[Ada] problem printing negative integer values in packed arrays.

Consider the following declarations:

   type Signed_Small is new Integer range - (2 ** 5) .. (2 ** 5 - 1);
   type Signed_Simple_Array is array (1 .. 4) of Signed_Small;
   pragma Pack (Signed_Simple_Array);
   SSA : Signed_Simple_Array := (-1, 2, -3, 4);

GDB currently print its value incorrectly for the elements that
are negative:

    (gdb) print ssa
    $1 = (65535, 2, 1048573, 4)
    (gdb) print ssa(1)
    $2 = 65535
    (gdb) print ssa(2)
    $3 = 2
    (gdb) print ssa(3)
    $4 = 1048573
    (gdb) print ssa(4)
    $5 = 4

What happens is that the sign-extension is not working because
we're trying to do left shift with a negative count. In
ada_value_primitive_packed_val, we have a loop which populates
the extra bits of the target (unpacked) value, after extraction
of the data from the original (packed) value:

        while (ntarg > 0)
          {
            accum |= sign << accumSize;
            unpacked[targ] = accum & ~(~0L << HOST_CHAR_BIT);
!!! ->      accumSize -= HOST_CHAR_BIT;
            accum >>= HOST_CHAR_BIT;
            ntarg -= 1;
            targ += delta;
          }

At each iteration, accumSize gets decremented by HOST_CHAR_BIT,
which can easily cause it to become negative, particularly on
little endian targets, where accumSize is at most HOST_CHAR_BIT - 1.
This causes us to perform a left-shift operation with a negative
accumSize at the next loop iteration, which is undefined, and
acutally does not produce the effect we wanted (value left untouched)
when the code is compiled with GCC.

This patch fixes the issue by simply setting accumSize to zero
if negative.

gdb/ChangeLog:

        * ada-lang.c (ada_value_primitive_packed_val): Make sure
        accumSize is never negative.

gdb/testsuite/ChangeLog:

        * gdb.ada/pckd_neg: New testcase.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2015-05-15  Joel Brobecker  <brobecker@adacore.com>
+
+	* ada-lang.c (ada_value_primitive_packed_val): Make sure
+	accumSize is never negative.
+
 2015-05-14  Patrick Palka  <patrick@parcs.ath.cx>
 
 	* tui/tui-command.c: Remove include of <ctype.h>.

gdb/ada-lang.c

@@ -2544,6 +2544,8 @@ ada_value_primitive_packed_val (struct value *obj, const gdb_byte *valaddr,
       accum |= sign << accumSize;
       unpacked[targ] = accum & ~(~0L << HOST_CHAR_BIT);
       accumSize -= HOST_CHAR_BIT;
+      if (accumSize < 0)
+	accumSize = 0;
       accum >>= HOST_CHAR_BIT;
       ntarg -= 1;
       targ += delta;

gdb/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2015-05-15  Joel Brobecker  <brobecker@adacore.com>
+
+	* gdb.ada/pckd_neg: New testcase.
+
 2015-05-13  Patrick Palka  <patrick@parcs.ath.cx>
 
 	PR gdb/17820

gdb/testsuite/gdb.ada/pckd_neg.exp

@@ -0,0 +1,44 @@
+# Copyright 2015 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 foo_o508_021
+
+if {[gdb_compile_ada "${srcfile}" "${binfile}" executable {debug}] != ""} {
+    return -1
+}
+
+clean_restart ${testfile}
+
+set bp_location [gdb_get_line_number "STOP" ${testdir}/foo_o508_021.adb]
+runto "foo_o508_021.adb:$bp_location"
+
+gdb_test "print SSA" \
+         "= \\(-1, 2, -3, 4\\)"
+
+gdb_test "print SSA(1)" \
+         "= -1"
+
+gdb_test "print SSA(2)" \
+         "= 2"
+
+gdb_test "print SSA(3)" \
+         "= -3"
+
+gdb_test "print SSA(4)" \
+         "= 4"

gdb/testsuite/gdb.ada/pckd_neg/foo_o508_021.adb

@@ -0,0 +1,22 @@
+--  Copyright (C) 2015 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 Foo_O508_021 is
+   SSA : Signed_Simple_Array := (-1, 2, -3, 4);
+begin
+   Update_Signed_Small (SSA (3));  -- STOP
+end Foo_O508_021;

gdb/testsuite/gdb.ada/pckd_neg/pck.adb

@@ -0,0 +1,21 @@
+--  Copyright (C) 2015 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 Update_Signed_Small (S : in out Signed_Small) is
+   begin
+      null;
+   end Update_Signed_Small;
+end Pck;

gdb/testsuite/gdb.ada/pckd_neg/pck.ads

@@ -0,0 +1,22 @@
+--  Copyright (C) 2015 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 Pck is
+   type Signed_Small is new Integer range - (2 ** 5) .. (2 ** 5 - 1);
+   type Signed_Simple_Array is array (1 .. 4) of Signed_Small;
+   pragma Pack (Signed_Simple_Array);
+
+   procedure Update_Signed_Small (S : in out Signed_Small);
+end Pck;