[flang-commits] [flang] [flang] Propagate INTENT(IN) dummy arguments as readonly (PR #207732)

[Eugene Epshteyn via flang-commits]

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@@ -187,6 +187,30 @@ asImplicitArg(Fortran::evaluate::characteristics::DummyDataObject &&dummy) {
                                                        std::move(shape)));
 }
 
+/// An INTENT(IN) data object passed by reference is not modified by the callee,
+/// so the LLVM `readonly` argument attribute can be set. Notes on the
+/// exclusions:
+///   - VALUE, POINTER, and ALLOCATABLE dummies require ABI- or
+///     descriptor-specific handling before `readonly` can be applied and are
+///     left out of scope;
+///   - ASYNCHRONOUS memory may change underneath the callee;
+///   - VOLATILE+INTENT(IN) is prohibited by the standard (C870); kept here
+///     defensively.
+/// TARGET and derived types are intentionally allowed: any write that
+/// INTENT(IN) still permits (e.g. writing the target of a POINTER component)
+/// goes through a pointer loaded from the object, not through the argument
+/// pointer, and hence does not violate LLVM `readonly` (which only constrains
+/// the argument and pointers based on it).
+static bool dummyArgIsReadOnly(
----------------
eugeneepshteyn wrote:

Also, what about this:
```
subroutine inner(y)
  integer :: y          ! no intent — writes y
  y = 99
end subroutine
subroutine outer(x)
  integer, intent(in) :: x
  call inner(x)
end subroutine
```


https://github.com/llvm/llvm-project/pull/207732