(Ada) fix handling of minimal symbols (UNOP_CAST and UNOP_ADDR)

Consider a program which provides a symbol without debugging
information. For instance, compiling the following code without -g:

    Some_Minimal_Symbol : Integer := 1234;
    pragma Export (C, Some_Minimal_Symbol, "some_minsym");

Trying to print this variable with GDB now causes an error, which
is now expected:

    (gdb) p some_minsym
    'some_minsym' has unknown type; cast it to its declared type

However, trying to cast this symbol, or to take its address
does not work:

    (gdb) p integer(some_minsym)
    'some_minsym' has unknown type; cast it to its declared type
    (gdb) p &some_minsym
    'some_minsym' has unknown type; cast it to its declared type

Another manisfestation of this issue can be seen when trying to
insert an Ada exception catchpoint for a specific standard exception
(this only occurs if the Ada runtime is built without debugging
information, which is the default).  For instance:

    $ (gdb) catch exception constraint_error
    warning: failed to reevaluate internal exception condition for catchpoint 0: 'constraint_error' has unknown type; cast it to its declared type

This is because, internally, the cachtpoint uses a condition referencing
a minimal symbol, more precisely:

   long_integer (e) = long_integer (&constraint_error)

This patch fixes all issues listed above:

  1. resolve_subexp: Special-case the handling of OP_VAR_MSYM_VALUE
     expression elements, where there are no ambiguities to be resolved
     in that situation;

  2. ada_evaluate_subexp: Enhance the handling of the UNOP_CAST
     handling so as to process the case where the target of
     the cast is a minimal symbol (as well as a symbol with debugging
     information). This mimics what's done in C.

gdb/ChangeLog:

        * ada-lang.c (resolve_subexp): Add handling of OP_VAR_MSYM_VALUE.
        (ada_evaluate_subexp_for_cast): New function.
        (ada_evaluate_subexp) <UNOP_CAST>: Replace code by call to
        ada_evaluate_subexp_for_cast.
        (ada_evaluate_subexp) <nosideret>: Replace code by call to
        eval_skip_value.
        * eval.c (evaluate_var_value): Make non-static.
        (evaluate_var_msym_value, eval_skip_value): Likewise.
        * value.h (evaluate_var_value, evaluate_var_msym_value)
        (eval_skip_value): Declare.

gdb/testsuite/ChangeLog:

        * gdb.ada/minsyms: New testcase.

Tested on x86_64-linux. No regression. Fixes the following failures:

    catch_ex.exp: continuing to Program_Error exception
    catch_ex.exp: continuing to failed assertion
    catch_ex.exp: continuing to unhandled exception
    catch_ex.exp: continuing to program completion
    complete.exp: p <Exported_Capitalized>
    complete.exp: p Exported_Capitalized
    complete.exp: p exported_capitalized
    mi_catch_ex.exp: catch Program_Error (unexpected output)
    mi_catch_ex.exp: continue to exception catchpoint hit (unknown output after running)
    mi_catch_ex.exp: continue to assert failure catchpoint hit (unknown output after running)
    mi_catch_ex.exp: continue to unhandled exception catchpoint hit (unknown output after running)
    mi_ex_cond.exp: catch C_E if i = 2 (unexpected output)

gdb/ChangeLog

@@ -1,3 +1,16 @@
+2017-11-17  Joel Brobecker  <brobecker@adacore.com>
+
+	* ada-lang.c (resolve_subexp): Add handling of OP_VAR_MSYM_VALUE.
+	(ada_evaluate_subexp_for_cast): New function.
+	(ada_evaluate_subexp) <UNOP_CAST>: Replace code by call to
+	ada_evaluate_subexp_for_cast.
+	(ada_evaluate_subexp) <nosideret>: Replace code by call to
+	eval_skip_value.
+	* eval.c (evaluate_var_value): Make non-static.
+	(evaluate_var_msym_value, eval_skip_value): Likewise.
+	* value.h (evaluate_var_value, evaluate_var_msym_value)
+	(eval_skip_value): Declare.
+
 2017-11-16  Joel Brobecker  <brobecker@adacore.com>
 
 	* ada-lang.c (ada_value_cast): Remove parameter "noside".

gdb/ada-lang.c

@@ -3603,6 +3603,11 @@ resolve_subexp (struct expression **expp, int *pos, int deprocedure_p,
     }
 
   *pos = pc;
+  if (exp->elts[pc].opcode == OP_VAR_MSYM_VALUE)
+    return evaluate_var_msym_value (EVAL_AVOID_SIDE_EFFECTS,
+				    exp->elts[pc + 1].objfile,
+				    exp->elts[pc + 2].msymbol);
+  else
     return evaluate_subexp_type (exp, pos);
 }
 
@@ -10313,6 +10318,58 @@ ada_value_cast (struct type *type, struct value *arg2)
     entity.  Results in this case are unpredictable, as we usually read
     past the buffer containing the data =:-o.  */
 
