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[flang][OpenMP] Don't allow DO CONCURRENT inside of a loop nest #144506

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Jun 18, 2025
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[flang][OpenMP] Don't allow DO CONCURRENT inside of a loop nest #144506

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3649ab1
[flang][OpenMP] Don't allow DO CONCURRENT inside of a loop nest
tblah Jun 17, 2025
ab3fba8
Allow do concurrent in loop constructs, but not loop nests
tblah Jun 17, 2025
eed0acb
Revert "Allow do concurrent in loop constructs, but not loop nests"
tblah Jun 17, 2025
5dbfe27
Allow do concurrent with LOOP construct
tblah Jun 17, 2025
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6 changes: 6 additions & 0 deletions flang/lib/Semantics/resolve-directives.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1952,6 +1952,12 @@ 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) {
if (loop->IsDoConcurrent()) {
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);
}
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@kparzysz kparzysz Jun 17, 2025

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

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@tblah tblah Jun 17, 2025

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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?

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@tblah tblah Jun 17, 2025

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

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@kparzysz kparzysz Jun 17, 2025

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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.

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@tblah tblah Jun 17, 2025

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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.

// go through all the nested do-loops and resolve index variables
const parser::Name *iv{GetLoopIndex(*loop)};
if (iv) {
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10 changes: 0 additions & 10 deletions flang/test/Lower/OpenMP/Todo/omp-doconcurrent.f90
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19 changes: 19 additions & 0 deletions flang/test/Semantics/OpenMP/do-concurrent-collapse.f90
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Original file line number Diff line number Diff line change
@@ -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
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