[Flang] Add perfect-nest and rectangular-loop semantic tests (#160283)

Add semantic tests of currently unsupported OpenMP canonical loops:

 * non-perfectly nested canonical loop nests
 * non-rectangular canonical loop nests

Both were introduced in OpenMP 5.0 and are not yet supported by Flang.

The message "Trip count must be computable and invariant" is the same
that OpenACC emits for non-rectangular loops in
`AccAttributeVisitor::CheckAssociatedLoop`. I considered reusing the
code, but calls OpenACC-only methods and has different behavior (e.g.
symbol resolution and does not check the step operand)

Author: Meinersbur

flang/lib/Semantics/resolve-directives.cpp

@@ -149,7 +149,24 @@ template <typename T> class DirectiveAttributeVisitor {
     dataSharingAttributeObjects_.clear();
   }
   bool HasDataSharingAttributeObject(const Symbol &);
+
+  /// Extract the iv and bounds of a DO loop:
+  /// 1. The loop index/induction variable
+  /// 2. The lower bound
+  /// 3. The upper bound
+  /// 4. The step/increment (or nullptr if not present)
+  ///
+  /// Each returned tuple value can be nullptr if not present. Diagnoses an
+  /// error if the the DO loop is a DO WHILE or DO CONCURRENT loop.
+  std::tuple<const parser::Name *, const parser::ScalarExpr *,
+      const parser::ScalarExpr *, const parser::ScalarExpr *>
+  GetLoopBounds(const parser::DoConstruct &);
+
+  /// Extract the loop index/induction variable from a DO loop. Diagnoses an
+  /// error if the the DO loop is a DO WHILE or DO CONCURRENT loop and returns
+  /// nullptr.
   const parser::Name *GetLoopIndex(const parser::DoConstruct &);
+
   const parser::DoConstruct *GetDoConstructIf(
       const parser::ExecutionPartConstruct &);
   Symbol *DeclareNewAccessEntity(const Symbol &, Symbol::Flag, Scope &);
@@ -953,6 +970,13 @@ class OmpAttributeVisitor : DirectiveAttributeVisitor<llvm::omp::Directive> {
     privateDataSharingAttributeObjects_.clear();
   }
 
+  /// Check that loops in the loop nest are perfectly nested, as well that lower
+  /// bound, upper bound, and step expressions do not use the iv
+  /// of a surrounding loop of the associated loops nest.
+  /// We do not support non-perfectly nested loops not non-rectangular loops yet
+  /// (both introduced in OpenMP 5.0)
+  void CheckPerfectNestAndRectangularLoop(const parser::OpenMPLoopConstruct &x);
+
   // Predetermined DSA rules
   void PrivatizeAssociatedLoopIndexAndCheckLoopLevel(
       const parser::OpenMPLoopConstruct &);
@@ -1028,23 +1052,30 @@ bool DirectiveAttributeVisitor<T>::HasDataSharingAttributeObject(
 }
 
 template <typename T>
-const parser::Name *DirectiveAttributeVisitor<T>::GetLoopIndex(
-    const parser::DoConstruct &x) {
+std::tuple<const parser::Name *, const parser::ScalarExpr *,
+    const parser::ScalarExpr *, const parser::ScalarExpr *>
+DirectiveAttributeVisitor<T>::GetLoopBounds(const parser::DoConstruct &x) {
   using Bounds = parser::LoopControl::Bounds;
   if (x.GetLoopControl()) {
     if (const Bounds * b{std::get_if<Bounds>(&x.GetLoopControl()->u)}) {
-      return &b->name.thing;
-    } else {
-      return nullptr;
+      auto &step = b->step;
+      return {&b->name.thing, &b->lower, &b->upper,
+          step.has_value() ? &step.value() : nullptr};
     }
   } else {
     context_
         .Say(std::get<parser::Statement<parser::NonLabelDoStmt>>(x.t).source,
             "Loop control is not present in the DO LOOP"_err_en_US)
         .Attach(GetContext().directiveSource,
             "associated with the enclosing LOOP construct"_en_US);
-    return nullptr;
   }
+  return {nullptr, nullptr, nullptr, nullptr};
+}
+
+template <typename T>
+const parser::Name *DirectiveAttributeVisitor<T>::GetLoopIndex(
+    const parser::DoConstruct &x) {
+  return std::get<const parser::Name *>(GetLoopBounds(x));
 }
 
