Make dwarf_expr_context::stack an std::vector

Replace the manually managed array with a vector.  It is mostly
straightforward, except maybe one thing in execute_stack_op, in the
handling of DW_OP_fbreg.  When the code stumbles on that opcode while
evaluating an expression, it needs to evaluate a subexpression to find
where the fb reg has been saved.  Rather than creating a new context, it
reuses the current context.  It saves the size of the stack before and
restores the stack to that size after.

I think we can do a little bit better by saving the current stack
locally and installing a new empty stack.  This way, if the
subexpression is malformed and underflows, we'll get an exception.
Before, it would have overwritten the top elements of the top-level
expression.  The evaluation of the top-level expression would have then
resumed with the same stack size, but possibly some corrupted elements.

gdb/ChangeLog:

	* dwarf2expr.h (dwarf_stack_value): Add constructor.
	(dwarf_expr_context) <~dwarf_expr_context>: Define as default.
	<stack>: Change type to std::vector.
	<stack_len, stack_allocated>: Remove.
	<grow_stack>: Remove.
	* dwarf2expr.c (dwarf_expr_context::dwarf_expr_context): Adjust.
	(dwarf_expr_context::~dwarf_expr_context): Remove.
	(dwarf_expr_context::grow_stack): Remove.
	(dwarf_expr_context::push): Adjust.
	(dwarf_expr_context::pop): Adjust.
	(dwarf_expr_context::fetch): Adjust.
	(dwarf_expr_context::fetch_in_stack_memory): Adjust.
	(dwarf_expr_context::stack_empty_p): Adjust.
	(dwarf_expr_context::execute_stack_op): Adjust.

3 files changed, 64 insertions, 78 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index f88cd6e1a45..c562f7b7207 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,22 @@
 2017-09-14  Simon Marchi  <simon.marchi@ericsson.com>
 
+	* dwarf2expr.h (dwarf_stack_value): Add constructor.
+	(dwarf_expr_context) <~dwarf_expr_context>: Define as default.
+	<stack>: Change type to std::vector.
+	<stack_len, stack_allocated>: Remove.
+	<grow_stack>: Remove.
+	* dwarf2expr.c (dwarf_expr_context::dwarf_expr_context): Adjust.
+	(dwarf_expr_context::~dwarf_expr_context): Remove.
+	(dwarf_expr_context::grow_stack): Remove.
+	(dwarf_expr_context::push): Adjust.
+	(dwarf_expr_context::pop): Adjust.
+	(dwarf_expr_context::fetch): Adjust.
+	(dwarf_expr_context::fetch_in_stack_memory): Adjust.
+	(dwarf_expr_context::stack_empty_p): Adjust.
+	(dwarf_expr_context::execute_stack_op): Adjust.
+
+2017-09-14  Simon Marchi  <simon.marchi@ericsson.com>
+
 	* dwarf2expr.h (dwarf_expr_context) <stack_empty_p>: Change
 	return type to bool.
 	* dwarf2expr.c (dwarf_expr_context::stack_empty_p): Likewise.
diff --git a/gdb/dwarf2expr.c b/gdb/dwarf2expr.c
index 3a388ac6fa1..f5e0e4c1fdb 100644
--- a/gdb/dwarf2expr.c
+++ b/gdb/dwarf2expr.c
@@ -88,10 +88,7 @@ dwarf_expr_context::address_type () const
 /* Create a new context for the expression evaluator.  */
 
 dwarf_expr_context::dwarf_expr_context ()
-: stack (NULL),
-  stack_len (0),
-  stack_allocated (10),
-  gdbarch (NULL),
+: gdbarch (NULL),
   addr_size (0),
   ref_addr_size (0),
   offset (0),
@@ -102,29 +99,6 @@ dwarf_expr_context::dwarf_expr_context ()
   data (NULL),
   initialized (0)
 {
-  this->stack = XNEWVEC (struct dwarf_stack_value, this->stack_allocated);
-}
-
-/* Clean up a dwarf_expr_context.  */
-
-dwarf_expr_context::~dwarf_expr_context ()
-{
-  xfree (this->stack);
-}
-
-/* Expand the memory allocated stack to contain at least
-   NEED more elements than are currently used.  */
-
-void
-dwarf_expr_context::grow_stack (size_t need)
-{
-  if (this->stack_len + need > this->stack_allocated)
-    {
-      size_t newlen = this->stack_len + need + 10;
-
-      this->stack = XRESIZEVEC (struct dwarf_stack_value, this->stack, newlen);
-      this->stack_allocated = newlen;
-    }
 }
 
