[GlobalISel][TableGen] Fix ConstrainOperandRC bug

[ehjogab]

Summary

TableGen would pick the largest RC for constraining the operands, which
could potentially be an unallocatable RC. This patch removes selection
of unallocatable RCs.

[ehjogab]

Fixed bad comment

[uabelho]

I pushed this for @ehjogab

llvm/test/TableGen/GlobalISelEmitterSubreg.td

 def DRegs : MyClass<32, [i32], (sequence "D%u", 0, 0)>;
 def ERegs : MyClass<32, [i32], (sequence "E%u", 0, 0)>;
 def SOP : RegisterOperand<SRegs>;
 def DOP : RegisterOperand<DRegs>;
 def SOME_INSN : I<(outs DRegs:$dst), (ins DOP:$src), []>;
 def SUBSOME_INSN : I<(outs SRegs:$dst), (ins SOP:$src), []>;
 def SUBSOME_INSN2 : I<(outs SRegs:$dst), (ins SOP:$src), []>;
 
+// Adding this enables the tests below to check that we are not using this class
+// for constraining the operand register classes, since it is unallocatable.
+let isAllocatable = 0 in
+def SuperDRegs : MyClass<32, [i32], (add DRegs, ERegs)>;
+
 // We should skip cases where we don't have a given register class for the
 // subregister source.
 // SKIPPED: def : Pat<(i32 (anyext i16:$src)), (INSERT_SUBREG (i32 (IMPLICIT_DEF)), i16:$src, sub0)>;
 // SKIPPED: def : Pat<(i32 (anyext i16:$src)), (SOME_INSN (INSERT_SUBREG (i32 (IMPLICIT_DEF)), i16:$src, sub0))>;
 def : Pat<(i32 (anyext i16:$src)), (INSERT_SUBREG (i32 (IMPLICIT_DEF)), i16:$src, sub0)>;
 def : Pat<(i32 (anyext i16:$src)), (SOME_INSN (INSERT_SUBREG (i32 (IMPLICIT_DEF)), i16:$src, sub0))>;
 
 // Test that we import INSERT_SUBREG when its subregister source has a given

llvm/utils/TableGen/CodeGenTarget.h

public:
 
   /// getRegBank - Return the register bank description.
   CodeGenRegBank &getRegBank() const;
 
   /// Return the largest register class on \p RegBank which supports \p Ty and
   /// covers \p SubIdx if it exists.
   Optional<CodeGenRegisterClass *>
   getSuperRegForSubReg(const ValueTypeByHwMode &Ty, CodeGenRegBank &RegBank,
-                       const CodeGenSubRegIndex *SubIdx) const;
+                       const CodeGenSubRegIndex *SubIdx,
+                       bool MustBeAllocatable = false) const;
 
   /// getRegisterByName - If there is a register with the specific AsmName,
   /// return it.
   const CodeGenRegister *getRegisterByName(StringRef Name) const;
 
   const std::vector<Record*> &getRegAltNameIndices() const {
     if (RegAltNameIndices.empty()) ReadRegAltNameIndices();
     return RegAltNameIndices;

llvm/utils/TableGen/CodeGenTarget.cpp

CodeGenRegBank &CodeGenTarget::getRegBank() const {
   if (!RegBank)
     RegBank = std::make_unique<CodeGenRegBank>(Records, getHwModes());
   return *RegBank;
 }
 
 Optional<CodeGenRegisterClass *>
 CodeGenTarget::getSuperRegForSubReg(const ValueTypeByHwMode &ValueTy,
                                     CodeGenRegBank &RegBank,
-                                    const CodeGenSubRegIndex *SubIdx) const {
+                                    const CodeGenSubRegIndex *SubIdx,
+                                    bool MustBeAllocatable) const {
   std::vector<CodeGenRegisterClass *> Candidates;
   auto &RegClasses = RegBank.getRegClasses();
 
   // Try to find a register class which supports ValueTy, and also contains
   // SubIdx.
   for (CodeGenRegisterClass &RC : RegClasses) {
     // Is there a subclass of this class which contains this subregister index?
     CodeGenRegisterClass *SubClassWithSubReg = RC.getSubClassWithSubReg(SubIdx);
     if (!SubClassWithSubReg)
       continue;
 
     // We have a class. Check if it supports this value type.
     if (!llvm::is_contained(SubClassWithSubReg->VTs, ValueTy))
       continue;
 
+    // If necessary, check that it is allocatable.
+    if (MustBeAllocatable && !SubClassWithSubReg->Allocatable)
+      continue;
+
     // We have a register class which supports both the value type and
     // subregister index. Remember it.
     Candidates.push_back(SubClassWithSubReg);
   }
 
   // If we didn't find anything, we're done.
   if (Candidates.empty())
     return None;

llvm/utils/TableGen/GlobalISelEmitter.cpp

GlobalISelEmitter::inferSuperRegisterClass(const TypeSetByHwMode &Ty,
   DefInit *SubRegInit = dyn_cast<DefInit>(SubRegIdxNode->getLeafValue());
   if (!SubRegInit)
     return None;
   CodeGenSubRegIndex *SubIdx = CGRegs.getSubRegIdx(SubRegInit->getDef());
 
   // Use the information we found above to find a minimal register class which
   // supports the subregister and type we want.
   auto RC =
-      Target.getSuperRegForSubReg(Ty.getValueTypeByHwMode(), CGRegs, SubIdx);
+      Target.getSuperRegForSubReg(Ty.getValueTypeByHwMode(), CGRegs, SubIdx,
+                                  /* MustBeAllocatable */ true);
   if (!RC)
     return None;
   return *RC;
 }
 
 Optional<const CodeGenRegisterClass *>
 GlobalISelEmitter::inferSuperRegisterClassForNode(
     const TypeSetByHwMode &Ty, TreePatternNode *SuperRegNode,