[RISCV][GISEL] Add support for scalable vector types in lowerReturnVal

Scalable vector types from LLVM IR are lowered into physical vector
registers in MIR based on calling convention.

This patch is stacked on llvm#70881.

llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp

@@ -359,7 +359,7 @@ bool IRTranslator::translateCompare(const User &U,
 bool IRTranslator::translateRet(const User &U, MachineIRBuilder &MIRBuilder) {
   const ReturnInst &RI = cast<ReturnInst>(U);
   const Value *Ret = RI.getReturnValue();
-  if (Ret && DL->getTypeStoreSize(Ret->getType()) == 0)
+  if (Ret && DL->getTypeStoreSize(Ret->getType()).isZero())
     Ret = nullptr;
 
   ArrayRef<Register> VRegs;

llvm/lib/CodeGen/MachineVerifier.cpp

@@ -1966,6 +1966,9 @@ void MachineVerifier::visitMachineInstrBefore(const MachineInstr *MI) {
     if (SrcReg.isVirtual() && DstReg.isPhysical() && SrcSize.isScalable() &&
         !DstSize.isScalable())
       break;
+    if (SrcReg.isVirtual() && DstReg.isPhysical() && SrcSize.isScalable() &&
+        !DstSize.isScalable())
+      break;
 
     if (SrcSize.isNonZero() && DstSize.isNonZero() && SrcSize != DstSize) {
       if (!DstOp.getSubReg() && !SrcOp.getSubReg()) {

llvm/lib/Target/RISCV/GISel/RISCVCallLowering.cpp

@@ -368,6 +368,10 @@ static bool isSupportedReturnType(Type *T, const RISCVSubtarget &Subtarget) {
     return true;
   }
 
+  if (T->isVectorTy() && Subtarget.hasVInstructions() && T->isScalableTy() &&
+      isLegalElementTypeForRVV(T->getScalarType(), Subtarget))
+    return true;
+
   return false;
 }
 

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -19638,12 +19638,15 @@ unsigned RISCVTargetLowering::getCustomCtpopCost(EVT VT,
 }
 
 bool RISCVTargetLowering::fallBackToDAGISel(const Instruction &Inst) const {
-  // We don't support scalable vectors in GISel.
+  // At the moment, the only scalable instruction GISel knows how to lower is
+  // ret with scalable argument.
+
   if (Inst.getType()->isScalableTy())
     return true;
 
   for (unsigned i = 0; i < Inst.getNumOperands(); ++i)
-    if (Inst.getOperand(i)->getType()->isScalableTy())
+    if (Inst.getOperand(i)->getType()->isScalableTy() &&
+        !isa<ReturnInst>(&Inst))
       return true;
 
   if (const AllocaInst *AI = dyn_cast<AllocaInst>(&Inst)) {

llvm/test/CodeGen/RISCV/GlobalISel/irtranslator/vec-ret-bf16-err.ll

@@ -0,0 +1,14 @@
+; RUN: not --crash llc -mtriple=riscv32 -mattr=+v -global-isel -stop-after=irtranslator \
+; RUN:   -verify-machineinstrs < %s 2>&1 | FileCheck %s
+; RUN: not --crash llc -mtriple=riscv64 -mattr=+v -global-isel -stop-after=irtranslator \
+; RUN:   -verify-machineinstrs < %s 2>&1 | FileCheck %s
+
+; The purpose of this test is to show that the compiler throws an error when
+; there is no support for bf16 vectors. If the compiler did not throw an error,
+; then it will try to scalarize the argument to an s32, which may drop elements.
+define <vscale x 1 x bfloat> @test_ret_nxv1bf16() {
+entry:
+  ret <vscale x 1 x bfloat> undef
+}
+
+; CHECK: LLVM ERROR: unable to translate instruction: ret (in function: test_ret_nxv1bf16)

llvm/test/CodeGen/RISCV/GlobalISel/irtranslator/vec-ret-f16-err.ll

@@ -0,0 +1,14 @@
+; RUN: not --crash llc -mtriple=riscv32 -mattr=+v -global-isel -stop-after=irtranslator \
+; RUN:   -verify-machineinstrs < %s 2>&1 | FileCheck %s
+; RUN: not --crash llc -mtriple=riscv64 -mattr=+v -global-isel -stop-after=irtranslator \
+; RUN:   -verify-machineinstrs < %s 2>&1 | FileCheck %s
+
+; The purpose of this test is to show that the compiler throws an error when
+; there is no support for f16 vectors. If the compiler did not throw an error,
+; then it will try to scalarize the argument to an s32, which may drop elements.
+define <vscale x 1 x half> @test_ret_nxv1f16() {
+entry:
+  ret <vscale x 1 x half> undef
+}
+
+; CHECK: LLVM ERROR: unable to translate instruction: ret (in function: test_ret_nxv1f16)