 /* Push VALUE onto the stack.  */
@@ -132,12 +106,7 @@ dwarf_expr_context::grow_stack (size_t need)
 void
 dwarf_expr_context::push (struct value *value, bool in_stack_memory)
 {
-  struct dwarf_stack_value *v;
-
-  grow_stack (1);
-  v = &this->stack[this->stack_len++];
-  v->value = value;
-  v->in_stack_memory = in_stack_memory;
+  stack.emplace_back (value, in_stack_memory);
 }
 
 /* Push VALUE onto the stack.  */
@@ -153,9 +122,10 @@ dwarf_expr_context::push_address (CORE_ADDR value, bool in_stack_memory)
 void
 dwarf_expr_context::pop ()
 {
-  if (this->stack_len <= 0)
+  if (stack.empty ())
     error (_("dwarf expression stack underflow"));
-  this->stack_len--;
+
+  stack.pop_back ();
 }
 
 /* Retrieve the N'th item on the stack.  */
@@ -163,11 +133,11 @@ dwarf_expr_context::pop ()
 struct value *
 dwarf_expr_context::fetch (int n)
 {
-  if (this->stack_len <= n)
+  if (stack.size () <= n)
      error (_("Asked for position %d of stack, "
-	      "stack only has %d elements on it."),
-	    n, this->stack_len);
-  return this->stack[this->stack_len - (1 + n)].value;
+	      "stack only has %zu elements on it."),
+	    n, stack.size ());
+  return stack[stack.size () - (1 + n)].value;
 }
 
 /* Require that TYPE be an integral type; throw an exception if not.  */
@@ -263,11 +233,11 @@ dwarf_expr_context::fetch_address (int n)
 bool
 dwarf_expr_context::fetch_in_stack_memory (int n)
 {
-  if (this->stack_len <= n)
+  if (stack.size () <= n)
      error (_("Asked for position %d of stack, "
-	      "stack only has %d elements on it."),
-	    n, this->stack_len);
-  return this->stack[this->stack_len - (1 + n)].in_stack_memory;
+	      "stack only has %zu elements on it."),
+	    n, stack.size ());
+  return stack[stack.size () - (1 + n)].in_stack_memory;
 }
 
 /* Return true if the expression stack is empty.  */
@@ -275,7 +245,7 @@ dwarf_expr_context::fetch_in_stack_memory (int n)
 bool
 dwarf_expr_context::stack_empty_p () const
 {
-  return this->stack_len == 0;
+  return stack.empty ();
 }
 
 /* Add a new piece to the dwarf_expr_context's piece list.  */
@@ -875,14 +845,16 @@ dwarf_expr_context::execute_stack_op (const gdb_byte *op_ptr,
 	  {
 	    const gdb_byte *datastart;
 	    size_t datalen;
-	    unsigned int before_stack_len;
 
 	    op_ptr = safe_read_sleb128 (op_ptr, op_end, &offset);
+
 	    /* Rather than create a whole new context, we simply
-	       record the stack length before execution, then reset it
-	       afterwards, effectively erasing whatever the recursive
-	       call put there.  */
-	    before_stack_len = this->stack_len;
+	       backup the current stack locally and install a new empty stack,
+	       then reset it afterwards, effectively erasing whatever the
+	       recursive call put there.  */
+	    std::vector<dwarf_stack_value> saved_stack = std::move (stack);
+	    stack.clear ();
+
 	    /* FIXME: cagney/2003-03-26: This code should be using
                get_frame_base_address(), and then implement a dwarf2
                specific this_base method.  */
@@ -898,7 +870,10 @@ dwarf_expr_context::execute_stack_op (const gdb_byte *op_ptr,
 	    result = result + offset;
 	    result_val = value_from_ulongest (address_type, result);
 	    in_stack_memory = true;
-	    this->stack_len = before_stack_len;
+
+	    /* Restore the content of the original stack.  */
+	    stack = std::move (saved_stack);
+
 	    this->location = DWARF_VALUE_MEMORY;
 	  }
 	  break;
@@ -920,16 +895,14 @@ dwarf_expr_context::execute_stack_op (const gdb_byte *op_ptr,
 	  
