[flang] Fix construct names on labeled DO (#67622)

Fortran requires that a DO construct with a construct name end with an
END DO statement bearing the same name. This is true even if the DO
construct begins with a label DO statement; e.g., "constrName: do 10
j=1,10" must end with "10 end do constrName".

The compiler presently basically ignores construct names that appear on
label DO statements, because only non-label DO statements can be parsed
as DO constructs. This causes us to miss some errors, and (worse) breaks
the usage of the construct name on CYCLE and EXIT statements.

To fix this, this patch changes the parse tree and parser so that a DO
construct name on a putative label DO statement causes it to be parsed
as a "non-label" DO statement... with a label. Only true old-style
labeled DO statements without construct names are now parsed as such.

I did not change the class name NonLabelDoStmt -- it's widely used
across the front-end, and is the name of a production in the standard's
grammar. But now it basically means DoConstructDoStmt.

Fixes llvm/llvm-project#67283.

flang/include/flang/Parser/parse-tree.h

@@ -2259,15 +2259,18 @@ struct LoopControl {
 };
 
 // R1121 label-do-stmt -> [do-construct-name :] DO label [loop-control]
+// A label-do-stmt with a do-construct-name is parsed as a non-label-do-stmt.
 struct LabelDoStmt {
   TUPLE_CLASS_BOILERPLATE(LabelDoStmt);
-  std::tuple<std::optional<Name>, Label, std::optional<LoopControl>> t;
+  std::tuple<Label, std::optional<LoopControl>> t;
 };
 
 // R1122 nonlabel-do-stmt -> [do-construct-name :] DO [loop-control]
 struct NonLabelDoStmt {
   TUPLE_CLASS_BOILERPLATE(NonLabelDoStmt);
-  std::tuple<std::optional<Name>, std::optional<LoopControl>> t;
+  std::tuple<std::optional<Name>, std::optional<Label>,
+      std::optional<LoopControl>>
+      t;
 };
 
 // R1132 end-do-stmt -> END DO [do-construct-name]

flang/lib/Parser/executable-parsers.cpp

@@ -279,13 +279,17 @@ TYPE_CONTEXT_PARSER("loop control"_en_US,
                 many(Parser<LocalitySpec>{})))))
 
 // R1121 label-do-stmt -> [do-construct-name :] DO label [loop-control]
+// A label-do-stmt with a do-construct-name is parsed as a nonlabel-do-stmt
+// with an optional label.
 TYPE_CONTEXT_PARSER("label DO statement"_en_US,
-    construct<LabelDoStmt>(
-        maybe(name / ":"), "DO" >> label, maybe(loopControl)))
+    construct<LabelDoStmt>("DO" >> label, maybe(loopControl)))
 
 // R1122 nonlabel-do-stmt -> [do-construct-name :] DO [loop-control]
 TYPE_CONTEXT_PARSER("nonlabel DO statement"_en_US,
-    construct<NonLabelDoStmt>(maybe(name / ":"), "DO" >> maybe(loopControl)))
+    construct<NonLabelDoStmt>(
+        name / ":", "DO" >> maybe(label), maybe(loopControl)) ||
+        construct<NonLabelDoStmt>(construct<std::optional<Name>>(),
+            construct<std::optional<Label>>(), "DO" >> maybe(loopControl)))
 
 // R1132 end-do-stmt -> END DO [do-construct-name]
 TYPE_CONTEXT_PARSER("END DO statement"_en_US,

flang/lib/Parser/unparse.cpp

@@ -1001,13 +1001,14 @@ class UnparseVisitor {
     Walk(std::get<Statement<EndDoStmt>>(x.t));
   }
   void Unparse(const LabelDoStmt &x) { // R1121
-    Walk(std::get<std::optional<Name>>(x.t), ": ");
     Word("DO "), Walk(std::get<Label>(x.t));
     Walk(" ", std::get<std::optional<LoopControl>>(x.t));
   }
   void Unparse(const NonLabelDoStmt &x) { // R1122
     Walk(std::get<std::optional<Name>>(x.t), ": ");
-    Word("DO "), Walk(std::get<std::optional<LoopControl>>(x.t));
+    Word("DO ");
+    Walk(std::get<std::optional<Label>>(x.t), " ");
+    Walk(std::get<std::optional<LoopControl>>(x.t));
   }
   void Unparse(const LoopControl &x) { // R1123
     common::visit(common::visitors{

