Add a global flag to disable the global dialect registry "process wise"

[mehdi_amini]

Summary

This is intended to ease the transition for client with a lot of
dependencies. It'll be removed in the coming weeks.

[mehdi_amini]

Build fix

[rriddle]

mlir/lib/IR/MLIRContext.cpp

520

This is backwards.

mlir/include/mlir/IR/Dialect.h

 private:
   MapTy registry;
 };
 
 /// Deprecated: this provides a global registry for convenience, while we're
 /// transitionning the registration mechanism to a stateless approach.
 DialectRegistry &getGlobalDialectRegistry();
 
+/// This controls globally whether the dialect registry is / isn't enabled.
+/// This is deprecated and only intended to help the transition. It'll be
+/// removed soon.
+void enableGlobalDialectRegistry(bool);
+bool isGlobalDialectRegistryEnabled();
+
 /// Registers all dialects from the global registries with the
 /// specified MLIRContext. This won't load the dialects in the context,
 /// but only make them available for lazy loading by name.
 /// Note: This method is not thread-safe.
+/// Deprecated: this method will be deleted soon.
 void registerAllDialects(MLIRContext *context);
 
 /// Register and return the dialect with the given namespace in the provided
 /// context. Returns nullptr is there is no constructor registered for this
 /// dialect.
 inline Dialect *registerDialect(StringRef name, MLIRContext *context) {
   return getGlobalDialectRegistry().loadByName(name, context);
 }

mlir/lib/IR/Dialect.cpp

 #include "llvm/Support/Regex.h"
 
 using namespace mlir;
 using namespace detail;
 
 DialectAsmParser::~DialectAsmParser() {}
 
 //===----------------------------------------------------------------------===//
-// Dialect Registration
+// Dialect Registration (DEPRECATED)
 //===----------------------------------------------------------------------===//
 
 /// Registry for all dialect allocation functions.
 static llvm::ManagedStatic<DialectRegistry> dialectRegistry;
 DialectRegistry &mlir::getGlobalDialectRegistry() { return *dialectRegistry; }
 
+// Note: deprecated, will be removed soon.
+static bool isGlobalDialectRegistryEnabledFlag = true;
+void mlir::enableGlobalDialectRegistry(bool enable) {
+  isGlobalDialectRegistryEnabledFlag = enable;
+}
+bool mlir::isGlobalDialectRegistryEnabled() {
+  return isGlobalDialectRegistryEnabledFlag;
+}
+
 void mlir::registerAllDialects(MLIRContext *context) {
   dialectRegistry->appendTo(context->getDialectRegistry());
 }
 
 Dialect *DialectRegistry::loadByName(StringRef name, MLIRContext *context) {
   auto it = registry.find(name.str());
   if (it == registry.end())
     return nullptr;

mlir/lib/IR/MLIRContext.cpp

#endif
   if (dialect->getTypeID() != dialectID)
     llvm::report_fatal_error("a dialect with namespace '" + dialectNamespace +
                              "' has already been registered");
 
   return dialect.get();
 }
 
 void MLIRContext::loadAllGloballyRegisteredDialects() {
+  if (!isGlobalDialectRegistryEnabled())
+    return;
   getGlobalDialectRegistry().loadAll(this);
 }
 
 bool MLIRContext::allowsUnregisteredDialects() {
   return impl->allowUnregisteredDialects;
 }
 
 void MLIRContext::allowUnregisteredDialects(bool allowing) {