 	case DW_OP_swap:
 	  {
-	    struct dwarf_stack_value t1, t2;
-
-	    if (this->stack_len < 2)
+	    if (stack.size () < 2)
 	       error (_("Not enough elements for "
-			"DW_OP_swap.  Need 2, have %d."),
-		      this->stack_len);
-	    t1 = this->stack[this->stack_len - 1];
-	    t2 = this->stack[this->stack_len - 2];
-	    this->stack[this->stack_len - 1] = t2;
-	    this->stack[this->stack_len - 2] = t1;
+			"DW_OP_swap.  Need 2, have %zu."),
+		      stack.size ());
+
+	    dwarf_stack_value &t1 = stack[stack.size () - 1];
+	    dwarf_stack_value &t2 = stack[stack.size () - 2];
+	    std::swap (t1, t2);
 	    goto no_push;
 	  }
 
@@ -940,18 +913,15 @@ dwarf_expr_context::execute_stack_op (const gdb_byte *op_ptr,
 
 	case DW_OP_rot:
 	  {
-	    struct dwarf_stack_value t1, t2, t3;
-
-	    if (this->stack_len < 3)
+	    if (stack.size () < 3)
 	       error (_("Not enough elements for "
-			"DW_OP_rot.  Need 3, have %d."),
-		      this->stack_len);
-	    t1 = this->stack[this->stack_len - 1];
-	    t2 = this->stack[this->stack_len - 2];
-	    t3 = this->stack[this->stack_len - 3];
-	    this->stack[this->stack_len - 1] = t2;
-	    this->stack[this->stack_len - 2] = t3;
-	    this->stack[this->stack_len - 3] = t1;
+			"DW_OP_rot.  Need 3, have %zu."),
+		      stack.size ());
+
+	    dwarf_stack_value temp = stack[stack.size () - 1];
+	    stack[stack.size () - 1] = stack[stack.size () - 2];
+	    stack[stack.size () - 2] = stack[stack.size () - 3];
+	    stack[stack.size () - 3] = temp;
 	    goto no_push;
 	  }
 
diff --git a/gdb/dwarf2expr.h b/gdb/dwarf2expr.h
index 0a57beef915..a8d6ae111dc 100644
--- a/gdb/dwarf2expr.h
+++ b/gdb/dwarf2expr.h
@@ -100,6 +100,10 @@ struct dwarf_expr_piece
 
 struct dwarf_stack_value
 {
+  dwarf_stack_value (struct value *value_, int in_stack_memory_)
+  : value (value_), in_stack_memory (in_stack_memory_)
+  {}
+
   struct value *value;
 
   /* True if the piece is in memory and is known to be on the program's stack.
@@ -114,7 +118,7 @@ struct dwarf_stack_value
 struct dwarf_expr_context
 {
   dwarf_expr_context ();
-  virtual ~dwarf_expr_context ();
+  virtual ~dwarf_expr_context () = default;
 
   void push_address (CORE_ADDR value, bool in_stack_memory);
   void eval (const gdb_byte *addr, size_t len);
@@ -122,12 +126,8 @@ struct dwarf_expr_context
   CORE_ADDR fetch_address (int n);
   bool fetch_in_stack_memory (int n);
 
-  /* The stack of values, allocated with xmalloc.  */
-  struct dwarf_stack_value *stack;
-
-  /* The number of values currently pushed on the stack, and the
-     number of elements allocated to the stack.  */
-  int stack_len, stack_allocated;
+  /* The stack of values.  */
+  std::vector<dwarf_stack_value> stack;
 
   /* Target architecture to use for address operations.  */
   struct gdbarch *gdbarch;
@@ -249,7 +249,6 @@ struct dwarf_expr_context
 private:
 
   struct type *address_type () const;
-  void grow_stack (size_t need);
   void push (struct value *value, bool in_stack_memory);
   bool stack_empty_p () const;
   void add_piece (ULONGEST size, ULONGEST offset);