+/* Evaluate a subexpression of EXP, at index *POS, and return a value
+   for that subexpression cast to TO_TYPE.  Advance *POS over the
+   subexpression.  */
+
+static value *
+ada_evaluate_subexp_for_cast (expression *exp, int *pos,
+			      enum noside noside, struct type *to_type)
+{
+  int pc = *pos;
+
+  if (exp->elts[pc].opcode == OP_VAR_MSYM_VALUE
+      || exp->elts[pc].opcode == OP_VAR_VALUE)
+    {
+      (*pos) += 4;
+
+      value *val;
+      if (exp->elts[pc].opcode == OP_VAR_MSYM_VALUE)
+        {
+          if (noside == EVAL_AVOID_SIDE_EFFECTS)
+            return value_zero (to_type, not_lval);
+
+          val = evaluate_var_msym_value (noside,
+                                         exp->elts[pc + 1].objfile,
+                                         exp->elts[pc + 2].msymbol);
+        }
+      else
+        val = evaluate_var_value (noside,
+                                  exp->elts[pc + 1].block,
+                                  exp->elts[pc + 2].symbol);
+
+      if (noside == EVAL_SKIP)
+        return eval_skip_value (exp);
+
+      val = ada_value_cast (to_type, val);
+
+      /* Follow the Ada language semantics that do not allow taking
+	 an address of the result of a cast (view conversion in Ada).  */
+      if (VALUE_LVAL (val) == lval_memory)
+        {
+          if (value_lazy (val))
+            value_fetch_lazy (val);
+          VALUE_LVAL (val) = not_lval;
+        }
+      return val;
+    }
+
+  value *val = evaluate_subexp (to_type, exp, pos, noside);
+  if (noside == EVAL_SKIP)
+    return eval_skip_value (exp);
+  return ada_value_cast (to_type, val);
+}
+
 /* Implement the evaluate_exp routine in the exp_descriptor structure
    for the Ada language.  */
 
@@ -10371,11 +10428,7 @@ ada_evaluate_subexp (struct type *expect_type, struct expression *exp,
     case UNOP_CAST:
       (*pos) += 2;
       type = exp->elts[pc + 1].type;
-      arg1 = evaluate_subexp (type, exp, pos, noside);
-      if (noside == EVAL_SKIP)
-        goto nosideret;
-      arg1 = ada_value_cast (type, arg1);
-      return arg1;
+      return ada_evaluate_subexp_for_cast (exp, pos, noside, type);
 
     case UNOP_QUAL:
       (*pos) += 2;
@@ -11347,7 +11400,7 @@ ada_evaluate_subexp (struct type *expect_type, struct expression *exp,
     }
 
 nosideret:
-  return value_from_longest (builtin_type (exp->gdbarch)->builtin_int, 1);
+  return eval_skip_value (exp);
 }
 
 

gdb/eval.c

@@ -705,7 +705,7 @@ fake_method::~fake_method ()
 
 /* Helper for evaluating an OP_VAR_VALUE.  */
 
-static value *
+value *
 evaluate_var_value (enum noside noside, const block *blk, symbol *var)
 {
   /* JYG: We used to just return value_zero of the symbol type if
@@ -738,7 +738,7 @@ evaluate_var_value (enum noside noside, const block *blk, symbol *var)
 
 /* Helper for evaluating an OP_VAR_MSYM_VALUE.  */
 
-static value *
+value *
 evaluate_var_msym_value (enum noside noside,
 			 struct objfile *objfile, minimal_symbol *msymbol)
 {
@@ -757,7 +757,7 @@ evaluate_var_msym_value (enum noside noside,
 
 /* Helper for returning a value when handling EVAL_SKIP.  */
 
-static value *
+value *
 eval_skip_value (expression *exp)
 {
   return value_from_longest (builtin_type (exp->gdbarch)->builtin_int, 1);

gdb/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2017-11-17  Joel Brobecker  <brobecker@adacore.com>
+
+	* gdb.ada/minsyms: New testcase.
+
 2017-11-16  Pedro Alves  <palves@redhat.com>
 
 	* gdb.base/bp-cmds-continue-ctrl-c.c: New file.

gdb/testsuite/gdb.ada/minsyms.exp

@@ -0,0 +1,41 @@
+# Copyright 2017 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"
+
+standard_ada_testfile foo_qb07_057
+
+if {[gdb_compile_ada "${srcfile}" "${binfile}" executable ""] != "" } {
+  return -1
+}
+
+clean_restart ${testfile}
+
+if ![runto "_ada_foo_qb07_057" ] then {
+  perror "Couldn't run ${testfile}"
+  return
+}
+
+gdb_test "print some_minsym" \
+         "'some_minsym' has unknown type; cast it to its declared type"
+
+gdb_test "print integer(some_minsym)" \
+         " = 1234"
+
+gdb_test "print &some_minsym" \
+         " = \\(access <data variable, no debug info>\\) 0x62c2f8 <some_minsym>"
+
+gdb_test "print /x integer(&some_minsym)" \
+         " = $hex"

gdb/testsuite/gdb.ada/minsyms/foo_qb07_057.adb

@@ -0,0 +1,20 @@
+--  Copyright 2017 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_QB07_057 is
+begin
+   Increment (Some_Minimal_Symbol);
+end Foo_QB07_057;

gdb/testsuite/gdb.ada/minsyms/pck.adb

@@ -0,0 +1,21 @@
+--  Copyright 2017 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 Increment (I: in out Integer) is
+   begin
+      I := I + 1;
+   end Increment;
+end Pck;

gdb/testsuite/gdb.ada/minsyms/pck.ads

@@ -0,0 +1,21 @@
+--  Copyright 2017 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
+   Some_Minimal_Symbol : Integer := 1234;
+   pragma Export (C, Some_Minimal_Symbol, "some_minsym");
+
+   procedure Increment (I: in out Integer);
+end Pck;

gdb/value.h

@@ -875,6 +875,15 @@ extern struct value *evaluate_subexp (struct type *expect_type,
 extern struct value *evaluate_subexpression_type (struct expression *exp,
 						  int subexp);
 
+extern value *evaluate_var_value (enum noside noside, const block *blk,
+				  symbol *var);
+
+extern value *evaluate_var_msym_value (enum noside noside,
+				       struct objfile *objfile,
+				       minimal_symbol *msymbol);
+
+extern value *eval_skip_value (expression *exp);
+
 extern void fetch_subexp_value (struct expression *exp, int *pc,
 				struct value **valp, struct value **resultp,
 				struct value **val_chain,