flang/lib/Semantics/canonicalize-do.cpp

@@ -117,16 +117,15 @@ class CanonicalizationOfDoLoops {
             std::get<ExecutableConstruct>(doLoop->u).u)};
         auto &loopControl{
             std::get<std::optional<LoopControl>>(labelDo.statement.value().t)};
-        auto &name{std::get<std::optional<Name>>(labelDo.statement.value().t)};
         Statement<NonLabelDoStmt> nonLabelDoStmt{std::move(labelDo.label),
-            NonLabelDoStmt{
-                std::make_tuple(common::Clone(name), std::move(loopControl))}};
+            NonLabelDoStmt{std::make_tuple(std::optional<Name>{},
+                std::optional<Label>{}, std::move(loopControl))}};
         nonLabelDoStmt.source = originalSource;
         std::get<ExecutableConstruct>(doLoop->u).u =
             common::Indirection<DoConstruct>{
                 std::make_tuple(std::move(nonLabelDoStmt), std::move(block),
-                    Statement<EndDoStmt>{
-                        std::optional<Label>{}, EndDoStmt{std::move(name)}})};
+                    Statement<EndDoStmt>{std::optional<Label>{},
+                        EndDoStmt{std::optional<Name>{}}})};
         stack.pop_back();
       } while (!stack.empty() && stack.back().label == currentLabel);
       i = --next;

flang/lib/Semantics/resolve-labels.cpp

@@ -275,7 +275,29 @@ class ParseTreeAnalyzer {
     return PushConstructName(criticalConstruct);
   }
   bool Pre(const parser::DoConstruct &doConstruct) {
-    return PushConstructName(doConstruct);
+    const auto &optionalName{std::get<std::optional<parser::Name>>(
+        std::get<parser::Statement<parser::NonLabelDoStmt>>(doConstruct.t)
+            .statement.t)};
+    if (optionalName) {
+      constructNames_.emplace_back(optionalName->ToString());
+    }
+    // Allow FORTRAN '66 extended DO ranges
+    PushScope().isExteriorGotoFatal = false;
+    // Process labels of the DO and END DO statements, but not the
+    // statements themselves, so that a non-construct END DO
+    // can be distinguished (below).
+    Pre(std::get<parser::Statement<parser::NonLabelDoStmt>>(doConstruct.t));
+    Walk(std::get<parser::Block>(doConstruct.t), *this);
+    Pre(std::get<parser::Statement<parser::EndDoStmt>>(doConstruct.t));
+    PopConstructName(doConstruct);
+    return false;
+  }
+  void Post(const parser::EndDoStmt &endDoStmt) {
+    // Visited only for non-construct labeled DO termination
+    if (const auto &name{endDoStmt.v}) {
+      context_.Say(name->source, "Unexpected DO construct name '%s'"_err_en_US,
+          name->source);
+    }
   }
   bool Pre(const parser::IfConstruct &ifConstruct) {
     return PushConstructName(ifConstruct);
@@ -330,9 +352,6 @@ class ParseTreeAnalyzer {
   void Post(const parser::CriticalConstruct &criticalConstruct) {
     PopConstructName(criticalConstruct);
   }
-  void Post(const parser::DoConstruct &doConstruct) {
-    PopConstructName(doConstruct);
-  }
   void Post(const parser::IfConstruct &ifConstruct) {
     PopConstructName(ifConstruct);
   }
@@ -716,6 +735,22 @@ class ParseTreeAnalyzer {
   // C1131
   void CheckName(const parser::DoConstruct &doConstruct) {
     CheckEndName<parser::NonLabelDoStmt, parser::EndDoStmt>("DO", doConstruct);
+    if (auto label{std::get<std::optional<parser::Label>>(
+            std::get<parser::Statement<parser::NonLabelDoStmt>>(doConstruct.t)
+                .statement.t)}) {
+      const auto &endDoStmt{
+          std::get<parser::Statement<parser::EndDoStmt>>(doConstruct.t)};
+      if (!endDoStmt.label || *endDoStmt.label != *label) {
+        context_
+            .Say(endDoStmt.source,
+                "END DO statement must have the label '%d' matching its DO statement"_err_en_US,
+                *label)
+            .Attach(std::get<parser::Statement<parser::NonLabelDoStmt>>(
+                        doConstruct.t)
+                        .source,
+                "corresponding DO statement"_en_US);
+      }
+    }
   }
   // C1035
   void CheckName(const parser::ForallConstruct &forallConstruct) {

flang/test/Semantics/dosemantics13.f90

@@ -0,0 +1,29 @@
+! RUN: %python %S/test_errors.py %s %flang_fc1
+program main
+
+  integer j, k
+
+  lab1: do j=1,10
+    cycle lab1
+    exit lab1
+  end do lab1
+
+  lab2: do 2 j=1,10
+    cycle lab2
+    exit lab2
+  2 end do lab2
+
+  lab3: do 3 j=1,10
+    cycle lab3
+    exit lab3
+    !ERROR: DO construct name required but missing
+  3 end do
+
+  do 4 j=1,10
+  !ERROR: Unexpected DO construct name 'lab4'
+  4 end do lab4
+
+  lab5: do 5 j=1,10
+  !ERROR: END DO statement must have the label '5' matching its DO statement
+  666 end do lab5
+end

flang/test/Semantics/dosemantics14.f90

@@ -0,0 +1,12 @@
+! RUN: %python %S/test_errors.py %s %flang_fc1
+subroutine bad1
+  lab1: do 1 j=1,10
+  1 continue
+!ERROR: expected 'END DO'
+end
+
+subroutine bad2
+  lab2: do 2 j=1,10
+  2 k = j
+!ERROR: expected 'END DO'
+end