[AMDGPU] Tail call support for whole wave functions

Support tail calls to whole wave functions (trivial) and from whole wave
functions (slightly more involved because we need a new pseudo for the
tail call return, that patches up the EXEC mask).

Move the expansion of whole wave function return pseudos (regular and
tail call returns) to prolog epilog insertion, since that's where we
patch up the EXEC mask.

Unnecessary register spills will be dealt with in a future patch.

Author: rovka

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -7977,13 +7977,18 @@ void SelectionDAGBuilder::visitIntrinsicCall(const CallInst &I,
   }
   case Intrinsic::amdgcn_call_whole_wave: {
     TargetLowering::ArgListTy Args;
+    bool isTailCall = I.isTailCall();
 
     // The first argument is the callee. Skip it when assembling the call args.
     TargetLowering::ArgListEntry Arg;
     for (unsigned Idx = 1; Idx < I.arg_size(); ++Idx) {
       Arg.Node = getValue(I.getArgOperand(Idx));
       Arg.Ty = I.getArgOperand(Idx)->getType();
       Arg.setAttributes(&I, Idx);
+
+      if (Arg.IsSRet && isa<Instruction>(I.getArgOperand(Idx)))
+        isTailCall = false;
+
       Args.push_back(Arg);
     }
 
@@ -7998,7 +8003,7 @@ void SelectionDAGBuilder::visitIntrinsicCall(const CallInst &I,
         .setChain(getRoot())
         .setCallee(CallingConv::AMDGPU_Gfx_WholeWave, I.getType(),
                    getValue(I.getArgOperand(0)), std::move(Args))
-        .setTailCall(false)
+        .setTailCall(isTailCall && canTailCall(I))
         .setIsPreallocated(
             I.countOperandBundlesOfType(LLVMContext::OB_preallocated) != 0)
         .setConvergent(I.isConvergent())
@@ -8879,6 +8884,29 @@ SelectionDAGBuilder::lowerInvokable(TargetLowering::CallLoweringInfo &CLI,
   return Result;
 }
 
+bool SelectionDAGBuilder::canTailCall(const CallBase &CB) const {
+  bool isMustTailCall = CB.isMustTailCall();
+
+  // Avoid emitting tail calls in functions with the disable-tail-calls
+  // attribute.
+  auto *Caller = CB.getParent()->getParent();
+  if (Caller->getFnAttribute("disable-tail-calls").getValueAsString() ==
+          "true" &&
+      !isMustTailCall)
+    return false;
+
+  // We can't tail call inside a function with a swifterror argument. Lowering
+  // does not support this yet. It would have to move into the swifterror
+  // register before the call.
+  if (DAG.getTargetLoweringInfo().supportSwiftError() &&
+      Caller->getAttributes().hasAttrSomewhere(Attribute::SwiftError))
+    return false;
+
+  // Check if target-independent constraints permit a tail call here.
+  // Target-dependent constraints are checked within TLI->LowerCallTo.
+  return isInTailCallPosition(CB, DAG.getTarget());
+}
+
 void SelectionDAGBuilder::LowerCallTo(const CallBase &CB, SDValue Callee,
                                       bool isTailCall, bool isMustTailCall,
                                       const BasicBlock *EHPadBB,
@@ -8893,21 +8921,8 @@ void SelectionDAGBuilder::LowerCallTo(const CallBase &CB, SDValue Callee,
   const Value *SwiftErrorVal = nullptr;
   const TargetLowering &TLI = DAG.getTargetLoweringInfo();
 
-  if (isTailCall) {
-    // Avoid emitting tail calls in functions with the disable-tail-calls
-    // attribute.
-    auto *Caller = CB.getParent()->getParent();
-    if (Caller->getFnAttribute("disable-tail-calls").getValueAsString() ==
-        "true" && !isMustTailCall)
-      isTailCall = false;
-
-    // We can't tail call inside a function with a swifterror argument. Lowering
-    // does not support this yet. It would have to move into the swifterror
-    // register before the call.
-    if (TLI.supportSwiftError() &&
-        Caller->getAttributes().hasAttrSomewhere(Attribute::SwiftError))
-      isTailCall = false;
-  }
+  if (isTailCall)
+    isTailCall = canTailCall(CB);
 
   for (auto I = CB.arg_begin(), E = CB.arg_end(); I != E; ++I) {
     TargetLowering::ArgListEntry Entry;
@@ -8952,11 +8967,6 @@ void SelectionDAGBuilder::LowerCallTo(const CallBase &CB, SDValue Callee,
     Args.push_back(Entry);
   }
 
