi Ulrich, thank you for your review. You are right that the non-zero frame traversal count is probably not very common. In the updated patch, I have removed the proposed -mattr=backchain changes. Is it better?

[uweigand]

LGTM, thanks.

llvm/trunk/lib/Target/SystemZ/SystemZISelLowering.h

SDValue lowerTLSGetOffset(GlobalAddressSDNode *Node,
                             SelectionDAG &DAG, unsigned Opcode,
                             SDValue GOTOffset) const;
   SDValue lowerGlobalTLSAddress(GlobalAddressSDNode *Node,
                                 SelectionDAG &DAG) const;
   SDValue lowerBlockAddress(BlockAddressSDNode *Node,
                             SelectionDAG &DAG) const;
   SDValue lowerJumpTable(JumpTableSDNode *JT, SelectionDAG &DAG) const;
   SDValue lowerConstantPool(ConstantPoolSDNode *CP, SelectionDAG &DAG) const;
+  SDValue lowerFRAMEADDR(SDValue Op, SelectionDAG &DAG) const;
+  SDValue lowerRETURNADDR(SDValue Op, SelectionDAG &DAG) const;
   SDValue lowerVASTART(SDValue Op, SelectionDAG &DAG) const;
   SDValue lowerVACOPY(SDValue Op, SelectionDAG &DAG) const;
   SDValue lowerDYNAMIC_STACKALLOC(SDValue Op, SelectionDAG &DAG) const;
   SDValue lowerSMUL_LOHI(SDValue Op, SelectionDAG &DAG) const;
   SDValue lowerUMUL_LOHI(SDValue Op, SelectionDAG &DAG) const;
   SDValue lowerSDIVREM(SDValue Op, SelectionDAG &DAG) const;
   SDValue lowerUDIVREM(SDValue Op, SelectionDAG &DAG) const;
   SDValue lowerBITCAST(SDValue Op, SelectionDAG &DAG) const;

llvm/trunk/lib/Target/SystemZ/SystemZISelLowering.cpp

SDValue SystemZTargetLowering::lowerConstantPool(ConstantPoolSDNode *CP,
   else
     Result = DAG.getTargetConstantPool(CP->getConstVal(), PtrVT,
                                        CP->getAlignment(), CP->getOffset());
 
   // Use LARL to load the address of the constant pool entry.
   return DAG.getNode(SystemZISD::PCREL_WRAPPER, DL, PtrVT, Result);
 }
 
+SDValue SystemZTargetLowering::lowerFRAMEADDR(SDValue Op,
+                                              SelectionDAG &DAG) const {
+  MachineFunction &MF = DAG.getMachineFunction();
+  MachineFrameInfo *MFI = MF.getFrameInfo();
+  MFI->setFrameAddressIsTaken(true);
+
+  SDLoc DL(Op);
+  unsigned Depth = cast<ConstantSDNode>(Op.getOperand(0))->getZExtValue();
+  EVT PtrVT = getPointerTy(DAG.getDataLayout());
+
+  // If the back chain frame index has not been allocated yet, do so.
+  SystemZMachineFunctionInfo *FI = MF.getInfo<SystemZMachineFunctionInfo>();
+  int BackChainIdx = FI->getFramePointerSaveIndex();
+  if (!BackChainIdx) {
+    // By definition, the frame address is the address of the back chain.
+    BackChainIdx = MFI->CreateFixedObject(8, -SystemZMC::CallFrameSize, false);
+    FI->setFramePointerSaveIndex(BackChainIdx);
+  }
+  SDValue BackChain = DAG.getFrameIndex(BackChainIdx, PtrVT);
+
+  // FIXME The frontend should detect this case.
+  if (Depth > 0) {
+    report_fatal_error("Unsupported stack frame traversal count");
+  }
+
+  return BackChain;
+}
+
+SDValue SystemZTargetLowering::lowerRETURNADDR(SDValue Op,
+                                               SelectionDAG &DAG) const {
+  MachineFunction &MF = DAG.getMachineFunction();
+  MachineFrameInfo *MFI = MF.getFrameInfo();
+  MFI->setReturnAddressIsTaken(true);
+
+  if (verifyReturnAddressArgumentIsConstant(Op, DAG))
+    return SDValue();
+
+  SDLoc DL(Op);
+  unsigned Depth = cast<ConstantSDNode>(Op.getOperand(0))->getZExtValue();
+  EVT PtrVT = getPointerTy(DAG.getDataLayout());
+
+  // FIXME The frontend should detect this case.
+  if (Depth > 0) {
+    report_fatal_error("Unsupported stack frame traversal count");
+  }
+
+  // Return R14D, which has the return address. Mark it an implicit live-in.
+  unsigned LinkReg = MF.addLiveIn(SystemZ::R14D, &SystemZ::GR64BitRegClass);
+  return DAG.getCopyFromReg(DAG.getEntryNode(), DL, LinkReg, PtrVT);
+}
+
 SDValue SystemZTargetLowering::lowerBITCAST(SDValue Op,
                                             SelectionDAG &DAG) const {
   SDLoc DL(Op);
   SDValue In = Op.getOperand(0);
   EVT InVT = In.getValueType();
   EVT ResVT = Op.getValueType();
 
   // Convert loads directly.  This is normally done by DAGCombiner,
SDValue SystemZTargetLowering::lowerShift(SDValue Op, SelectionDAG &DAG,
 
