[AddOrUpdateCommonBlockUse] Implemented selection of largest COMMON block in case of uses from multiple modules, except that it doesn't work: COMMON block sizes are computed after the name resolution

Author: eugeneepshteyn

flang/include/flang/Semantics/scope.h

@@ -194,7 +194,7 @@ class Scope {
   const mapType &crayPointers() const { return crayPointers_; }
   void add_crayPointer(const SourceName &, Symbol &);
   Symbol &MakeCommonBlock(SourceName, SourceName location);
-  bool AddCommonBlockUse(const SourceName &name, Symbol &cbSymbol);
+  bool AddOrUpdateCommonBlockUse(const SourceName &name, Attrs attrs, Symbol& cbUltimate);
 
   // Find COMMON block that is declared in the current scope
   Symbol *FindCommonBlock(const SourceName &name) const {

flang/lib/Semantics/resolve-names.cpp

@@ -3631,22 +3631,14 @@ void ModuleVisitor::Post(const parser::UseStmt &x) {
   // their USE association to the current scope's USE-associated COMMON blocks.
   for (const auto &[name, symbol] : useModuleScope_->commonBlocks()) {
     if (!currScope().FindCommonBlockInVisibleScopes(name)) {
-      // Make a symbol, but don't add it to the Scope, since it needs to
-      // be added to the USE-associated COMMON blocks
-      Symbol &localCB{currScope().MakeSymbol(
-          name, symbol->attrs(), UseDetails{name, symbol->GetUltimate()})};
-      currScope().AddCommonBlockUse(name, localCB);
+      currScope().AddOrUpdateCommonBlockUse(name, symbol->attrs(), symbol->GetUltimate());
     }
   }
   // Go through the list of USE-associated COMMON block symbols in the module
   // scope and add USE associations to their ultimate symbols to the current
   // scope's USE-associated COMMON blocks.
   for (const auto &[name, symbol] : useModuleScope_->commonBlockUses()) {
-    // Make a symbol, but don't add it to the Scope, since it needs to
-    // be added to the USE-associated COMMON blocks
-    Symbol &localCB{currScope().MakeSymbol(
-        name, symbol->attrs(), UseDetails{name, symbol->GetUltimate()})};
-    currScope().AddCommonBlockUse(name, localCB);
+    currScope().AddOrUpdateCommonBlockUse(name, symbol->attrs(), symbol->GetUltimate());
   }
   useModuleScope_ = nullptr;
 }

flang/lib/Semantics/scope.cpp

@@ -183,8 +183,24 @@ Scope *Scope::FindSubmodule(const SourceName &name) const {
   }
 }
 
-bool Scope::AddCommonBlockUse(const SourceName &name, Symbol &cbSymbol) {
-  return commonBlockUses_.emplace(name, cbSymbol).second;
+bool Scope::AddOrUpdateCommonBlockUse(const SourceName &name, Attrs attrs, Symbol& cbUltimate) {
+  CHECK(cbUltimate.has<CommonBlockDetails>());
+  if (Symbol *cb{FindCommonBlockUse(name)}) {
+    // We already stored UseDetails from the previous encounter with the
+    // same named COMMON block (perhaps it came from a different module).
+    // Ensure that we store USE association to the largest COMMON block,
+    // which is presumably is better defined.
+    Symbol &cbExistingUltimate{cb->GetUltimate()};
+    if (cbExistingUltimate.size() < cbUltimate.size()) {
+      cb->set_details(UseDetails{name, cbUltimate});
+    }
+    return true;
+  } else {
+    // Make a symbol, but don't add it to the Scope, since it needs to
+    // be added to the USE-associated COMMON blocks
+    Symbol &useCB{MakeSymbol(name, attrs, UseDetails{name, cbUltimate})};
+    return commonBlockUses_.emplace(name, useCB).second;
+  }
 }
 
 bool Scope::AddSubmodule(const SourceName &name, Scope &submodule) {

flang/lib/Semantics/symbol.cpp

@@ -373,7 +373,16 @@ bool Symbol::CanReplaceDetails(const Details &details) const {
             },
             [&](const UseDetails &x) {
               const auto *use{this->detailsIf<UseDetails>()};
-              return use && use->symbol() == x.symbol();
+              if (!use) {
+                return false;
+              } else if (use->symbol().detailsIf<CommonBlockDetails>() &&
+                  x.symbol().detailsIf<CommonBlockDetails>()) {
+                // Allow to replace UseDetails, if they both refer to COMMON
+                // symbol with the same name.
+                return use->symbol().name() == x.symbol().name();
+              } else {
+                return use->symbol() == x.symbol();
+              }
             },
             [&](const HostAssocDetails &) { return has<HostAssocDetails>(); },
             [&](const UserReductionDetails &) {