[flang-commits] [flang] [llvm] [flang] Enhance show_descriptor intrinsic to avoid extra copies and extra descriptor creation (PR #173461)

[Slava Zakharin via flang-commits]

================
@@ -103,20 +85,17 @@ subroutine test_logical
   call show_descriptor(l2)
   pla2 => la2
 ! CHECK:           %[[DECLARE_3:.*]] = fir.declare %[[ALLOCA_0]] {fortran_attrs = #fir.var_attrs<pointer>, uniq_name = "_QMtest_show_descriptorFtest_logicalEpla2"} : (!fir.ref<!fir.box<!fir.ptr<!fir.array<?x!fir.logical<2>>>>>) -> !fir.ref<!fir.box<!fir.ptr<!fir.array<?x!fir.logical<2>>>>>
-! CHECK:           %[[EMBOX_1:.*]] = fir.embox %[[DECLARE_0]] : (!fir.ref<!fir.logical<1>>) -> !fir.box<!fir.logical<1>>
-! CHECK:           fir.call @_FortranAShowDescriptor(%[[EMBOX_1]]) fastmath<contract> : (!fir.box<!fir.logical<1>>) -> ()
-! CHECK:           %[[EMBOX_2:.*]] = fir.embox %[[DECLARE_1]] : (!fir.ref<!fir.logical<2>>) -> !fir.box<!fir.logical<2>>
-! CHECK:           fir.call @_FortranAShowDescriptor(%[[EMBOX_2]]) fastmath<contract> : (!fir.box<!fir.logical<2>>) -> ()
+! CHECK:           %[[ZERO_BITS_3:.*]] = fir.zero_bits !fir.box<none>
+! CHECK:           fir.call @_FortranAShowDescriptor(%[[ZERO_BITS_3]]) fastmath<contract> : (!fir.box<none>) -> ()
+! CHECK:           fir.call @_FortranAShowDescriptor(%[[ZERO_BITS_3]]) fastmath<contract> : (!fir.box<none>) -> ()
 
   call show_descriptor(la2)
   call show_descriptor(pla2)
 ! CHECK:           %[[CONVERT_0:.*]] = fir.convert %[[DECLARE_2]] : (!fir.ref<!fir.array<2x!fir.logical<2>>>) -> !fir.ref<!fir.array<?x!fir.logical<2>>>
 ! CHECK:           %[[EMBOX_3:.*]] = fir.embox %[[CONVERT_0]](%[[SHAPE_0]]) : (!fir.ref<!fir.array<?x!fir.logical<2>>>, !fir.shape<1>) -> !fir.box<!fir.ptr<!fir.array<?x!fir.logical<2>>>>
 ! CHECK:           fir.store %[[EMBOX_3]] to %[[DECLARE_3]] : !fir.ref<!fir.box<!fir.ptr<!fir.array<?x!fir.logical<2>>>>>
-! CHECK:           %[[EMBOX_4:.*]] = fir.embox %[[DECLARE_2]](%[[SHAPE_0]]) : (!fir.ref<!fir.array<2x!fir.logical<2>>>, !fir.shape<1>) -> !fir.box<!fir.array<2x!fir.logical<2>>>
-! CHECK:           fir.call @_FortranAShowDescriptor(%[[EMBOX_4]]) fastmath<contract> : (!fir.box<!fir.array<2x!fir.logical<2>>>) -> ()
-! CHECK:           %[[LOAD_0:.*]] = fir.load %[[DECLARE_3]] : !fir.ref<!fir.box<!fir.ptr<!fir.array<?x!fir.logical<2>>>>>
-! CHECK:           fir.call @_FortranAShowDescriptor(%[[LOAD_0]]) fastmath<contract> : (!fir.box<!fir.ptr<!fir.array<?x!fir.logical<2>>>>) -> ()
+! CHECK:           fir.call @_FortranAShowDescriptor(%[[ZERO_BITS_3]]) fastmath<contract> : (!fir.box<none>) -> ()
+! CHECK:           fir.call @_FortranAShowDescriptor(%[[DECLARE_3]]) fastmath<contract> : (!fir.ref<!fir.box<!fir.ptr<!fir.array<?x!fir.logical<2>>>>>) -> ()
----------------
vzakhari wrote:

It is not a big deal, I guess, but I would prefer keeping the ABI of this function consistent, e.g. that it always takes `!fir.ref<!fir.box<none>>` argument.

I think this should be possible if we cast all `!fir.ref<!fir.box<>` actual arguments to `!fir.ref<!fir.box<none>>`. For non-descriptor argument, please use `fir.absent !fir.ref<!fir.box<none>>` instead of `fir.zero_bits`.

Unfortunately, passing `!fir.ref<!fir.box<none>>` makes an impression that the actual argument may be modifed by the call, but we can always handle this call specially in `flang/Optimizer/Transforms/RuntimeFunctions.inc`.

https://github.com/llvm/llvm-project/pull/173461