[flang][OpenMP] Set isNewBlock directly on OpenMP constructs

[kparzysz]

When the PFT builder decides that an evaluation needs a new block it checks if the evaluation has nested evaluations. In such case it sets the flag on the first nested evaluation. This works under the assuption that such an evaluation only serves as a container, and does not, by itself, generate any code.

This fails for OpenMP constructs that contain nested evaluations because the top-level evaluation does generate code that wraps the code from the nested evaluations. In such cases, the code for the top-level evaluation may be emitted in a wrong place.

When setting the isNewBlock flag, recognize OpenMP directives, and treat them accordingly.

This fixes #139071

[llvmbot]

@llvm/pr-subscribers-flang-openmp

@llvm/pr-subscribers-flang-fir-hlfir

Author: Krzysztof Parzyszek (kparzysz)

Changes

When the PFT builder decides that an evaluation needs a new block it checks if the evaluation has nested evaluations. In such case it sets the flag on the first nested evaluation. This works under the assuption that such an evaluation only serves as a container, and does not, by itself, generate any code.

This fails for OpenMP constructs that contain nested evaluations because the top-level evaluation does generate code that wraps the code from the nested evaluations. In such cases, the code for the top-level evaluation may be emitted in a wrong place.

When setting the isNewBlock flag, recognize OpenMP directives, and treat them accordingly.

This fixes #139071

---

Full diff: https://github.com/llvm/llvm-project/pull/144593.diff

3 Files Affected:

• (modified) flang/include/flang/Lower/PFTBuilder.h (+10)
• (modified) flang/lib/Lower/PFTBuilder.cpp (+3-1)
• (added) flang/test/Lower/OpenMP/multiple-entry-points.f90 (+46)

diff --git a/flang/include/flang/Lower/PFTBuilder.h b/flang/include/flang/Lower/PFTBuilder.h
index 42d6546b77553..ac87fcd7c3b9b 100644
--- a/flang/include/flang/Lower/PFTBuilder.h
+++ b/flang/include/flang/Lower/PFTBuilder.h
@@ -184,6 +184,11 @@ static constexpr bool isExecutableDirective{common::HasMember<
     A, std::tuple<parser::CompilerDirective, parser::OpenACCConstruct,
                   parser::OpenMPConstruct, parser::CUFKernelDoConstruct>>};
 
+template <typename A>
+static constexpr bool isOpenMPDirective{
+    common::HasMember<A, std::tuple<parser::OpenMPConstruct,
+                                    parser::OpenMPDeclarativeConstruct>>};
+
 template <typename A>
 static constexpr bool isFunctionLike{common::HasMember<
     A, std::tuple<parser::MainProgram, parser::FunctionSubprogram,
@@ -267,6 +272,11 @@ struct Evaluation : EvaluationVariant {
       return pft::isExecutableDirective<std::decay_t<decltype(r)>>;
     }});
   }
+  constexpr bool isOpenMPDirective() const {
+    return visit(common::visitors{[](auto &r) {
+      return pft::isOpenMPDirective<std::decay_t<decltype(r)>>;
+    }});
+  }
 
   /// Return the predicate:  "This is a non-initial, non-terminal construct
   /// statement."  For an IfConstruct, this is ElseIfStmt and ElseStmt.
diff --git a/flang/lib/Lower/PFTBuilder.cpp b/flang/lib/Lower/PFTBuilder.cpp
index 2cc458cb6130d..68023610c3c50 100644
--- a/flang/lib/Lower/PFTBuilder.cpp
+++ b/flang/lib/Lower/PFTBuilder.cpp
@@ -1096,7 +1096,9 @@ class PFTBuilder {
 
     // The first executable statement in the subprogram is preceded by a
     // branch to the entry point, so it starts a new block.
-    if (initialEval->hasNestedEvaluations())
+    // OpenMP directives can generate code around the nested evaluations.
+    if (initialEval->hasNestedEvaluations() &&
+        !initialEval->isOpenMPDirective())
       initialEval = &initialEval->getFirstNestedEvaluation();
     else if (initialEval->isA<Fortran::parser::EntryStmt>())
       initialEval = initialEval->lexicalSuccessor;
diff --git a/flang/test/Lower/OpenMP/multiple-entry-points.f90 b/flang/test/Lower/OpenMP/multiple-entry-points.f90
new file mode 100644
index 0000000000000..2b8caa79eaa15
--- /dev/null
+++ b/flang/test/Lower/OpenMP/multiple-entry-points.f90
@@ -0,0 +1,46 @@
+!RUN: %flang_fc1 -emit-hlfir -fopenmp %s -o - | FileCheck %s
+
+! Check the first entry point
+!CHECK: func.func @_QPprocess_a
+!CHECK: omp.parallel
+!CHECK: omp.wsloop
+!CHECK: %[[V0:[0-9]+]] = fir.load %{{[0-9]+}} : !fir.ref<f32>
+!CHECK: %[[V1:[a-z_0-9]+]] = arith.constant 2.000000e+00 : f32
+!CHECK:   = arith.mulf %[[V0]], %[[V1]] fastmath<contract> : f32
+!CHECK: omp.terminator
+!CHECK-NOT: omp
+!CHECK: return
+
+! Check the second entry point
+!CHECK: func.func @_QPprocess_b
+!CHECK: omp.parallel
+!CHECK: fir.do_loop
+!CHECK: %[[V3:[0-9]+]] = fir.load %[[V2:[0-9]+]]#0 : !fir.ref<i32>
+!CHECK: %[[V4:[0-9]+]] = fir.load %[[V2]]#0 : !fir.ref<i32>
+!CHECK:   = arith.muli %[[V3]], %[[V4]] : i32
+!CHECK: omp.terminator
+!CHECK-NOT: omp
+!CHECK: return
+
+subroutine process_a(n, a)
+  integer, intent(in) :: n
+  real, intent(inout) :: a(n)
+  integer :: i
+
+  !$omp parallel do
+  do i = 1, n
+    a(i) = a(i) * 2.0
+  end do
+  !$omp end parallel do
+
+  return
+
+  entry process_b(n, b)
+    
+  !$omp parallel
+  do i = 1, n
+    a(i) = i * i
+  end do
+  !$omp end parallel
+
+end subroutine process_a