[GlobalISel][TableGen] Fix ConstrainOperandRC bug

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

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D93945

test/TableGen/GlobalISelEmitterSubreg.td

@@ -36,6 +36,11 @@ 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)>;

utils/TableGen/CodeGenTarget.cpp

@@ -344,7 +344,8 @@ CodeGenRegBank &CodeGenTarget::getRegBank() const {
 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();
 
@@ -360,6 +361,10 @@ CodeGenTarget::getSuperRegForSubReg(const ValueTypeByHwMode &ValueTy,
     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);

utils/TableGen/CodeGenTarget.h

@@ -111,7 +111,8 @@ public:
   /// 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.

utils/TableGen/GlobalISelEmitter.cpp

@@ -4998,7 +4998,8 @@ GlobalISelEmitter::inferSuperRegisterClass(const TypeSetByHwMode &Ty,
   // 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;