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[Flang] Fix handling of unlimited polymorphic arrays #159624

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[Flang] Fix handling of unlimited polymorphic arrays #159624

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[Flang] Fix handling of unlimited polymorphic arrays
ceseo Sep 18, 2025
b982f6f
Simplified condition and updated comments in AssignOpConversion
ceseo Sep 22, 2025
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5 changes: 3 additions & 2 deletions flang/lib/Optimizer/HLFIR/Transforms/ConvertToFIR.cpp
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Expand Up @@ -126,8 +126,9 @@ class AssignOpConversion : public mlir::OpRewritePattern<hlfir::AssignOp> {
// assignment is the copy of the contents, because the dynamic
// types of the LHS and the RHS must match already. We use the
// runtime in this case so that the polymorphic (including
// unlimited) content is copied properly.
(lhs.isPolymorphic() && assignOp.isTemporaryLHS())) {
// unlimited) content is copied properly. This is also needed
// for unlimited polymorphic arrays in WHERE statements.
(lhs.isPolymorphic())) {
// Use the runtime for simplicity. An optimization pass will be added to
// inline array assignment when profitable.
mlir::Value from = emboxRHS(rhsExv);
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25 changes: 25 additions & 0 deletions flang/test/HLFIR/unlimited-polymorphic-where-construct.f90
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@@ -0,0 +1,25 @@
! RUN: flang -fc1 -emit-hlfir %s -o - | FileCheck %s
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Is this test testing the fix? Since the fix in the patch happens at the HLFIR to FIR level, I do not think -emit-hlfir exercises it.

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I think it is because if the polymorphic assignment was going through the scalar assignment path as before, we would see a fir.store instead of a hlfir.region_assign.


module m1
type x
end type x
logical,parameter::t=.true.,f=.false.
logical::mask(3)=[t,f,t]
end module m1

subroutine s1
use m1
class(*),allocatable::v(:),u(:)
allocate(x::v(3))
allocate(x::u(3))
where(mask)
u=v
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Tricky case. Your fix implies that the LHS and RHS dynamic type is the same at runtime to work properly. I do not know what is the standard required behavior here....

In, assignment-stmt polymorphic LHS are required to be allocatable in F2023 10.2.1.2 (1) so that the LHS can be reallocated to the RHS dynamic type if it is not the same.

But here, inside a masked assignment, the reallocation logic is not really specified. Lowering is not generating it because the LHS/RHS shapes are supposed to match already.

I do think it would be cleaner to forbid assignments to whole polymorphic allocatables inside WHERE statements.

Section 10.2.3 does not really say anything about it allocatable assignements inside WHERE statements, and especially in the case of polymorphic LHS, this opens the doors to many interpretation/behaviors.

@klausler, what do you think? Should this case be clarified with the J3?

end where
! CHECK: hlfir.region_assign
! CHECK: !fir.ref<!fir.class<!fir.heap<!fir.array<?xnone>>>>
end subroutine s1

program main
call s1
print *,'pass'
end program main