 template <typename T>
@@ -1990,6 +2021,10 @@ bool OmpAttributeVisitor::Pre(const parser::OpenMPLoopConstruct &x) {
       }
     }
   }
+
+  // Must be done before iv privatization
+  CheckPerfectNestAndRectangularLoop(x);
+
   PrivatizeAssociatedLoopIndexAndCheckLoopLevel(x);
   ordCollapseLevel = GetNumAffectedLoopsFromLoopConstruct(x) + 1;
   return true;
@@ -2185,6 +2220,116 @@ void OmpAttributeVisitor::CollectNumAffectedLoopsFromClauses(
   }
 }
 
+void OmpAttributeVisitor::CheckPerfectNestAndRectangularLoop(
+    const parser::OpenMPLoopConstruct &x) {
+  auto &dirContext{GetContext()};
+  std::int64_t dirDepth{dirContext.associatedLoopLevel};
+  if (dirDepth <= 0)
+    return;
+
+  auto checkExprHasSymbols = [&](llvm::SmallVector<Symbol *> &ivs,
+                                 const parser::ScalarExpr *bound) {
+    if (ivs.empty())
+      return;
+    auto boundExpr{semantics::AnalyzeExpr(context_, *bound)};
+    if (!boundExpr)
+      return;
+    semantics::UnorderedSymbolSet boundSyms{
+        evaluate::CollectSymbols(*boundExpr)};
+    if (boundSyms.empty())
+      return;
+    for (Symbol *iv : ivs) {
+      if (boundSyms.count(*iv) != 0) {
+        // TODO: Point to occurence of iv in boundExpr, directiveSource as a
+        //       note
+        context_.Say(dirContext.directiveSource,
+            "Trip count must be computable and invariant"_err_en_US);
+      }
+    }
+  };
+
+  // Find the associated region by skipping nested loop-associated constructs
+  // such as loop transformations
+  const parser::NestedConstruct *innermostAssocRegion{nullptr};
+  const parser::OpenMPLoopConstruct *innermostConstruct{&x};
+  while (const auto &innerAssocStmt{
+      std::get<std::optional<parser::NestedConstruct>>(
+          innermostConstruct->t)}) {
+    innermostAssocRegion = &(innerAssocStmt.value());
+    if (const auto *innerConstruct{
+            std::get_if<common::Indirection<parser::OpenMPLoopConstruct>>(
+                innermostAssocRegion)}) {
+      innermostConstruct = &innerConstruct->value();
+    } else {
+      break;
+    }
+  }
+
+  if (!innermostAssocRegion)
+    return;
+  const auto &outer{std::get_if<parser::DoConstruct>(innermostAssocRegion)};
+  if (!outer)
+    return;
+
+  llvm::SmallVector<Symbol *> ivs;
+  int curLevel{0};
+  const parser::DoConstruct *loop{outer};
+  while (true) {
+    auto [iv, lb, ub, step] = GetLoopBounds(*loop);
+
+    if (lb)
+      checkExprHasSymbols(ivs, lb);
+    if (ub)
+      checkExprHasSymbols(ivs, ub);
+    if (step)
+      checkExprHasSymbols(ivs, step);
+    if (iv) {
+      if (auto *symbol{currScope().FindSymbol(iv->source)})
+        ivs.push_back(symbol);
+    }
+
+    // Stop after processing all affected loops
+    if (curLevel + 1 >= dirDepth)
+      break;
+
+    // Recurse into nested loop
+    const auto &block{std::get<parser::Block>(loop->t)};
+    if (block.empty()) {
+      // Insufficient number of nested loops already reported by
+      // CheckAssocLoopLevel()
+      break;
+    }
+
+    loop = GetDoConstructIf(block.front());
+    if (!loop) {
+      // Insufficient number of nested loops already reported by
+      // CheckAssocLoopLevel()
+      break;
+    }
+
+    auto checkPerfectNest = [&, this]() {
+      auto blockSize = block.size();
+      if (blockSize <= 1)
+        return;
+
+      if (parser::Unwrap<parser::ContinueStmt>(x))
+        blockSize -= 1;
+
+      if (blockSize <= 1)
+        return;
+
+      // Non-perfectly nested loop
+      // TODO: Point to non-DO statement, directiveSource as a note
+      context_.Say(dirContext.directiveSource,
+          "Canonical loop nest must be perfectly nested."_err_en_US);
+    };
+
+    checkPerfectNest();
+
+    ++curLevel;
+  }
+}
+
 // 2.15.1.1 Data-sharing Attribute Rules - Predetermined
 //   - The loop iteration variable(s) in the associated do-loop(s) of a do,
 //     parallel do, taskloop, or distribute construct is (are) private.

