[X86][NFC] structure-return simplificiation

The X86 backend only needs to know whether structure return is via an
sret pointer.  This removes the categorization enumeration and
adjusts, templatizes and renames the related functions.

Differential Revision: https://reviews.llvm.org/D109966

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -3339,38 +3339,40 @@ SDValue X86TargetLowering::LowerCallResult(
 //  For info on fast calling convention see Fast Calling Convention (tail call)
 //  implementation LowerX86_32FastCCCallTo.
 
-/// CallIsStructReturn - Determines whether a call uses struct return
-/// semantics.
-enum StructReturnType {
-  NotStructReturn,
-  RegStructReturn,
-  StackStructReturn
-};
-static StructReturnType
-callIsStructReturn(ArrayRef<ISD::OutputArg> Outs, bool IsMCU) {
-  if (Outs.empty())
-    return NotStructReturn;
+/// Determines whether Args, either a set of outgoing arguments to a call, or a
+/// set of incoming args of a call, contains an sret pointer that the callee
+/// pops
+template <typename T>
+static bool hasCalleePopSRet(const SmallVectorImpl<T> &Args,
+                             const X86Subtarget &Subtarget) {
+  // Not C++20 (yet), so no concepts available.
+  static_assert(std::is_same<T, ISD::OutputArg>::value ||
+                    std::is_same<T, ISD::InputArg>::value,
+                "requires ISD::OutputArg or ISD::InputArg");
 
-  const ISD::ArgFlagsTy &Flags = Outs[0].Flags;
-  if (!Flags.isSRet())
-    return NotStructReturn;
-  if (Flags.isInReg() || IsMCU)
-    return RegStructReturn;
-  return StackStructReturn;
-}
+  // Only 32-bit pops the sret.  It's a 64-bit world these days, so early-out
+  // for most compilations.
+  if (!Subtarget.is32Bit())
+    return false;
+
+  if (Args.empty())
+    return false;
 
-/// Determines whether a function uses struct return semantics.
-static StructReturnType
-argsAreStructReturn(ArrayRef<ISD::InputArg> Ins, bool IsMCU) {
-  if (Ins.empty())
-    return NotStructReturn;
+  // Most calls do not have an sret argument, check the arg next.
+  const ISD::ArgFlagsTy &Flags = Args[0].Flags;
+  if (!Flags.isSRet() || Flags.isInReg())
+    return false;
 
-  const ISD::ArgFlagsTy &Flags = Ins[0].Flags;
-  if (!Flags.isSRet())
-    return NotStructReturn;
-  if (Flags.isInReg() || IsMCU)
-    return RegStructReturn;
-  return StackStructReturn;
+  // The MSVCabi does not pop the sret.
+  if (Subtarget.getTargetTriple().isOSMSVCRT())
+    return false;
+
+  // MCUs don't pop the sret
+  if (Subtarget.isTargetMCU())
+    return false;
+
+  // Callee pops argument
+  return true;
 }
 
 /// Make a copy of an aggregate at address specified by "Src" to address
@@ -4004,9 +4006,7 @@ SDValue X86TargetLowering::LowerFormalArguments(
   } else {
     FuncInfo->setBytesToPopOnReturn(0); // Callee pops nothing.
     // If this is an sret function, the return should pop the hidden pointer.
-    if (!Is64Bit && !canGuaranteeTCO(CallConv) &&
-        !Subtarget.getTargetTriple().isOSMSVCRT() &&
-        argsAreStructReturn(Ins, Subtarget.isTargetMCU()) == StackStructReturn)
+    if (!canGuaranteeTCO(CallConv) && hasCalleePopSRet(Ins, Subtarget))
       FuncInfo->setBytesToPopOnReturn(4);
   }
 
