[clang] [CIR][NFC] Refactor constant pointer l-value handling (PR #144165)

[Amr Hesham via cfe-commits]

================
@@ -329,6 +329,222 @@ emitArrayConstant(CIRGenModule &cgm, mlir::Type desiredType,
   return {};
 }
 
+//===----------------------------------------------------------------------===//
+//                          ConstantLValueEmitter
+//===----------------------------------------------------------------------===//
+
+namespace {
+/// A struct which can be used to peephole certain kinds of finalization
+/// that normally happen during l-value emission.
+struct ConstantLValue {
+  llvm::PointerUnion<mlir::Value, mlir::Attribute> value;
+  bool hasOffsetApplied;
+
+  ConstantLValue(std::nullptr_t) : value(nullptr), hasOffsetApplied(false) {}
+  ConstantLValue() : value(nullptr), hasOffsetApplied(false) {}
+};
+
+/// A helper class for emitting constant l-values.
+class ConstantLValueEmitter
+    : public ConstStmtVisitor<ConstantLValueEmitter, ConstantLValue> {
+  CIRGenModule &cgm;
+  ConstantEmitter &emitter;
+  const APValue &value;
+  QualType destType;
+
+  // Befriend StmtVisitorBase so that we don't have to expose Visit*.
+  friend StmtVisitorBase;
+
+public:
+  ConstantLValueEmitter(ConstantEmitter &emitter, const APValue &value,
+                        QualType destType)
+      : cgm(emitter.cgm), emitter(emitter), value(value), destType(destType) {}
+
+  mlir::Attribute tryEmit();
+
+private:
+  mlir::Attribute tryEmitAbsolute(mlir::Type destTy);
+  ConstantLValue tryEmitBase(const APValue::LValueBase &base);
+
+  ConstantLValue VisitStmt(const Stmt *s) { return nullptr; }
+  ConstantLValue VisitConstantExpr(const ConstantExpr *e);
+  ConstantLValue VisitCompoundLiteralExpr(const CompoundLiteralExpr *e);
+  ConstantLValue VisitStringLiteral(const StringLiteral *e);
+  ConstantLValue VisitObjCBoxedExpr(const ObjCBoxedExpr *e);
+  ConstantLValue VisitObjCEncodeExpr(const ObjCEncodeExpr *e);
+  ConstantLValue VisitObjCStringLiteral(const ObjCStringLiteral *e);
+  ConstantLValue VisitPredefinedExpr(const PredefinedExpr *e);
+  ConstantLValue VisitAddrLabelExpr(const AddrLabelExpr *e);
+  ConstantLValue VisitCallExpr(const CallExpr *e);
+  ConstantLValue VisitBlockExpr(const BlockExpr *e);
+  ConstantLValue VisitCXXTypeidExpr(const CXXTypeidExpr *e);
+  ConstantLValue
+  VisitMaterializeTemporaryExpr(const MaterializeTemporaryExpr *e);
+};
+
+} // namespace
+
+mlir::Attribute ConstantLValueEmitter::tryEmit() {
+  const APValue::LValueBase &base = value.getLValueBase();
+
+  // The destination type should be a pointer or reference
+  // type, but it might also be a cast thereof.
+  //
+  // FIXME: the chain of casts required should be reflected in the APValue.
+  // We need this in order to correctly handle things like a ptrtoint of a
+  // non-zero null pointer and addrspace casts that aren't trivially
+  // represented in LLVM IR.
+  mlir::Type destTy = cgm.getTypes().convertTypeForMem(destType);
+  assert(mlir::isa<cir::PointerType>(destTy));
+
+  // If there's no base at all, this is a null or absolute pointer,
+  // possibly cast back to an integer type.
+  if (!base)
+    return tryEmitAbsolute(destTy);
+
+  // Otherwise, try to emit the base.
+  ConstantLValue result = tryEmitBase(base);
+
+  // If that failed, we're done.
+  llvm::PointerUnion<mlir::Value, mlir::Attribute> &value = result.value;
+  if (!value)
+    return {};
+
+  // Apply the offset if necessary and not already done.
+  if (!result.hasOffsetApplied) {
+    cgm.errorNYI("ConstantLValueEmitter: apply offset");
+    return {};
+  }
+
+  // Convert to the appropriate type; this could be an lvalue for
+  // an integer. FIXME: performAddrSpaceCast
+  if (mlir::isa<cir::PointerType>(destTy)) {
+    if (auto attr = mlir::dyn_cast<mlir::Attribute>(value))
+      return attr;
+    cgm.errorNYI("ConstantLValueEmitter: non-attribute pointer");
+    return {};
+  }
+
+  cgm.errorNYI("ConstantLValueEmitter: other?");
+  return {};
+}
+
+/// Try to emit an absolute l-value, such as a null pointer or an integer
+/// bitcast to pointer type.
+mlir::Attribute ConstantLValueEmitter::tryEmitAbsolute(mlir::Type destTy) {
+  // If we're producing a pointer, this is easy.
+  auto destPtrTy = mlir::cast<cir::PointerType>(destTy);
+  return cgm.getBuilder().getConstPtrAttr(
+      destPtrTy, value.getLValueOffset().getQuantity());
+}
+
+ConstantLValue
+ConstantLValueEmitter::tryEmitBase(const APValue::LValueBase &base) {
+  // Handle values.
+  if (const ValueDecl *d = base.dyn_cast<const ValueDecl *>()) {
+    // The constant always points to the canonical declaration. We want to look
+    // at properties of the most recent declaration at the point of emission.
+    d = cast<ValueDecl>(d->getMostRecentDecl());
+
+    if (d->hasAttr<WeakRefAttr>()) {
+      cgm.errorNYI(d->getSourceRange(),
+                   "ConstantLValueEmitter: emit pointer base for weakref");
+      return {};
+    }
+
+    if (auto *fd = dyn_cast<FunctionDecl>(d)) {
+      cgm.errorNYI(fd->getSourceRange(),
+                   "ConstantLValueEmitter: function decl");
+      return {};
+    }
+
+    if (auto *vd = dyn_cast<VarDecl>(d)) {
+      cgm.errorNYI(vd->getSourceRange(), "ConstantLValueEmitter: var decl");
+      return {};
+    }
+  }
+
+  // Handle typeid(T).
+  if (TypeInfoLValue TI = base.dyn_cast<TypeInfoLValue>()) {
----------------
AmrDeveloper wrote:

NIT: `TI` is unused

```
if (base.dyn_cast<TypeInfoLValue>()) {
}
```

https://github.com/llvm/llvm-project/pull/144165