Use bit stride when taking slice of array

Testing with -fgnat-encodings=minimal showed that the Ada code failed
to use the bit stride of an array when taking a slice.  This patch
fixes the oversight.

gdb/ChangeLog
2020-11-04  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (ada_value_slice_from_ptr): Use bit size.

gdb/testsuite/ChangeLog
2020-11-04  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/array_of_variant.exp: New file.
	* gdb.ada/array_of_variant/p.adb: New file.
	* gdb.ada/array_of_variant/pck.ads: New file.
	* gdb.ada/array_of_variant/pck.adb: New file.

7 files changed, 153 insertions, 3 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index a98f89d..d4ea435 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,9 @@
 2020-11-04  Tom Tromey  <tromey@adacore.com>
 
+	* ada-lang.c (ada_value_slice_from_ptr): Use bit size.
+
+2020-11-04  Tom Tromey  <tromey@adacore.com>
+
 	* dwarf2/read.c (read_array_type): Only apply stride to innermost
 	array.
 
diff --git a/gdb/ada-lang.c b/gdb/ada-lang.c
index 7613e19..fe3ea70 100644
--- a/gdb/ada-lang.c
+++ b/gdb/ada-lang.c
@@ -2828,9 +2828,11 @@ ada_value_slice_from_ptr (struct value *array_ptr, struct type *type,
       base_low_pos = base_low;
     }
 
-  base = value_as_address (array_ptr)
-    + ((low_pos - base_low_pos)
-       * TYPE_LENGTH (TYPE_TARGET_TYPE (type0)));
+  ULONGEST stride = TYPE_FIELD_BITSIZE (slice_type, 0) / 8;
+  if (stride == 0)
+    stride = TYPE_LENGTH (TYPE_TARGET_TYPE (type0));
+
+  base = value_as_address (array_ptr) + (low_pos - base_low_pos) * stride;
   return value_at_lazy (slice_type, base);
 }
 
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index d18d88f..fe97b07 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,12 @@
 2020-11-04  Tom Tromey  <tromey@adacore.com>
 