@@ -4125,10 +4125,10 @@ X86TargetLowering::LowerCall(TargetLowering::CallLoweringInfo &CLI,
   MachineFunction &MF = DAG.getMachineFunction();
   bool Is64Bit        = Subtarget.is64Bit();
   bool IsWin64        = Subtarget.isCallingConvWin64(CallConv);
-  StructReturnType SR = callIsStructReturn(Outs, Subtarget.isTargetMCU());
   bool IsSibcall      = false;
   bool IsGuaranteeTCO = MF.getTarget().Options.GuaranteedTailCallOpt ||
       CallConv == CallingConv::Tail || CallConv == CallingConv::SwiftTail;
+  bool IsCalleePopSRet = !IsGuaranteeTCO && hasCalleePopSRet(Outs, Subtarget);
   X86MachineFunctionInfo *X86Info = MF.getInfo<X86MachineFunctionInfo>();
   bool HasNCSR = (CB && isa<CallInst>(CB) &&
                   CB->hasFnAttr("no_caller_saved_registers"));
@@ -4154,12 +4154,11 @@ X86TargetLowering::LowerCall(TargetLowering::CallLoweringInfo &CLI,
       isTailCall = false;
   }
 
-
   if (isTailCall && !IsMustTail) {
     // Check if it's really possible to do a tail call.
     isTailCall = IsEligibleForTailCallOptimization(
-        Callee, CallConv, SR == StackStructReturn, isVarArg, CLI.RetTy, Outs,
-        OutVals, Ins, DAG);
+        Callee, CallConv, IsCalleePopSRet, isVarArg, CLI.RetTy, Outs, OutVals,
+        Ins, DAG);
 
     // Sibcalls are automatically detected tailcalls which do not require
     // ABI changes.
@@ -4660,20 +4659,14 @@ X86TargetLowering::LowerCall(TargetLowering::CallLoweringInfo &CLI,
       DAG.addHeapAllocSite(Chain.getNode(), HeapAlloc);
 
   // Create the CALLSEQ_END node.
-  unsigned NumBytesForCalleeToPop;
+  unsigned NumBytesForCalleeToPop = 0; // Callee pops nothing.
   if (X86::isCalleePop(CallConv, Is64Bit, isVarArg,
                        DAG.getTarget().Options.GuaranteedTailCallOpt))
     NumBytesForCalleeToPop = NumBytes;    // Callee pops everything
-  else if (!Is64Bit && !canGuaranteeTCO(CallConv) &&
-           !Subtarget.getTargetTriple().isOSMSVCRT() &&
-           SR == StackStructReturn)
-    // If this is a call to a struct-return function, the callee
-    // pops the hidden struct pointer, so we have to push it back.
-    // This is common for Darwin/X86, Linux & Mingw32 targets.
-    // For MSVC Win32 targets, the caller pops the hidden struct pointer.
+  else if (!canGuaranteeTCO(CallConv) && IsCalleePopSRet)
+    // If this call passes a struct-return pointer, the callee
+    // pops that struct pointer.
     NumBytesForCalleeToPop = 4;
-  else
-    NumBytesForCalleeToPop = 0;  // Callee pops nothing.
 
   // Returns a flag for retval copy to use.
   if (!IsSibcall) {
@@ -4832,7 +4825,7 @@ bool MatchingStackOffset(SDValue Arg, unsigned Offset, ISD::ArgFlagsTy Flags,
 /// Check whether the call is eligible for tail call optimization. Targets
 /// that want to do tail call optimization should implement this function.
 bool X86TargetLowering::IsEligibleForTailCallOptimization(
-    SDValue Callee, CallingConv::ID CalleeCC, bool IsCalleeStackStructRet,
+    SDValue Callee, CallingConv::ID CalleeCC, bool IsCalleePopSRet,
     bool isVarArg, Type *RetTy, const SmallVectorImpl<ISD::OutputArg> &Outs,
     const SmallVectorImpl<SDValue> &OutVals,
     const SmallVectorImpl<ISD::InputArg> &Ins, SelectionDAG &DAG) const {
@@ -4885,9 +4878,9 @@ bool X86TargetLowering::IsEligibleForTailCallOptimization(
     // needs to be (a) an sret function itself and (b) we pass our sret as its
     // sret. Condition #b is harder to determine.
     return false;
-  } else if (Subtarget.is32Bit() && IsCalleeStackStructRet)
-    // In the i686 ABI, the sret pointer is callee-pop, so we cannot tail-call,
-    // as our caller doesn't expect that.
+  } else if (IsCalleePopSRet)
+    // The callee pops an sret, so we cannot tail-call, as our caller doesn't
+    // expect that.
     return false;
 
   // Do not sibcall optimize vararg calls unless all arguments are passed via