[Ada] Zero-size slices

[Pierre-Marie de Rodat]

Fix a bug in slices, where a zero-sized slice causes an invalid read
detected by valgrind

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

        * libgnat/s-bituti.ads (Small_Size): Do not include 0 in this
        type.
        * libgnat/s-bituti.adb (Copy_Bitfield): Do nothing for 0-bit
        bitfields.
diff --git a/gcc/ada/libgnat/s-bituti.adb b/gcc/ada/libgnat/s-bituti.adb
--- a/gcc/ada/libgnat/s-bituti.adb
+++ b/gcc/ada/libgnat/s-bituti.adb
@@ -402,11 +402,22 @@ package body System.Bitfield_Utils is
          pragma Assert (Al_Src_Address mod Val'Alignment = 0);
          pragma Assert (Al_Dest_Address mod Val'Alignment = 0);
       begin
+         --  Optimized small case
+
          if Size in Small_Size then
             Copy_Small_Bitfield
               (Al_Src_Address, Al_Src_Offset,
                Al_Dest_Address, Al_Dest_Offset,
                Size);
+
+         --  Do nothing for zero size. This is necessary to avoid doing invalid
+         --  reads, which are detected by valgrind.
+
+         elsif Size = 0 then
+            null;
+
+         --  Large case
+
          else
             Copy_Large_Bitfield
               (Al_Src_Address, Al_Src_Offset,


diff --git a/gcc/ada/libgnat/s-bituti.ads b/gcc/ada/libgnat/s-bituti.ads
--- a/gcc/ada/libgnat/s-bituti.ads
+++ b/gcc/ada/libgnat/s-bituti.ads
@@ -98,9 +98,9 @@ package System.Bitfield_Utils is
       pragma Assert (Val_Array'Component_Size = Val'Size);
 
       subtype Bit_Size is Natural; -- Size in bits of a bit field
-      subtype Small_Size is Bit_Size range 0 .. Val'Size;
+      subtype Small_Size is Bit_Size range 1 .. Val'Size;
       --  Size of a small one
-      subtype Bit_Offset is Small_Size range 0 .. Val'Size - 1;
+      subtype Bit_Offset is Small_Size'Base range 0 .. Val'Size - 1;
       --  Starting offset
       subtype Bit_Offset_In_Byte is Bit_Offset range 0 .. Storage_Unit - 1;