[Ada] Refine conditions for calling Copy_Bitfield

Avoid calling Copy_Bitfield if there are volatile or independent
components that might be read or written. The test is conservative.

2019-09-17  Bob Duff  <duff@adacore.com>

gcc/ada/

	* exp_ch5.adb (Expand_Assign_Array_Loop_Or_Bitfield): Add tests
	for potential volatile or independent components.
	* libgnat/s-bituti.adb (Copy_Small_Bitfield,
	Copy_Large_Bitfield): Move declarations to more appropriate
	place.

From-SVN: r275768

1 files changed, 22 insertions, 7 deletions

diff --git a/gcc/ada/exp_ch5.adb b/gcc/ada/exp_ch5.adb
index ba0b793..6ef3fb2 100644
--- a/gcc/ada/exp_ch5.adb
+++ b/gcc/ada/exp_ch5.adb
@@ -1451,20 +1451,35 @@ package body Exp_Ch5 is
    begin
       --  Determine whether Copy_Bitfield is appropriate (will work, and will
       --  be more efficient than component-by-component copy). Copy_Bitfield
-      --  doesn't work for reversed storage orders. It is efficient only for
-      --  slices of bit-packed arrays.
-
-      --  Note that Expand_Assign_Array_Bitfield is disabled for now
-
-      if False -- ???
+      --  doesn't work for reversed storage orders. It is efficient for slices
+      --  of bit-packed arrays. Copy_Bitfield can read and write bits that are
+      --  not part of the objects being copied, so we don't want to use it if
+      --  there are volatile or independent components. If the Prefix of the
+      --  slice is a selected component (etc, see below), then it might be a
+      --  component of an object with some other volatile or independent
+      --  components, so we disable the optimization in that case as well.
+      --  We could complicate this code by actually looking for such volatile
+      --  and independent components.
+
+      --  Note that Expand_Assign_Array_Bitfield is disabled for now.
+
+      if False and then -- ???
+        RTE_Available (RE_Copy_Bitfield)
         and then Is_Bit_Packed_Array (L_Type)
         and then Is_Bit_Packed_Array (R_Type)
-        and then RTE_Available (RE_Copy_Bitfield)
         and then not Reverse_Storage_Order (L_Type)
         and then not Reverse_Storage_Order (R_Type)
         and then Ndim = 1
         and then not Rev
         and then Slices
+        and then not Has_Volatile_Component (L_Type)
+        and then not Has_Volatile_Component (R_Type)
+        and then not Has_Independent_Components (L_Type)
+        and then not Has_Independent_Components (R_Type)
+        and then not Nkind_In (Prefix (Name (N)),
+                               N_Selected_Component,
+                               N_Indexed_Component,
+                               N_Slice)
       then
          return Expand_Assign_Array_Bitfield
            (N, Larray, Rarray, L_Type, R_Type, Rev);