[MLIR] Keep but deprecate old autogenerated pass base classes

[mscuttari]

Summary

Restore the generation of the old pass base classes for better transitioning of external projects relying on them. They were eliminated with 67d0d7ac0acb0665d6a09f61278fbcf51f0114c2.

[mehdi_amini]

Thanks! Can you start a thread (tagged [PSA]) on discord announcing the deprecation and including the instructions for downstream to upgrade within X weeks? (anything between 2 to 6 weeks is likely fine)

[mscuttari]

In D133027#3762562, @mehdi_amini wrote:

Thanks! Can you start a thread (tagged [PSA]) on discord announcing the deprecation and including the instructions for downstream to upgrade within X weeks? (anything between 2 to 6 weeks is likely fine)

Opened, I proposed a 6 weeks window.

mlir/tools/mlir-tblgen/PassGen.cpp

 static void emitPass(const Pass &pass, raw_ostream &os) {
   StringRef passName = pass.getDef()->getName();
   os << llvm::formatv(passHeader, passName);
 
   emitPassDecls(pass, os);
   emitPassDefs(pass, os);
 }
 
+// TODO: Drop old pass declarations.
+// The old pass base class is being kept until all the passes have switched to
+// the new decls/defs design.
+const char *const oldPassDeclBegin = R"(
+template <typename DerivedT>
+class {0}Base : public {1} {
+public:
+  using Base = {0}Base;
+
+  {0}Base() : {1}(::mlir::TypeID::get<DerivedT>()) {{}
+  {0}Base(const {0}Base &other) : {1}(other) {{}
+
+  /// Returns the command-line argument attached to this pass.
+  static constexpr ::llvm::StringLiteral getArgumentName() {
+    return ::llvm::StringLiteral("{2}");
+  }
+  ::llvm::StringRef getArgument() const override { return "{2}"; }
+
+  ::llvm::StringRef getDescription() const override { return "{3}"; }
+
+  /// Returns the derived pass name.
+  static constexpr ::llvm::StringLiteral getPassName() {
+    return ::llvm::StringLiteral("{0}");
+  }
+  ::llvm::StringRef getName() const override { return "{0}"; }
+
+  /// Support isa/dyn_cast functionality for the derived pass class.
+  static bool classof(const ::mlir::Pass *pass) {{
+    return pass->getTypeID() == ::mlir::TypeID::get<DerivedT>();
+  }
+
+  /// A clone method to create a copy of this pass.
+  std::unique_ptr<::mlir::Pass> clonePass() const override {{
+    return std::make_unique<DerivedT>(*static_cast<const DerivedT *>(this));
+  }
+
+  /// Return the dialect that must be loaded in the context before this pass.
+  void getDependentDialects(::mlir::DialectRegistry &registry) const override {
+    {4}
+  }
+
+  /// Explicitly declare the TypeID for this class. We declare an explicit private
+  /// instantiation because Pass classes should only be visible by the current
+  /// library.
+  MLIR_DEFINE_EXPLICIT_INTERNAL_INLINE_TYPE_ID({0}Base<DerivedT>)
+
+protected:
+)";
+
+// TODO: Drop old pass declarations.
+/// Emit a backward-compatible declaration of the pass base class.
+static void emitOldPassDecl(const Pass &pass, raw_ostream &os) {
+  StringRef defName = pass.getDef()->getName();
+  std::string dependentDialectRegistrations;
+  {
+    llvm::raw_string_ostream dialectsOs(dependentDialectRegistrations);
+    for (StringRef dependentDialect : pass.getDependentDialects())
+      dialectsOs << llvm::formatv(dialectRegistrationTemplate,
+                                  dependentDialect);
+  }
+  os << llvm::formatv(oldPassDeclBegin, defName, pass.getBaseClass(),
+                      pass.getArgument(), pass.getSummary(),
+                      dependentDialectRegistrations);
+  emitPassOptionDecls(pass, os);
+  emitPassStatisticDecls(pass, os);
+  os << "};\n";
+}
+
 static void emitPasses(const llvm::RecordKeeper &recordKeeper,
                        raw_ostream &os) {
   std::vector<Pass> passes = getPasses(recordKeeper);
   os << "/* Autogenerated by mlir-tblgen; don't manually edit */\n";
 
   for (const Pass &pass : passes)
     emitPass(pass, os);
 
   emitRegistrations(passes, os);
+
+  // TODO: Drop old pass declarations.
+  // Emit the old code until all the passes have switched to the new design.
+  os << "// Deprecated. Please use the new per-pass macros.\n";
+  os << "#ifdef GEN_PASS_CLASSES\n";
+  for (const Pass &pass : passes)
+    emitOldPassDecl(pass, os);
+  os << "#undef GEN_PASS_CLASSES\n";
+  os << "#endif // GEN_PASS_CLASSES\n";
 }
 
 static mlir::GenRegistration
     genPassDecls("gen-pass-decls", "Generate pass declarations",
                  [](const llvm::RecordKeeper &records, raw_ostream &os) {
                    emitPasses(records, os);
                    return false;
                  });