[AArch64][GlobalISel] Fall back when translating musttail calls

[paquette]

Summary

These are currently translated as normal functions calls in AArch64. This is incorrect. Until we have proper tail call lowering support, we shouldn't translate these.

Add a IsMustTailCall bit in CallLoweringInfo to start plumbing along the information necessary for tail call optimization.

[aemerson]

LGTM.

llvm/include/llvm/CodeGen/GlobalISel/CallLowering.h

struct CallLoweringInfo {
     /// List of descriptors of the arguments passed to the function.
     SmallVector<ArgInfo, 8> OrigArgs;
 
     /// Valid if the call has a swifterror inout parameter, and contains the
     /// vreg that the swifterror should be copied into after the call.
     Register SwiftErrorVReg = 0;
 
     MDNode *KnownCallees = nullptr;
+
+    /// True if the call must be tail call optimized.
+    bool IsMustTailCall = false;
   };
 
   /// Argument handling is mostly uniform between the four places that
   /// make these decisions: function formal arguments, call
   /// instruction args, call instruction returns and function
   /// returns. However, once a decision has been made on where an
   /// arugment should go, exactly what happens can vary slightly. This
   /// class abstracts the differences.

llvm/lib/CodeGen/GlobalISel/CallLowering.cpp

bool CallLowering::lowerCall(MachineIRBuilder &MIRBuilder, ImmutableCallSite CS,
   Info.OrigRet = ArgInfo{ResRegs, CS.getType(), ISD::ArgFlagsTy{}};
   if (!Info.OrigRet.Ty->isVoidTy())
     setArgFlags(Info.OrigRet, AttributeList::ReturnIndex, DL, CS);
 
   Info.KnownCallees =
       CS.getInstruction()->getMetadata(LLVMContext::MD_callees);
   Info.CallConv = CS.getCallingConv();
   Info.SwiftErrorVReg = SwiftErrorVReg;
+  Info.IsMustTailCall = CS.isMustTailCall();
 
   return lowerCall(MIRBuilder, Info);
 }
 
 template <typename FuncInfoTy>
 void CallLowering::setArgFlags(CallLowering::ArgInfo &Arg, unsigned OpIdx,
                                const DataLayout &DL,
                                const FuncInfoTy &FuncInfo) const {

llvm/lib/Target/AArch64/AArch64CallLowering.cpp

 
 bool AArch64CallLowering::lowerCall(MachineIRBuilder &MIRBuilder,
                                     CallLoweringInfo &Info) const {
   MachineFunction &MF = MIRBuilder.getMF();
   const Function &F = MF.getFunction();
   MachineRegisterInfo &MRI = MF.getRegInfo();
   auto &DL = F.getParent()->getDataLayout();
 
+  if (Info.IsMustTailCall) {
+    LLVM_DEBUG(dbgs() << "Cannot lower musttail calls yet.\n");
+    return false;
+  }
+
   SmallVector<ArgInfo, 8> SplitArgs;
   for (auto &OrigArg : Info.OrigArgs) {
     splitToValueTypes(OrigArg, SplitArgs, DL, MRI, Info.CallConv);
     // AAPCS requires that we zero-extend i1 to 8 bits by the caller.
     if (OrigArg.Ty->isIntegerTy(1))
       SplitArgs.back().Flags.setZExt();
   }
 

llvm/test/CodeGen/AArch64/GlobalISel/call-translator-musttail.ll

+; RUN: not llc %s -mtriple aarch64-unknown-unknown -debug-only=aarch64-call-lowering -global-isel -o - 2>&1 | FileCheck %s
+
+; CHECK: Cannot lower musttail calls yet.
+; CHECK-NEXT: LLVM ERROR: unable to translate instruction: call (in function: foo)
+declare void @must_callee(i8*)
+define void @foo(i32*) {
+  musttail call void @must_callee(i8* null)
+  ret void
+}