-  // Check if target-independent constraints permit a tail call here.
-  // Target-dependent constraints are checked within TLI->LowerCallTo.
-  if (isTailCall && !isInTailCallPosition(CB, DAG.getTarget()))
-    isTailCall = false;
-
   // Disable tail calls if there is an swifterror argument. Targets have not
   // been updated to support tail calls.
   if (TLI.supportSwiftError() && SwiftErrorVal)

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h

@@ -408,6 +408,10 @@ class SelectionDAGBuilder {
                    bool IsMustTailCall, const BasicBlock *EHPadBB = nullptr,
                    const TargetLowering::PtrAuthInfo *PAI = nullptr);
 
+  // Check some of the target-independent constraints for tail calls. This does
+  // not iterate over the call arguments.
+  bool canTailCall(const CallBase &CB) const;
+
   // Lower range metadata from 0 to N to assert zext to an integer of nearest
   // floor power of two.
   SDValue lowerRangeToAssertZExt(SelectionDAG &DAG, const Instruction &I,

llvm/lib/Target/AMDGPU/AMDGPUCallLowering.cpp

@@ -993,8 +993,14 @@ static unsigned getCallOpcode(const MachineFunction &CallerF, bool IsIndirect,
     return IsWave32 ? AMDGPU::SI_CS_CHAIN_TC_W32 : AMDGPU::SI_CS_CHAIN_TC_W64;
   }
 
-  return CC == CallingConv::AMDGPU_Gfx ? AMDGPU::SI_TCRETURN_GFX :
-                                         AMDGPU::SI_TCRETURN;
+  if (CallerF.getFunction().getCallingConv() ==
+      CallingConv::AMDGPU_Gfx_WholeWave)
+    return AMDGPU::SI_TCRETURN_GFX_WholeWave;
+
+  if (CC == CallingConv::AMDGPU_Gfx || CC == CallingConv::AMDGPU_Gfx_WholeWave)
+    return AMDGPU::SI_TCRETURN_GFX;
+
+  return AMDGPU::SI_TCRETURN;
 }
 
 // Add operands to call instruction to track the callee.
@@ -1273,6 +1279,13 @@ bool AMDGPUCallLowering::lowerTailCall(
   unsigned Opc = getCallOpcode(MF, Info.Callee.isReg(), /*IsTailCall*/ true,
                                ST.isWave32(), CalleeCC, IsDynamicVGPRChainCall);
   auto MIB = MIRBuilder.buildInstrNoInsert(Opc);
+
+  if (FuncInfo->isWholeWaveFunction())
+    addOriginalExecToReturn(MF, MIB);
+
+  // Keep track of the index of the next operand to be added to the call
+  unsigned CalleeIdx = MIB->getNumOperands();
+
   if (!addCallTargetOperands(MIB, MIRBuilder, Info, IsDynamicVGPRChainCall))
     return false;
 
@@ -1390,7 +1403,7 @@ bool AMDGPUCallLowering::lowerTailCall(
   // If we have -tailcallopt, we need to adjust the stack. We'll do the call
   // sequence start and end here.
   if (!IsSibCall) {
-    MIB->getOperand(1).setImm(FPDiff);
+    MIB->getOperand(CalleeIdx + 1).setImm(FPDiff);
     CallSeqStart.addImm(NumBytes).addImm(0);
     // End the call sequence *before* emitting the call. Normally, we would
     // tidy the frame up after the call. However, here, we've laid out the
@@ -1402,16 +1415,24 @@ bool AMDGPUCallLowering::lowerTailCall(
   // Now we can add the actual call instruction to the correct basic block.
   MIRBuilder.insertInstr(MIB);
 
+  // If this is a whole wave tail call, we need to constrain the register for
+  // the original EXEC.
+  if (MIB->getOpcode() == AMDGPU::SI_TCRETURN_GFX_WholeWave) {
+    MIB->getOperand(0).setReg(
+        constrainOperandRegClass(MF, *TRI, MRI, *TII, *ST.getRegBankInfo(),
+                                 *MIB, MIB->getDesc(), MIB->getOperand(0), 0));
+  }
+
   // If Callee is a reg, since it is used by a target specific
   // instruction, it must have a register class matching the
   // constraint of that instruction.
 
