[mlir] Avoid pontentially ambiguous class name

[Kayjukh]

Summary

The Pass class exists in both the mlir and the llvm namespaces. Use the fully qualified class name to avoid any ambiguities.

[stephenneuendorffer]

Thanks.. I actually just ran into this the other day. I wonder if the standalone dialect example needs to get pushed into a separate namespace (I think it's still in mlir?) to find other places where there is an assumed namespace? I think all tablegen'd code would be alot safer if it always explicitly referred to MLIR classes through ::mlir.

[Kayjukh]

Yes, it's still in the mlir namespace. Moving it out of mlir would probably help, but I don't think there is enough coverage in it right now to have a meaningful impact. The issue fixed in this patch would not have been detected for example, since there are no custom passes in examples/standalone yet.

mlir/tools/mlir-tblgen/PassGen.cpp

public:
   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<Pass> clonePass() const override {{
+  std::unique_ptr<::mlir::Pass> clonePass() const override {{
     return std::make_unique<DerivedT>(*static_cast<const DerivedT *>(this));
   }
 
 protected:
 )";
 
 /// Emit the declarations for each of the pass options.
 static void emitPassOptionDecls(const Pass &pass, raw_ostream &os) {
os << llvm::formatv("#define GEN_PASS_REGISTRATION_{0}\n",
                         pass.getDef()->getName());
   }
   os << "#endif // GEN_PASS_REGISTRATION\n";
 
   for (const Pass &pass : passes) {
     os << llvm::formatv("#ifdef GEN_PASS_REGISTRATION_{0}\n",
                         pass.getDef()->getName());
     os << llvm::formatv("::mlir::registerPass(\"{0}\", \"{1}\", []() -> "
-                        "std::unique_ptr<Pass> {{ return {2}; });\n",
+                        "std::unique_ptr<::mlir::Pass> {{ return {2}; });\n",
                         pass.getArgument(), pass.getSummary(),
                         pass.getConstructor());
     os << llvm::formatv("#endif // GEN_PASS_REGISTRATION_{0}\n",
                         pass.getDef()->getName());
     os << llvm::formatv("#undef GEN_PASS_REGISTRATION_{0}\n",
                         pass.getDef()->getName());
   }