[mlir][flang] Improve handling of fortran module variables. (#91604)

Currently, only those global variables which are at compile unit scope
are added to the 'globals' list of the DICompileUnit. This does not work
for languages which support modules (e.g. Fortran) where hierarchy
can be
variable -> module -> compile unit.

To fix this, if a variable scope points to a module, we walk one level
up and see if module is in the compile unit scope.

This was initially part of #91582 which adds debug information for
Fortran module variables. @kiranchandramohan pointed out that MLIR
changes should go in separate PRs.

mlir/lib/Target/LLVMIR/ModuleTranslation.cpp

@@ -1030,10 +1030,21 @@ LogicalResult ModuleTranslation::convertGlobals() {
       llvm::DIGlobalVariable *diGlobalVar = diGlobalExpr->getVariable();
       var->addDebugInfo(diGlobalExpr);
 
+      // There is no `globals` field in DICompileUnitAttr which can be directly
+      // assigned to DICompileUnit. We have to build the list by looking at the
+      // dbgExpr of all the GlobalOps. The scope of the variable is used to get
+      // the DICompileUnit in which to add it. But for the languages that
+      // support modules, the scope hierarchy can be
+      // variable -> module -> compile unit
+      // If a variable scope points to the module then we use the scope of the
+      // module to get the compile unit.
+      llvm::DIScope *scope = diGlobalVar->getScope();
+      if (llvm::DIModule *mod = dyn_cast_if_present<llvm::DIModule>(scope))
+        scope = mod->getScope();
+
       // Get the compile unit (scope) of the the global variable.
       if (llvm::DICompileUnit *compileUnit =
-              dyn_cast_if_present<llvm::DICompileUnit>(
-                  diGlobalVar->getScope())) {
+              dyn_cast_if_present<llvm::DICompileUnit>(scope)) {
         // Update the compile unit with this incoming global variable expression
         // during the finalizing step later.
         allGVars[compileUnit].push_back(diGlobalExpr);

mlir/test/Target/LLVMIR/llvmir-debug.mlir

@@ -311,6 +311,25 @@ llvm.mlir.global external @global_with_expr_2() {addr_space = 0 : i32, dbg_expr
 
 // -----
 
+// CHECK: @module_global = external global i64, !dbg {{.*}}
+// CHECK: !llvm.module.flags = !{{{.*}}}
+// CHECK: !llvm.dbg.cu = !{{{.*}}}
+// CHECK-DAG: ![[FILE:.*]] = !DIFile(filename: "test.f90", directory: "existence")
+// CHECK-DAG: ![[TYPE:.*]] = !DIBasicType(name: "integer", size: 64, encoding: DW_ATE_signed)
+// CHECK-DAG: ![[SCOPE:.*]] = distinct !DICompileUnit(language: DW_LANG_Fortran95, file: ![[FILE]], producer: "MLIR", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, globals: ![[GVALS:.*]])
+// CHECK-DAG: ![[SCOPE1:.*]] = !DIModule(scope: ![[SCOPE]], name: "module2", file: ![[FILE]], line: 120)
+// CHECK-DAG: ![[GVAR:.*]] = distinct !DIGlobalVariable(name: "module_global", linkageName: "module_global", scope: ![[SCOPE1]], file: ![[FILE]], line: 121, type: ![[TYPE]], isLocal: false, isDefinition: true)
+// CHECK-DAG: ![[GEXPR:.*]] = !DIGlobalVariableExpression(var: ![[GVAR]], expr: !DIExpression())
+// CHECK-DAG: ![[GVALS]] = !{![[GEXPR]]}
+
+#di_file = #llvm.di_file<"test.f90" in "existence">
+#di_compile_unit = #llvm.di_compile_unit<id = distinct[0]<>, sourceLanguage = DW_LANG_Fortran95, file = #di_file, producer = "MLIR", isOptimized = true, emissionKind = Full>
+#di_basic_type = #llvm.di_basic_type<tag = DW_TAG_base_type, name = "integer", sizeInBits = 64, encoding = DW_ATE_signed>
+#di_module = #llvm.di_module<file = #di_file, scope = #di_compile_unit, name = "module2", configMacros = "", includePath = "", apinotes = "", line = 120, isDecl = false >
+llvm.mlir.global external @module_global() {dbg_expr = #llvm.di_global_variable_expression<var = <scope = #di_module, name = "module_global", linkageName = "module_global", file = #di_file, line = 121, type = #di_basic_type, isLocalToUnit = false, isDefined = true>, expr = <>>} : i64
+
+// -----
+
 // Nameless and scopeless global constant.
 
 // CHECK-LABEL: @.str.1 = external constant [10 x i8]