   // FIXME: We should define regbankselectable call instructions to handle
   // divergent call targets.
-  if (MIB->getOperand(0).isReg()) {
-    MIB->getOperand(0).setReg(
-        constrainOperandRegClass(MF, *TRI, MRI, *TII, *ST.getRegBankInfo(),
-                                 *MIB, MIB->getDesc(), MIB->getOperand(0), 0));
+  if (MIB->getOperand(CalleeIdx).isReg()) {
+    MIB->getOperand(CalleeIdx).setReg(constrainOperandRegClass(
+        MF, *TRI, MRI, *TII, *ST.getRegBankInfo(), *MIB, MIB->getDesc(),
+        MIB->getOperand(CalleeIdx), CalleeIdx));
   }
 
   MF.getFrameInfo().setHasTailCall();

llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp

@@ -5578,6 +5578,7 @@ const char* AMDGPUTargetLowering::getTargetNodeName(unsigned Opcode) const {
   NODE_NAME_CASE(CALL)
   NODE_NAME_CASE(TC_RETURN)
   NODE_NAME_CASE(TC_RETURN_GFX)
+  NODE_NAME_CASE(TC_RETURN_GFX_WholeWave)
   NODE_NAME_CASE(TC_RETURN_CHAIN)
   NODE_NAME_CASE(TC_RETURN_CHAIN_DVGPR)
   NODE_NAME_CASE(TRAP)

llvm/lib/Target/AMDGPU/AMDGPUISelLowering.h

@@ -402,6 +402,7 @@ enum NodeType : unsigned {
   CALL,
   TC_RETURN,
   TC_RETURN_GFX,
+  TC_RETURN_GFX_WholeWave,
   TC_RETURN_CHAIN,
   TC_RETURN_CHAIN_DVGPR,
   TRAP,

llvm/lib/Target/AMDGPU/AMDGPUInstrInfo.td

@@ -94,6 +94,10 @@ def AMDGPUtc_return_gfx: SDNode<"AMDGPUISD::TC_RETURN_GFX", AMDGPUTCReturnTP,
 [SDNPHasChain, SDNPOptInGlue, SDNPVariadic]
 >;
 
+def AMDGPUtc_return_gfx_ww: SDNode<"AMDGPUISD::TC_RETURN_GFX_WholeWave", AMDGPUTCReturnTP,
+[SDNPHasChain, SDNPOptInGlue, SDNPVariadic]
+>;
+
 def AMDGPUtc_return_chain: SDNode<"AMDGPUISD::TC_RETURN_CHAIN",
   SDTypeProfile<0, -1, [SDTCisPtrTy<0>]>,
   [SDNPHasChain, SDNPOptInGlue, SDNPVariadic]

llvm/lib/Target/AMDGPU/SIFrameLowering.cpp

@@ -1125,9 +1125,18 @@ void SIFrameLowering::emitCSRSpillRestores(
     RestoreWWMRegisters(WWMCalleeSavedRegs);
 
     // The original EXEC is the first operand of the return instruction.
-    const MachineInstr &Return = MBB.instr_back();
-    assert(Return.getOpcode() == AMDGPU::SI_WHOLE_WAVE_FUNC_RETURN &&
-           "Unexpected return inst");
+    MachineInstr &Return = MBB.instr_back();
+    unsigned Opcode = Return.getOpcode();
+    switch (Opcode) {
+    case AMDGPU::SI_WHOLE_WAVE_FUNC_RETURN:
+      Opcode = AMDGPU::SI_RETURN;
+      break;
+    case AMDGPU::SI_TCRETURN_GFX_WholeWave:
+      Opcode = AMDGPU::SI_TCRETURN_GFX;
+      break;
+    default:
+      llvm_unreachable("Unexpected return inst");
+    }
     Register OrigExec = Return.getOperand(0).getReg();
 
     if (!WWMScratchRegs.empty()) {
@@ -1141,6 +1150,11 @@ void SIFrameLowering::emitCSRSpillRestores(
     // Restore original EXEC.
     unsigned MovOpc = ST.isWave32() ? AMDGPU::S_MOV_B32 : AMDGPU::S_MOV_B64;
     BuildMI(MBB, MBBI, DL, TII->get(MovOpc), TRI.getExec()).addReg(OrigExec);
+
+    // Drop the first operand and update the opcode.
+    Return.removeOperand(0);
+    Return.setDesc(TII->get(Opcode));
+
     return;
   }
 

llvm/lib/Target/AMDGPU/SIISelLowering.cpp

@@ -4130,6 +4130,11 @@ SDValue SITargetLowering::LowerCall(CallLoweringInfo &CLI,
       break;
     }
 
