[mlir][Interfaces] Generate a struct containing function pointers instead of a class with vtables

[rriddle]

Summary

When compiling for code size, the use of a vtable causes a destructor(and constructor in certain cases) to be generated for the class. Interface models don't need a complex constructor or a destructor, so this can lead to many megabytes of code size increase(even in opt). This revision switches to a simpler struct of function pointers approach that accomplishes the same API requirements as before. This change requires no updates to user code, or any other code aside from the generator, as the user facing API is still exactly the same.

Depends On D90084

[mehdi_amini]

Impressively simple!

[jpienaar]

mlir/tools/mlir-tblgen/OpInterfacesGen.cpp

204

Why 2 lines here? (e.g., os << ");\n"; doesn't seem more difficult to read).

[rriddle]

Rebase

mlir/tools/mlir-tblgen/OpInterfacesGen.cpp

bool InterfaceGenerator::emitInterfaceDefs() {
   return false;
 }
 
 //===----------------------------------------------------------------------===//
 // GEN: Interface declarations
 //===----------------------------------------------------------------------===//
 
 void InterfaceGenerator::emitConceptDecl(Interface &interface) {
-  os << "  class Concept {\n"
-     << "  public:\n"
-     << "    virtual ~Concept() = default;\n";
+  os << "  struct Concept {\n";
 
   // Insert each of the pure virtual concept methods.
   for (auto &method : interface.getMethods()) {
-    os << "    virtual ";
+    os << "    ";
     emitCPPType(method.getReturnType(), os);
-    emitMethodNameAndArgs(method, os, valueType,
-                          /*addThisArg=*/!method.isStatic(), /*addConst=*/true);
-    os << " = 0;\n";
+    os << "(*" << method.getName() << ")(";
+    if (!method.isStatic())
+      emitCPPType(valueType, os) << (method.arg_empty() ? "" : ", ");
+    llvm::interleaveComma(
+        method.getArguments(), os,
+        [&](const InterfaceMethod::Argument &arg) { os << arg.type; });
+    os << ");\n";
   }
   os << "  };\n";
 }
 
 void InterfaceGenerator::emitModelDecl(Interface &interface) {
   os << "  template<typename " << valueTemplate << ">\n";
   os << "  class Model : public Concept {\n  public:\n";
+  os << "    Model() : Concept{";
+  llvm::interleaveComma(
+      interface.getMethods(), os,
+      [&](const InterfaceMethod &method) { os << method.getName(); });
+  os << "} {}\n\n";
 
   // Insert each of the virtual method overrides.
   for (auto &method : interface.getMethods()) {
-    emitCPPType(method.getReturnType(), os << "    ");
+    emitCPPType(method.getReturnType(), os << "    static ");
     emitMethodNameAndArgs(method, os, valueType,
-                          /*addThisArg=*/!method.isStatic(), /*addConst=*/true);
-    os << " final {\n      ";
+                          /*addThisArg=*/!method.isStatic(),
+                          /*addConst=*/false);
+    os << " {\n      ";
 
     // Check for a provided body to the function.
     if (Optional<StringRef> body = method.getBody()) {
       if (method.isStatic())
         os << body->trim();
       else
         os << tblgen::tgfmt(body->trim(), &nonStaticMethodFmt);
       os << "\n    }\n";