[flang][openacc] Relax constraint on OpenACC declare statement

[akuhlens]

OpenACC declare statements are restricted from having having clauses that reference assumed size arrays. It should be the case that we can implement deviceptr and present clauses for assumed-size arrays. This is a first step towards relaxing this restriction.

Note running flang on the following example results in an error in lowering.

$ cat t.f90
subroutine vadd (a, b, c, n)
   real(8) :: a(*), b(*), c(*)
!$acc declare deviceptr(a, b, c)
!$acc parallel loop
   do i = 1,n
      c(i) = a(i) + b(i)
   enddo
end subroutine

$ flang -fopenacc -c t.f90
error: loc("/home/akuhlenschmi/work/p4/ta/tests/openacc/src/t.f90":3:7): expect declare attribute on variable in declare operation
error: Lowering to LLVM IR failed
error: loc("/home/akuhlenschmi/work/p4/ta/tests/openacc/src/t.f90":4:7): unsupported OpenACC operation: acc.private.recipe
error: loc("/home/akuhlenschmi/work/p4/ta/tests/openacc/src/t.f90":4:7): LLVM Translation failed for operation: acc.private.recipe
error: failed to create the LLVM module

I would like to to share this code, because others are currently working on the implementation of deviceptr, but it is obviously not running end-to-end. I think the cleanest approach to this would be to put this exception to the rule behind some feature flag, but I am not certain what the precedence for that is.

[llvmbot]

@llvm/pr-subscribers-flang-semantics
@llvm/pr-subscribers-openacc

@llvm/pr-subscribers-flang-openmp

Author: Andre Kuhlenschmidt (akuhlens)

Changes

OpenACC declare statements are restricted from having having clauses that reference assumed size arrays. It should be the case that we can implement deviceptr and present clauses for assumed-size arrays. This is a first step towards relaxing this restriction.

Note running flang on the following example results in an error in lowering.

$ cat t.f90
subroutine vadd (a, b, c, n)
   real(8) :: a(*), b(*), c(*)
!$acc declare deviceptr(a, b, c)
!$acc parallel loop
   do i = 1,n
      c(i) = a(i) + b(i)
   enddo
end subroutine

$ flang -fopenacc -c t.f90
error: loc("/home/akuhlenschmi/work/p4/ta/tests/openacc/src/t.f90":3:7): expect declare attribute on variable in declare operation
error: Lowering to LLVM IR failed
error: loc("/home/akuhlenschmi/work/p4/ta/tests/openacc/src/t.f90":4:7): unsupported OpenACC operation: acc.private.recipe
error: loc("/home/akuhlenschmi/work/p4/ta/tests/openacc/src/t.f90":4:7): LLVM Translation failed for operation: acc.private.recipe
error: failed to create the LLVM module

I would like to to share this code, because others are currently working on the implementation of deviceptr, but it is obviously not running end-to-end. I think the cleanest approach to this would be to put this exception to the rule behind some feature flag, but I am not certain what the precedence for that is.

---

Full diff: https://github.com/llvm/llvm-project/pull/135238.diff

2 Files Affected:

• (modified) flang/lib/Semantics/resolve-directives.cpp (+8-1)
• (modified) flang/test/Semantics/OpenACC/acc-declare-validity.f90 (+13-1)

diff --git a/flang/lib/Semantics/resolve-directives.cpp b/flang/lib/Semantics/resolve-directives.cpp
index a5b3391859500..4a3994dcffa1b 100644
--- a/flang/lib/Semantics/resolve-directives.cpp
+++ b/flang/lib/Semantics/resolve-directives.cpp
@@ -953,7 +953,14 @@ void AccAttributeVisitor::Post(
   const auto &clauseList = std::get<parser::AccClauseList>(x.t);
   for (const auto &clause : clauseList.v) {
     // Restriction - line 2414
-    DoNotAllowAssumedSizedArray(GetAccObjectList(clause));
+    // We assume the restriction is present because clauses that require
+    // moving data would require the size of the data to be present, but
+    // the deviceptr and present clauses do not require moving data and
+    // thus we permit them.
+    if (!std::holds_alternative<parser::AccClause::Deviceptr>(clause.u) &&
+        !std::holds_alternative<parser::AccClause::Present>(clause.u)) {
+      DoNotAllowAssumedSizedArray(GetAccObjectList(clause));
+    }
   }
 }
 
diff --git a/flang/test/Semantics/OpenACC/acc-declare-validity.f90 b/flang/test/Semantics/OpenACC/acc-declare-validity.f90
index 2e7e651815d4a..7cdc69436704b 100644
--- a/flang/test/Semantics/OpenACC/acc-declare-validity.f90
+++ b/flang/test/Semantics/OpenACC/acc-declare-validity.f90
@@ -62,9 +62,21 @@ end function fct1
   subroutine sub2(cc)
     real(8), dimension(*) :: cc
     !ERROR: Assumed-size dummy arrays may not appear on the DECLARE directive
-    !$acc declare present(cc)
+    !$acc declare copyin(cc)
   end subroutine sub2
 
+  subroutine sub2e1(cc)
+    real(8), dimension(*) :: cc
+    !OK
+    !$acc declare present(cc)
+  end subroutine sub2e1
+
+  subroutine sub2e2(cc)
+    real(8), dimension(*) :: cc
+    !OK
+    !$acc declare deviceptr(cc)
+  end subroutine sub2e2
+
   subroutine sub3()
     real :: aa(100)
     !ERROR: The ZERO modifier is not allowed for the COPYOUT clause on the DECLARE directive

[klausler]

LGTM

[clementval]

Can you add a line in flang/docs/OpenACC.md. There is a section where we list the intentional deviation from the standard.

[razvanlupusoru]

Thank you! Great work!