[flang-commits] [flang] [flang][OpenMP][Semantics] Don't allow reduction of derived type comp… (PR #125480)

[Tom Eccles via flang-commits]

https://github.com/tblah created https://github.com/llvm/llvm-project/pull/125480

…onents

Before this patch, reduction of derived type components crashed the compiler when trying to create the omp.declare_reduction.

In OpenMP 3.1 the standard says "a list item that appears in a reduction clause must be a named variable of intrinsic type" (page 106). As I understand it, a derived type component is not a variable.

OpenMP 4.0 added declare reduction, partly so that users could define their own reductions on derived types. The above wording was removed from the standard but derived type components were never explicitly allowed.

OpenMP 5.0 added "A variable that is part of another variable, with the exception of array elements, cannot appear in17 a reduction clause".

As we do not currently support derived type components, and these are only implicitly permitted in OpenMP 4.x, I think it is better to disallow them in semantics rather than only printing a TODO for openmp 4.x. This is also what gfortran and classic-flang do.

Fixes #125445

>From 0b6f2e176feda4cee6000ab83ef36512d44df138 Mon Sep 17 00:00:00 2001
From: Tom Eccles <tom.eccles at arm.com>
Date: Mon, 3 Feb 2025 11:10:05 +0000
Subject: [PATCH] [flang][OpenMP][Semantics] Don't allow reduction of derived
 type components

Before this patch, reduction of derived type components crashed the
compiler when trying to create the omp.declare_reduction.

In OpenMP 3.1 the standard says "a list item that appears in a reduction
clause must be a named variable of intrinsic type" (page 106). As I
understand it, a derived type component is not a variable.

OpenMP 4.0 added declare reduction, partly so that users could define
their own reductions on derived types. The above wording was removed
from the standard but derived type components were never explicitly
allowed.

OpenMP 5.0 added "A variable that is part of another variable, with
the exception of array elements, cannot appear in17 a reduction
clause".

As we do not currently support derived type components, and these are
only implicitly permitted in OpenMP 4.x, I think it is better to
disallow them in semantics rather than only printing a TODO for openmp
4.x. This is also what gfortran and classic-flang do.

Fixes #125445
---
 flang/lib/Semantics/check-omp-structure.cpp            |  8 ++++++--
 .../Semantics/OpenMP/reduction-derived-component.f90   | 10 ++++++++++
 2 files changed, 16 insertions(+), 2 deletions(-)
 create mode 100644 flang/test/Semantics/OpenMP/reduction-derived-component.f90

diff --git a/flang/lib/Semantics/check-omp-structure.cpp b/flang/lib/Semantics/check-omp-structure.cpp
index 035064ecf3a46e..ad36ce2bde0db9 100644
--- a/flang/lib/Semantics/check-omp-structure.cpp
+++ b/flang/lib/Semantics/check-omp-structure.cpp
@@ -3224,9 +3224,13 @@ void OmpStructureChecker::CheckReductionObjects(
     }
   }
 
+  // Disallowed in standards before 4.0 and in 5.0 and later. Not explicitly
+  // allowed in 4.0. Keep this as an error until/unless structure component
+  // reduction is implemented.
+  // Object cannot be a part of another object (except array elements)
+  CheckStructureComponent(objects, clauseId);
+
   if (version >= 50) {
-    // Object cannot be a part of another object (except array elements)
-    CheckStructureComponent(objects, clauseId);
     // If object is an array section or element, the base expression must be
     // a language identifier.
     for (const parser::OmpObject &object : objects.v) {
diff --git a/flang/test/Semantics/OpenMP/reduction-derived-component.f90 b/flang/test/Semantics/OpenMP/reduction-derived-component.f90
new file mode 100644
index 00000000000000..87e5f1acda159b
--- /dev/null
+++ b/flang/test/Semantics/OpenMP/reduction-derived-component.f90
@@ -0,0 +1,10 @@
+! RUN: %python %S/../test_errors.py %s %flang_fc1 -fopenmp
+subroutine intrinsic_reduction
+  type local
+     integer alpha
+  end type local
+  type(local) a
+!ERROR: A variable that is part of another variable cannot appear on the REDUCTION clause
+!$omp parallel reduction(+:a%alpha)
+!$omp end parallel
+end subroutine intrinsic_reduction