+    // If the caller is a whole wave function, we need to use a special opcode
+    // so we can patch up EXEC.
+    if (Info->isWholeWaveFunction())
+      OPC = AMDGPUISD::TC_RETURN_GFX_WholeWave;
+
     return DAG.getNode(OPC, DL, MVT::Other, Ops);
   }
 
@@ -5871,6 +5876,7 @@ SITargetLowering::EmitInstrWithCustomInserter(MachineInstr &MI,
     MI.eraseFromParent();
     return SplitBB;
   }
+  case AMDGPU::SI_TCRETURN_GFX_WholeWave:
   case AMDGPU::SI_WHOLE_WAVE_FUNC_RETURN: {
     assert(MFI->isWholeWaveFunction());
 

llvm/lib/Target/AMDGPU/SIInstrInfo.cpp

@@ -2515,7 +2515,6 @@ bool SIInstrInfo::expandPostRAPseudo(MachineInstr &MI) const {
     MI.setDesc(get(ST.isWave32() ? AMDGPU::S_MOV_B32 : AMDGPU::S_MOV_B64));
     break;
   }
-  case AMDGPU::SI_WHOLE_WAVE_FUNC_RETURN:
   case AMDGPU::SI_RETURN: {
     const MachineFunction *MF = MBB.getParent();
     const GCNSubtarget &ST = MF->getSubtarget<GCNSubtarget>();

llvm/lib/Target/AMDGPU/SIInstructions.td

@@ -670,6 +670,33 @@ def SI_WHOLE_WAVE_FUNC_RETURN : SPseudoInstSI <
 def : GCNPat<
   (AMDGPUwhole_wave_return), (SI_WHOLE_WAVE_FUNC_RETURN (i1 (IMPLICIT_DEF)))>;
 
+// Restores the previous EXEC and otherwise behaves entirely like a SI_TCRETURN.
+// This is used for tail calls *from* a whole wave function. Tail calls to
+// a whole wave function may use the usual opcodes, depending on the calling
+// convention of the caller.
+def SI_TCRETURN_GFX_WholeWave : SPseudoInstSI <
+  (outs),
+  (ins SReg_1:$orig_exec, Gfx_CCR_SGPR_64:$src0, unknown:$callee, i32imm:$fpdiff)> {
+  let isCall = 1;
+  let isTerminator = 1;
+  let isReturn = 1;
+  let isBarrier = 1;
+  let UseNamedOperandTable = 1;
+  let SchedRW = [WriteBranch];
+  let isConvergent = 1;
+
+  // We're going to use custom handling to set the $orig_exec to the correct value.
+  let usesCustomInserter = 1;
+}
+
+// Generate a SI_TCRETURN_GFX_WholeWave pseudo with a placeholder for its
+// argument. It will be filled in by the custom inserter.
+def : GCNPat<
+  (AMDGPUtc_return_gfx_ww i64:$src0, tglobaladdr:$callee, i32:$fpdiff),
+  (SI_TCRETURN_GFX_WholeWave (i1 (IMPLICIT_DEF)), Gfx_CCR_SGPR_64:$src0,
+   tglobaladdr:$callee, i32:$fpdiff)>;
+
+
 // Return for returning shaders to a shader variant epilog.
 def SI_RETURN_TO_EPILOG : SPseudoInstSI <
   (outs), (ins variable_ops), [(AMDGPUreturn_to_epilog)]> {

llvm/lib/Target/AMDGPU/Utils/AMDGPUBaseInfo.h

@@ -1412,6 +1412,7 @@ constexpr bool mayTailCallThisCC(CallingConv::ID CC) {
   switch (CC) {
   case CallingConv::C:
   case CallingConv::AMDGPU_Gfx:
+  case CallingConv::AMDGPU_Gfx_WholeWave:
     return true;
   default:
     return canGuaranteeTCO(CC);