[flang][OpenMP] Don't allow DO CONCURRENT inside of a loop nest

[tblah]

I don't think DO CONCURRENT fits the definition of a Canonical Loop Nest (OpenMP 6.0 section 6.4.1).
It is however explicitly allowed for the LOOP construct (6.0 section 13.8).

There's some obscure language in OpenMP 6.0 for the LOOP construct:

If the collapsed loop is a DO CONCURRENT loop, neither the
data-sharing attribute clauses nor the collapse clause may be specified.

From the surrounding context, I think "collapsed loop" just means the
loop that the LOOP construct applies to. So I will interpret this to
mean that DO CONCURRENT can only be used with the LOOP construct if it
does not contain the COLLAPSE clause.

This also fixes a bug where the associated clause was never cleared
after it was set.

Fixes #144178

[kparzysz]

DO CONCURRENT is allowed in the LOOP construct, although, IIUC, only as the top-level loop.

[tblah]

ab3fba8 allows do concurrent as a single loop construct but not as part of a loop nest. Does that match your understanding of the standard?

[tblah]

The test for this is https://github.com/llvm/llvm-project/blob/main/flang/test/Lower/OpenMP/Todo/omp-doconcurrent.f90

[kparzysz]

I don't think it's allowed in any worksharing-loop construct, only in the LOOP construct [6.0:424:1] "The collapsed loop may be a DO CONCURRENT loop." No other constructs in that chapter have this annotation.

[tblah]

Thanks for pointing this out.

If the collapsed loop is a DO CONCURRENT loop, neither the data-sharing attribute clauses nor the collapse clause may be specified.

I am taking this to mean that DO CONCURRENT is allowed with LOOP, but not when LOOP has a COLLAPSE clause. "Collapsed loop" seems elsewhere to refer to anything enclosed by the LOOP construct.

[llvmbot]

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Author: Tom Eccles (tblah)

Changes

I don't think DO CONCURRENT fits the definition of a Canonical Loop Nest (OpenMP 6.0 section 6.4.1).

Fixes #144178

---

Full diff: https://github.com/llvm/llvm-project/pull/144506.diff

2 Files Affected:

• (modified) flang/lib/Semantics/resolve-directives.cpp (+7)
• (added) flang/test/Semantics/OpenMP/do-concurrent-collapse.f90 (+19)

diff --git a/flang/lib/Semantics/resolve-directives.cpp b/flang/lib/Semantics/resolve-directives.cpp
index b5f8667fe36f2..05568cc0ba0ae 100644
--- a/flang/lib/Semantics/resolve-directives.cpp
+++ b/flang/lib/Semantics/resolve-directives.cpp
@@ -1952,6 +1952,13 @@ void OmpAttributeVisitor::PrivatizeAssociatedLoopIndexAndCheckLoopLevel(
   const auto &outer{std::get<std::optional<parser::DoConstruct>>(x.t)};
   if (outer.has_value()) {
     for (const parser::DoConstruct *loop{&*outer}; loop && level > 0; --level) {
+      // DO CONCURRENT is allowed for loop constructs but not loop nests
+      if (loop->IsDoConcurrent() && GetContext().associatedLoopLevel != 1) {
+        auto &stmt =
+            std::get<parser::Statement<parser::NonLabelDoStmt>>(loop->t);
+        context_.Say(stmt.source,
+            "DO CONCURRENT loops cannot form part of a loop nest."_err_en_US);
+      }
       // go through all the nested do-loops and resolve index variables
       const parser::Name *iv{GetLoopIndex(*loop)};
       if (iv) {
diff --git a/flang/test/Semantics/OpenMP/do-concurrent-collapse.f90 b/flang/test/Semantics/OpenMP/do-concurrent-collapse.f90
new file mode 100644
index 0000000000000..71e1928e1f245
--- /dev/null
+++ b/flang/test/Semantics/OpenMP/do-concurrent-collapse.f90
@@ -0,0 +1,19 @@
+!RUN: %python %S/../test_errors.py %s %flang -fopenmp
+
+integer :: i, j
+!$omp parallel do collapse(2)
+do i = 1, 1
+  ! ERROR: DO CONCURRENT loops cannot form part of a loop nest.
+  do concurrent (j = 1:2)
+    print *, j
+  end do
+end do
+
+!$omp parallel do
+do i = 1, 1
+  ! This should not lead to an error because it is not part of a loop nest:
+  do concurrent (j = 1:2)
+    print *, j
+  end do
+end do
+end