[RegisterBank] Provide a way to check if a register bank is valid.

Change the default constructor to create invalid object.
The target will have to properly initialize the register banks before
using them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@265460 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/GlobalISel/RegisterBank.h

@@ -30,10 +30,17 @@ private:
   unsigned ID;
   const char *Name;
   unsigned Size;
-  BitVector ContainedRegClass;
+  BitVector ContainedRegClasses;
+
+  /// Sentinel value used to recognize register bank not properly
+  /// initialized yet.
+  static const unsigned InvalidID;
 
   /// Only the RegisterBankInfo can create RegisterBank.
-  RegisterBank() = default;
+  /// The default constructor will leave the object in
+  /// an invalid state. I.e. isValid() == false.
+  /// The field must be updated to fix that.
+  RegisterBank();
 
   friend RegisterBankInfo;
 
@@ -48,6 +55,9 @@ public:
   /// Get the maximal size in bits that fits in this register bank.
   unsigned getSize() const { return Size; }
 
+  /// Check whether this instance is ready to be used.
+  bool isValid() const;
+
   /// Check if this register bank is valid. In other words,
   /// if it has been properly constructed.
   void verify(const TargetRegisterInfo &TRI) const;
@@ -55,6 +65,7 @@ public:
   /// Check whether this register bank contains \p RC.
   /// In other words, check if this register bank fully covers
   /// the registers that \p RC contains.
+  /// \pre isValid()
   bool contains(const TargetRegisterClass &RC) const;
 
   /// Check whether \p OtherRB is the same as this.

lib/CodeGen/GlobalISel/RegisterBank.cpp

@@ -18,6 +18,10 @@
 
 using namespace llvm;
 
+const unsigned RegisterBank::InvalidID = UINT_MAX;
+
+RegisterBank::RegisterBank() : ID(InvalidID), Name(nullptr), Size(0) {}
+
 void RegisterBank::verify(const TargetRegisterInfo &TRI) const {
   // Verify that the Size of the register bank is big enough to cover all the
   // register classes it covers.
@@ -26,7 +30,14 @@ void RegisterBank::verify(const TargetRegisterInfo &TRI) const {
 }
 
 bool RegisterBank::contains(const TargetRegisterClass &RC) const {
-  return ContainedRegClass.test(RC.getID());
+  assert(isValid() && "RB hasn't been initialized yet");
+  return ContainedRegClasses.test(RC.getID());
+}
+
+bool RegisterBank::isValid() const {
+  return ID != InvalidID && Name != nullptr && Size != 0 &&
+         // A register bank that does not cover anything is useless.
+         !ContainedRegClasses.empty();
 }
 
 bool RegisterBank::operator==(const RegisterBank &OtherRB) const {