+	* gdb.ada/array_of_variant.exp: New file.
+	* gdb.ada/array_of_variant/p.adb: New file.
+	* gdb.ada/array_of_variant/pck.ads: New file.
+	* gdb.ada/array_of_variant/pck.adb: New file.
+
+2020-11-04  Tom Tromey  <tromey@adacore.com>
+
 	* gdb.ada/enum_idx_packed.exp: Add test.
 	* gdb.ada/enum_idx_packed/foo.adb (Multi_Access):
 	New variable.
diff --git a/gdb/testsuite/gdb.ada/array_of_variant.exp b/gdb/testsuite/gdb.ada/array_of_variant.exp
new file mode 100644
index 0000000..f2c343f
--- /dev/null
+++ b/gdb/testsuite/gdb.ada/array_of_variant.exp
@@ -0,0 +1,52 @@
+# Copyright 2020 Free Software Foundation, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+load_lib "ada.exp"
+
+if { [skip_ada_tests] } { return -1 }
+
+standard_ada_testfile p
+
+foreach_with_prefix scenario {all minimal} {
+    set flags [list debug additional_flags=-fgnat-encodings=$scenario]
+
+    if {[gdb_compile_ada "${srcfile}" "${binfile}" executable $flags] != ""} {
+	return -1
+    }
+
+    clean_restart ${testfile}
+
+    set bp_location [gdb_get_line_number "START" ${testdir}/p.adb]
+    runto "p.adb:$bp_location"
+
+    set v1 "(tag => object, values => (2, 2, 2, 2, 2))"
+    set v2 "(tag => unused)"
+
+    gdb_test "print objects" \
+	[string_to_regexp " = ($v1, $v2)"] \
+	"print entire array"
+    gdb_test "print objects(1)" \
+	[string_to_regexp " = $v1"] \
+	"print first array element"
+    gdb_test "print objects(1 .. 1)" \
+	[string_to_regexp " = ($v1)"] \
+	"print first array slice"
+    gdb_test "print objects(2)" \
+	[string_to_regexp " = $v2"] \
+	"print second array element"
+    gdb_test "print objects(2 .. 2)" \
+	[string_to_regexp " = (2 => $v2)"] \
+	"print second array slice"
+}
diff --git a/gdb/testsuite/gdb.ada/array_of_variant/p.adb b/gdb/testsuite/gdb.ada/array_of_variant/p.adb
new file mode 100644
index 0000000..d9974ef
--- /dev/null
+++ b/gdb/testsuite/gdb.ada/array_of_variant/p.adb
@@ -0,0 +1,39 @@
+--  Copyright 2020 Free Software Foundation, Inc.
+--
+--  This program is free software; you can redistribute it and/or modify
+--  it under the terms of the GNU General Public License as published by
+--  the Free Software Foundation; either version 3 of the License, or
+--  (at your option) any later version.
+--
+--  This program is distributed in the hope that it will be useful,
+--  but WITHOUT ANY WARRANTY; without even the implied warranty of
+--  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+--  GNU General Public License for more details.
+--
+--  You should have received a copy of the GNU General Public License
+--  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+with Pck; use Pck;
+
+procedure P is
+
+   type Tag_T is (Unused, Object);
+
+   type Array_T is array (1 .. Five) of Integer;
+
+   type Payload_T (Tag : Tag_T := Unused) is
+      record
+         case Tag is
+            when Object =>
+               Values : Array_T := (others => 1);
+            when Unused =>
+               null;
+         end case;
+      end record;
+
+   Objects : array (1 .. 2) of Payload_T;
+
+begin
+   Objects (1) := (Tag => Object, Values => (others => 2));
+   Do_Nothing (Objects'Address);  --  START
+end P;
diff --git a/gdb/testsuite/gdb.ada/array_of_variant/pck.adb b/gdb/testsuite/gdb.ada/array_of_variant/pck.adb
new file mode 100644
index 0000000..af4fe6d
--- /dev/null
+++ b/gdb/testsuite/gdb.ada/array_of_variant/pck.adb
@@ -0,0 +1,23 @@
+--  Copyright 2020 Free Software Foundation, Inc.
+--
+--  This program is free software; you can redistribute it and/or modify
+--  it under the terms of the GNU General Public License as published by
+--  the Free Software Foundation; either version 3 of the License, or
+--  (at your option) any later version.
+--
+--  This program is distributed in the hope that it will be useful,
+--  but WITHOUT ANY WARRANTY; without even the implied warranty of
+--  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+--  GNU General Public License for more details.
+--
+--  You should have received a copy of the GNU General Public License
+--  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package body Pck is
+
+   procedure Do_Nothing (A : System.Address) is
+   begin
+      null;
+   end Do_Nothing;
+
+end Pck;
diff --git a/gdb/testsuite/gdb.ada/array_of_variant/pck.ads b/gdb/testsuite/gdb.ada/array_of_variant/pck.ads
new file mode 100644
index 0000000..f2a676c
--- /dev/null
+++ b/gdb/testsuite/gdb.ada/array_of_variant/pck.ads
@@ -0,0 +1,23 @@
+--  Copyright 2020 Free Software Foundation, Inc.
+--
+--  This program is free software; you can redistribute it and/or modify
+--  it under the terms of the GNU General Public License as published by
+--  the Free Software Foundation; either version 3 of the License, or
+--  (at your option) any later version.
+--
+--  This program is distributed in the hope that it will be useful,
+--  but WITHOUT ANY WARRANTY; without even the implied warranty of
+--  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+--  GNU General Public License for more details.
+--
+--  You should have received a copy of the GNU General Public License
+--  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+with System;
+
+package Pck is
+   Five : Integer := 5;
+
+   procedure Do_Nothing (A : System.Address);
+
+end Pck;