[flang][hlfir] Lower NULL(MOLD) to a variable

[jeanPerier]

Summary

HLFIR lowering promotes intrinsic results lowered in memory to
hlfir.expr to underline their read-only aspect once they are created.
NULL(MOLD) should not be promoted to an hlfir.expr, it is the NULL
variable (we need to see it as an address).

[clementval]

LGTM

flang/lib/Lower/ConvertCall.cpp

genIntrinsicRefCore(PreparedActualArguments &loweredActuals,
   // Let the intrinsic library lower the intrinsic procedure call.
   auto [resultExv, mustBeFreed] = genIntrinsicCall(
       callContext.getBuilder(), loc, intrinsicName, scalarResultType, operands);
   if (!fir::getBase(resultExv))
     return std::nullopt;
   hlfir::EntityWithAttributes resultEntity = extendedValueToHlfirEntity(
       loc, builder, resultExv, ".tmp.intrinsic_result");
   // Move result into memory into an hlfir.expr since they are immutable from
-  // that point, and the result storage is some temp.
-  if (resultEntity.isVariable()) {
+  // that point, and the result storage is some temp. "Null" is special: it
+  // returns a null pointer variable that should not be transformed into a value
+  // (what matters is the memory address).
+  if (resultEntity.isVariable() && intrinsicName != "null") {
     hlfir::AsExprOp asExpr;
     // Character/Derived MERGE lowering returns one of its argument address
     // (this is the only intrinsic implemented in that way so far). The
     // ownership of this address cannot be taken here since it may not be a
     // temp.
     if (intrinsicName == "merge")
       asExpr = builder.create<hlfir::AsExprOp>(loc, resultEntity);
     else

flang/test/Lower/HLFIR/null.f90

+! Test lowering of NULL(MOLD) to HLFIR.
+! RUN: bbc -emit-fir -hlfir -o - %s | FileCheck %s
+subroutine test(mold)
+  integer, pointer :: mold(:)
+  interface
+    subroutine takes_ptr(p)
+      integer, pointer :: p(:)
+    end subroutine
+  end interface
+  call takes_ptr(null(mold))
+end subroutine
+! CHECK-LABEL:   func.func @_QPtest(
+! CHECK:  %[[VAL_1:.*]] = fir.alloca !fir.box<!fir.ptr<!fir.array<?xi32>>>
+! CHECK:  %[[VAL_3:.*]] = fir.zero_bits !fir.ptr<!fir.array<?xi32>>
+! CHECK:  %[[VAL_4:.*]] = arith.constant 0 : index
+! CHECK:  %[[VAL_5:.*]] = fir.shape %[[VAL_4]] : (index) -> !fir.shape<1>
+! CHECK:  %[[VAL_6:.*]] = fir.embox %[[VAL_3]](%[[VAL_5]]) : (!fir.ptr<!fir.array<?xi32>>, !fir.shape<1>) -> !fir.box<!fir.ptr<!fir.array<?xi32>>>
+! CHECK:  fir.store %[[VAL_6]] to %[[VAL_1]] : !fir.ref<!fir.box<!fir.ptr<!fir.array<?xi32>>>>
+! CHECK:  %[[VAL_7:.*]]:2 = hlfir.declare %[[VAL_1]] {uniq_name = ".tmp.intrinsic_result"} : (!fir.ref<!fir.box<!fir.ptr<!fir.array<?xi32>>>>) -> (!fir.ref<!fir.box<!fir.ptr<!fir.array<?xi32>>>>, !fir.ref<!fir.box<!fir.ptr<!fir.array<?xi32>>>>)
+! CHECK:  fir.call @_QPtakes_ptr(%[[VAL_7]]#0) fastmath<contract> : (!fir.ref<!fir.box<!fir.ptr<!fir.array<?xi32>>>>) -> ()