flang/test/Semantics/OpenMP/do08.f90

@@ -61,6 +61,7 @@ program omp
   !$omp end do
 
 
+  !ERROR: Canonical loop nest must be perfectly nested.
   !ERROR: The value of the parameter in the COLLAPSE or ORDERED clause must not be larger than the number of nested loops following the construct.
   !$omp do collapse(3)
   do 60 i=2,200,2

flang/test/Semantics/OpenMP/do13.f90

@@ -59,6 +59,7 @@ program omp
   !$omp end do
 
 
+  !ERROR: Canonical loop nest must be perfectly nested.
   !ERROR: The value of the parameter in the COLLAPSE or ORDERED clause must not be larger than the number of nested loops following the construct.
   !$omp do collapse(3)
   do 60 i=1,10

flang/test/Semantics/OpenMP/do22.f90

@@ -0,0 +1,73 @@
+! RUN: %python %S/../test_errors.py %s %flang -fopenmp
+! Check for existence of loop following a DO directive
+
+subroutine do_imperfectly_nested_before
+  integer i, j
+
+  !ERROR: The value of the parameter in the COLLAPSE or ORDERED clause must not be larger than the number of nested loops following the construct.
+  !$omp do collapse(2)
+  do i = 1, 10
+    print *, i
+    do j = 1, 10
+      print *, i, j
+    end do
+  end do
+  !$omp end do
+end subroutine
+
+
+subroutine do_imperfectly_nested_behind
+  integer i, j
+
+  !ERROR: Canonical loop nest must be perfectly nested.
+  !$omp do collapse(2)
+  do i = 1, 10
+    do j = 1, 10
+      print *, i, j
+    end do
+    print *, i
+  end do
+  !$omp end do
+end subroutine
+
+
+subroutine do_nonrectangular_lb
+  integer i, j
+
+  !ERROR: Trip count must be computable and invariant
+  !$omp do collapse(2)
+  do i = 1, 10
+    do j = i, 10
+      print *, i, j
+    end do
+  end do
+  !$omp end do
+end subroutine
+
+
+subroutine do_nonrectangular_ub
+  integer i, j
+
+  !ERROR: Trip count must be computable and invariant
+  !$omp do collapse(2)
+  do i = 1, 10
+    do j = 0, i
+      print *, i, j
+    end do
+  end do
+  !$omp end do
+end subroutine
+
+
+subroutine do_nonrectangular_step
+  integer i, j
+
+  !ERROR: Trip count must be computable and invariant
+  !$omp do collapse(2)
+  do i = 1, 10
+    do j = 1, 10, i
+      print *, i, j
+    end do
+  end do
+  !$omp end do
+end subroutine