[flang] Lower BIND(C) assumed length to CFI descriptor

Outside of BIND(C), assumed length character scalar and explicit shape
are passed by address + an extra length argument (fir.boxchar in FIR).

The standard mandates that they be passed via CFI descriptor in
BIND(C) interface (fir.box in FIR). This patch fix the handling for
this case.

Author: jeanPerier

flang/lib/Lower/CallInterface.cpp

@@ -702,7 +702,7 @@ class Fortran::lower::CallInterfaceImpl {
               [&](const Fortran::evaluate::characteristics::DummyDataObject
                       &dummy) {
                 const auto &entity = getDataObjectEntity(std::get<1>(pair));
-                if (dummy.CanBePassedViaImplicitInterface())
+                if (!isBindC && dummy.CanBePassedViaImplicitInterface())
                   handleImplicitDummy(&argCharacteristics, dummy, entity);
                 else
                   handleExplicitDummy(&argCharacteristics, dummy, entity,
@@ -871,7 +871,8 @@ class Fortran::lower::CallInterfaceImpl {
 
   // Define when an explicit argument must be passed in a fir.box.
   bool dummyRequiresBox(
-      const Fortran::evaluate::characteristics::DummyDataObject &obj) {
+      const Fortran::evaluate::characteristics::DummyDataObject &obj,
+      bool isBindC) {
     using ShapeAttr = Fortran::evaluate::characteristics::TypeAndShape::Attr;
     using ShapeAttrs = Fortran::evaluate::characteristics::TypeAndShape::Attrs;
     constexpr ShapeAttrs shapeRequiringBox = {
@@ -888,6 +889,8 @@ class Fortran::lower::CallInterfaceImpl {
       if (const Fortran::semantics::Scope *scope = derived->scope())
         // Need to pass length type parameters in fir.box if any.
         return scope->IsDerivedTypeWithLengthParameter();
+    if (isBindC && obj.type.type().IsAssumedLengthCharacter())
+      return true; // Fortran 2018 18.3.6 point 2 (5)
     return false;
   }
 
@@ -973,7 +976,7 @@ class Fortran::lower::CallInterfaceImpl {
       addFirOperand(boxRefType, nextPassedArgPosition(), Property::MutableBox,
                     attrs);
       addPassedArg(PassEntityBy::MutableBox, entity, characteristics);
-    } else if (dummyRequiresBox(obj)) {
+    } else if (dummyRequiresBox(obj, isBindC)) {
       // Pass as fir.box or fir.class
       if (isValueAttr)
         TODO(loc, "assumed shape dummy argument with VALUE attribute");

flang/test/Lower/HLFIR/bindc-assumed-length.f90

@@ -0,0 +1,18 @@
+! Test that assumed length character scalars and explicit shape arrays are passed via
+! CFI descriptor (fir.box) in BIND(C) procedures. They are passed only by address
+! and length  in non BIND(C) procedures. See Fortran 2018 standard 18.3.6 point 2(5).
+! RUN: bbc -emit-hlfir -o - %s 2>&1 | FileCheck %s
+
+! CHECK: func.func @foo(
+! CHECK-SAME: %{{[^:]*}}: !fir.box<!fir.char<1,?>>
+! CHECK-SAME: %{{[^:]*}}: !fir.box<!fir.array<100x!fir.char<1,?>>>
+subroutine foo(c1, c3) bind(c)
+  character(*) :: c1,  c3(100)
+end subroutine
+
+! CHECK: func.func @_QPnot_bindc(
+! CHECK-SAME: %{{[^:]*}}: !fir.boxchar<1>
+! CHECK-SAME: %{{[^:]*}}: !fir.boxchar<1>
+subroutine not_bindc(c1, c3)
+  character(*) :: c1,  c3(100)
+end subroutine