   // Otherwise just treat the current form as legal.
   return Op;
 }
 
 SDValue SystemZTargetLowering::LowerOperation(SDValue Op,
                                               SelectionDAG &DAG) const {
   switch (Op.getOpcode()) {
+  case ISD::FRAMEADDR:
+    return lowerFRAMEADDR(Op, DAG);
+  case ISD::RETURNADDR:
+    return lowerRETURNADDR(Op, DAG);
   case ISD::BR_CC:
     return lowerBR_CC(Op, DAG);
   case ISD::SELECT_CC:
     return lowerSELECT_CC(Op, DAG);
   case ISD::SETCC:
     return lowerSETCC(Op, DAG);
   case ISD::GlobalAddress:
     return lowerGlobalAddress(cast<GlobalAddressSDNode>(Op), DAG);

llvm/trunk/lib/Target/SystemZ/SystemZMachineFunctionInfo.h

 class SystemZMachineFunctionInfo : public MachineFunctionInfo {
   virtual void anchor();
   unsigned LowSavedGPR;
   unsigned HighSavedGPR;
   unsigned VarArgsFirstGPR;
   unsigned VarArgsFirstFPR;
   unsigned VarArgsFrameIndex;
   unsigned RegSaveFrameIndex;
+  int FramePointerSaveIndex;
   bool ManipulatesSP;
   unsigned NumLocalDynamics;
 
 public:
   explicit SystemZMachineFunctionInfo(MachineFunction &MF)
     : LowSavedGPR(0), HighSavedGPR(0), VarArgsFirstGPR(0), VarArgsFirstFPR(0),
-      VarArgsFrameIndex(0), RegSaveFrameIndex(0), ManipulatesSP(false),
-      NumLocalDynamics(0) {}
+      VarArgsFrameIndex(0), RegSaveFrameIndex(0), FramePointerSaveIndex(0),
+      ManipulatesSP(false), NumLocalDynamics(0) {}
 
   // Get and set the first call-saved GPR that should be saved and restored
   // by this function.  This is 0 if no GPRs need to be saved or restored.
   unsigned getLowSavedGPR() const { return LowSavedGPR; }
   void setLowSavedGPR(unsigned Reg) { LowSavedGPR = Reg; }
 
   // Get and set the last call-saved GPR that should be saved and restored
   // by this function.
public:
   unsigned getVarArgsFrameIndex() const { return VarArgsFrameIndex; }
   void setVarArgsFrameIndex(unsigned FI) { VarArgsFrameIndex = FI; }
 
   // Get and set the frame index of the register save area
   // (i.e. the incoming stack pointer).
   unsigned getRegSaveFrameIndex() const { return RegSaveFrameIndex; }
   void setRegSaveFrameIndex(unsigned FI) { RegSaveFrameIndex = FI; }
 
+  // Get and set the frame index of where the old frame pointer is stored.
+  int getFramePointerSaveIndex() const { return FramePointerSaveIndex; }
+  void setFramePointerSaveIndex(int Idx) { FramePointerSaveIndex = Idx; }
+
   // Get and set whether the function directly manipulates the stack pointer,
   // e.g. through STACKSAVE or STACKRESTORE.
   bool getManipulatesSP() const { return ManipulatesSP; }
   void setManipulatesSP(bool MSP) { ManipulatesSP = MSP; }
 
   // Count number of local-dynamic TLS symbols used.
   unsigned getNumLocalDynamicTLSAccesses() const { return NumLocalDynamics; }
   void incNumLocalDynamicTLSAccesses() { ++NumLocalDynamics; }
 };
 
 } // end namespace llvm
 
 #endif

llvm/trunk/test/CodeGen/SystemZ/frameaddr-01.ll

+; RUN: llc < %s -mtriple=s390x-linux-gnu | FileCheck %s
+
+; The current function's frame address is the address of
+; the optional back chain slot.
+define i8* @fp0() nounwind {
+entry:
+; CHECK-LABEL: fp0:
+; CHECK: la   %r2, 0(%r15)
+; CHECK: br   %r14
+  %0 = tail call i8* @llvm.frameaddress(i32 0)
+  ret i8* %0
+}
+
+; Check that the frame address is correct in a presence
+; of a stack frame.
+define i8* @fp0f() nounwind {
+entry:
+; CHECK-LABEL: fp0f:
+; CHECK: aghi %r15, -168
+; CHECK: la   %r2, 168(%r15)
+; CHECK: aghi %r15, 168
+; CHECK: br   %r14
+  %0 = alloca i64, align 8
+  %1 = tail call i8* @llvm.frameaddress(i32 0)
+  ret i8* %1
+}
+
+declare i8* @llvm.frameaddress(i32) nounwind readnone

llvm/trunk/test/CodeGen/SystemZ/ret-addr-01.ll

+; Test support for the llvm.returnaddress intrinsic.
+; 
+; RUN: llc < %s -mtriple=s390x-linux-gnu | FileCheck %s
+
+; The current function's return address is in the link register.
+define i8* @rt0() norecurse nounwind readnone {
+entry:
+; CHECK-LABEL: rt0:
+; CHECK: lgr  %r2, %r14
+; CHECK: br   %r14
+  %0 = tail call i8* @llvm.returnaddress(i32 0)
+  ret i8* %0
+}
+
+declare i8* @llvm.returnaddress(i32) nounwind readnone