[llvm] [SandboxIR] Implement UndefValue (PR #107628)

[via llvm-commits]

================
@@ -1020,10 +1021,61 @@ class ConstantPointerNull final : public Constant {
 #endif
 };
 
-// TODO: Inherit from UndefValue.
-class PoisonValue final : public Constant {
+// TODO: Inherit from ConstantData.
+class UndefValue : public Constant {
+protected:
+  UndefValue(llvm::UndefValue *C, Context &Ctx)
+      : Constant(ClassID::UndefValue, C, Ctx) {}
+  UndefValue(ClassID ID, llvm::Constant *C, Context &Ctx)
+      : Constant(ID, C, Ctx) {}
+  friend class Context; // For constructor.
+
+public:
+  /// Static factory methods - Return an 'undef' object of the specified type.
+  static UndefValue *get(Type *T);
+
+  /// If this Undef has array or vector type, return a undef with the right
+  /// element type.
+  UndefValue *getSequentialElement() const;
+
+  /// If this undef has struct type, return a undef with the right element type
+  /// for the specified element.
+  UndefValue *getStructElement(unsigned Elt) const;
+
+  /// Return an undef of the right value for the specified GEP index if we can,
+  /// otherwise return null (e.g. if C is a ConstantExpr).
+  UndefValue *getElementValue(Constant *C) const;
+
+  /// Return an undef of the right value for the specified GEP index.
+  UndefValue *getElementValue(unsigned Idx) const;
+
+  /// Return the number of elements in the array, vector, or struct.
+  unsigned getNumElements() const {
+    return cast<llvm::UndefValue>(Val)->getNumElements();
+  }
+
+  /// For isa/dyn_cast.
+  static bool classof(const sandboxir::Value *From) {
+    return From->getSubclassID() == ClassID::UndefValue ||
+           From->getSubclassID() == ClassID::PoisonValue;
----------------
vporpo wrote:

> isa<PoisonValue>(Undef) should/must be false.

We are already testing that: ` EXPECT_FALSE(isa<sandboxir::PoisonValue>(Val: Undef)); // Undef is not Poison  `

https://github.com/llvm/llvm-project/pull/107628