[ADT] Fix specialization of ValueIsPresent for PointerUnion (#121847)

Two instances of `PointerUnion` with different active members and null
value compare unequal. Currently, this results in counterintuitive
behavior when using functions from `Casting.h`, e.g.:

```C++
  PointerUnion<int *, float *> U;
  // U = (int *)nullptr;
  dyn_cast<int *>(U); // Aborts
  dyn_cast<float *>(U); // Aborts
  U = (float *)nullptr;
  dyn_cast<int *>(U); // OK
  dyn_cast<float *>(U); // OK
```

`dyn_cast` should abort in all cases because the argument is null.
Currently, it aborts only if the first member is active. This happens
because the partial template specialization of `ValueIsPresent` for
nullable types compares the union with a union constructed from nullptr,
and the two unions compare equal only if their active members are the
same.

This patch changed the specialization of `ValueIsPresent` for nullable
types to make `isPresent()` return false for all possible null values of
a PointerUnion, and fixes two places where the old behavior was
exploited.

Pull Request: llvm/llvm-project#121847

llvm/include/llvm/Support/Casting.h

@@ -614,12 +614,12 @@ template <typename T> struct ValueIsPresent<std::optional<T>> {
   static inline decltype(auto) unwrapValue(std::optional<T> &t) { return *t; }
 };
 
-// If something is "nullable" then we just compare it to nullptr to see if it
-// exists.
+// If something is "nullable" then we just cast it to bool to see if it exists.
 template <typename T>
-struct ValueIsPresent<T, std::enable_if_t<IsNullable<T>>> {
+struct ValueIsPresent<
+    T, std::enable_if_t<IsNullable<T> && std::is_constructible_v<bool, T>>> {
   using UnwrappedType = T;
-  static inline bool isPresent(const T &t) { return t != T(nullptr); }
+  static inline bool isPresent(const T &t) { return static_cast<bool>(t); }
   static inline decltype(auto) unwrapValue(T &t) { return t; }
 };
 

llvm/lib/CodeGen/RegisterBankInfo.cpp

@@ -134,10 +134,10 @@ const TargetRegisterClass *RegisterBankInfo::constrainGenericRegister(
 
   // If the register already has a class, fallback to MRI::constrainRegClass.
   auto &RegClassOrBank = MRI.getRegClassOrRegBank(Reg);
-  if (isa<const TargetRegisterClass *>(RegClassOrBank))
+  if (isa_and_present<const TargetRegisterClass *>(RegClassOrBank))
     return MRI.constrainRegClass(Reg, &RC);
 
-  const RegisterBank *RB = cast<const RegisterBank *>(RegClassOrBank);
+  const auto *RB = dyn_cast_if_present<const RegisterBank *>(RegClassOrBank);
   // Otherwise, all we can do is ensure the bank covers the class, and set it.
   if (RB && !RB->covers(RC))
     return nullptr;

llvm/lib/Target/AMDGPU/SIRegisterInfo.cpp

@@ -3708,10 +3708,10 @@ const TargetRegisterClass *
 SIRegisterInfo::getConstrainedRegClassForOperand(const MachineOperand &MO,
                                          const MachineRegisterInfo &MRI) const {
   const RegClassOrRegBank &RCOrRB = MRI.getRegClassOrRegBank(MO.getReg());
-  if (const RegisterBank *RB = dyn_cast<const RegisterBank *>(RCOrRB))
+  if (const auto *RB = dyn_cast_if_present<const RegisterBank *>(RCOrRB))
     return getRegClassForTypeOnBank(MRI.getType(MO.getReg()), *RB);
 
-  if (const auto *RC = dyn_cast<const TargetRegisterClass *>(RCOrRB))
+  if (const auto *RC = dyn_cast_if_present<const TargetRegisterClass *>(RCOrRB))
     return getAllocatableClass(RC);
 
   return nullptr;

llvm/unittests/ADT/PointerUnionTest.cpp

@@ -208,6 +208,11 @@ TEST_F(PointerUnionTest, NewCastInfra) {
   EXPECT_FALSE(isa<float *>(d4null));
   EXPECT_FALSE(isa<long long *>(d4null));
 
+  EXPECT_FALSE(isa_and_present<int *>(i4null));
+  EXPECT_FALSE(isa_and_present<float *>(f4null));
+  EXPECT_FALSE(isa_and_present<long long *>(l4null));
+  EXPECT_FALSE(isa_and_present<double *>(d4null));
+
   // test cast<>
   EXPECT_EQ(cast<float *>(a), &f);
   EXPECT_EQ(cast<int *>(b), &i);