Introduce class operation

This patch introduces class operation, the new base class for all
expression operations.

In the new approach, an operation is simply a class that presents a
certain interface.  Operations own their operands, and management is
done via unique_ptr.

The operation interface is largely ad hoc, based on the evolution of
expression handling in GDB.  Parts (for example,
evaluate_with_coercion) are probably redundant; however I took this
approach to try to avoid mixing different kinds of refactorings.

In some specific situations, rather than add a generic method across
the entire operation class hierarchy, I chose instead to use
dynamic_cast and specialized methods on certain concrete subclasses.
This will appear in some subsequent patches.

One goal of this work is to avoid the kinds of easy-to-make errors
that affected the old implementation.  To this end, some helper
subclasses are also added here.  These helpers automate the
implementation of the 'dump', 'uses_objfile', and 'constant_p'
methods.  Nearly every concrete operation that is subsequently added
will use these facilities.  (Note that the 'dump' implementation is
only outlined here, the body appears in the next patch.)

gdb/ChangeLog
2021-03-08  Tom Tromey  <tom@tromey.com>

	* expression.h (expr::operation): New class.
	(expr::make_operation): New function.
	(expr::operation_up): New typedef.
	* expop.h: New file.
	* eval.c (operation::evaluate_for_cast)
	(operation::evaluate_for_address, operation::evaluate_for_sizeof):
	New methods.
	* ax-gdb.c (operation::generate_ax): New method.

gdb/ChangeLog

@@ -1,3 +1,14 @@
+2021-03-08  Tom Tromey  <tom@tromey.com>
+
+	* expression.h (expr::operation): New class.
+	(expr::make_operation): New function.
+	(expr::operation_up): New typedef.
+	* expop.h: New file.
+	* eval.c (operation::evaluate_for_cast)
+	(operation::evaluate_for_address, operation::evaluate_for_sizeof):
+	New methods.
+	* ax-gdb.c (operation::generate_ax): New method.
+
 2021-03-08  Tom Tromey  <tom@tromey.com>
 
 	* ax-gdb.c (gen_expr_binop_rest): Remove "pc" parameter.

gdb/ax-gdb.c

@@ -2270,6 +2270,32 @@ gen_expr (struct expression *exp, union exp_element **pc,
     }
 }
 
+namespace expr
+{
+
+void
+operation::generate_ax (struct expression *exp,
+			struct agent_expr *ax,
+			struct axs_value *value,
+			struct type *cast_type)
+{
+  if (constant_p ())
+    {
+      struct value *v = evaluate (nullptr, exp, EVAL_AVOID_SIDE_EFFECTS);
+      ax_const_l (ax, value_as_long (v));
+      value->kind = axs_rvalue;
+      value->type = check_typedef (value_type (v));
+    }
+  else
+    {
+      do_generate_ax (exp, ax, value, cast_type);
+      if (cast_type != nullptr)
+	gen_cast (ax, value, cast_type);
+    }
+}
+
+}
+
 /* This handles the middle-to-right-side of code generation for binary
    expressions, which is shared between regular binary operations and
    assign-modify (+= and friends) expressions.  */

gdb/eval.c

@@ -40,6 +40,7 @@
 #include "objfiles.h"
 #include "typeprint.h"
 #include <ctype.h>
+#include "expop.h"
 
 /* Prototypes for local functions.  */
 
@@ -3267,6 +3268,29 @@ evaluate_subexp_for_address (struct expression *exp, int *pos,
     }
 }
 
+namespace expr
+{
+
+value *
+operation::evaluate_for_cast (struct type *expect_type,
+			      struct expression *exp,
+			      enum noside noside)
+{
+  value *val = evaluate (expect_type, exp, noside);
+  if (noside == EVAL_SKIP)
+    return eval_skip_value (exp);
+  return value_cast (expect_type, val);
+}
+
+value *
+operation::evaluate_for_address (struct expression *exp, enum noside noside)
+{
+  value *val = evaluate (nullptr, exp, noside);
+  return evaluate_subexp_for_address_base (exp, noside, val);
+}
+
+}
+
 /* Evaluate like `evaluate_subexp' except coercing arrays to pointers.
    When used in contexts where arrays will be coerced anyway, this is
    equivalent to `evaluate_subexp' but much faster because it avoids
@@ -3455,6 +3479,18 @@ evaluate_subexp_for_sizeof (struct expression *exp, int *pos,
   return evaluate_subexp_for_sizeof_base (exp, type);
 }
 
+namespace expr
+{
+
+value *
+operation::evaluate_for_sizeof (struct expression *exp, enum noside noside)
+{
+  value *val = evaluate (nullptr, exp, EVAL_AVOID_SIDE_EFFECTS);
+  return evaluate_subexp_for_sizeof_base (exp, value_type (val));
+}
+
+}
+
 /* Evaluate a subexpression of EXP, at index *POS, and return a value
    for that subexpression cast to TO_TYPE.  Advance *POS over the
    subexpression.  */

gdb/expop.h

@@ -0,0 +1,315 @@
+/* Definitions for expressions in GDB
+
+   Copyright (C) 2020 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   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/>.  */
+
+#ifndef EXPOP_H
+#define EXPOP_H
+
+#include "block.h"
+#include "c-lang.h"
+#include "cp-abi.h"
+#include "expression.h"
+#include "objfiles.h"
+#include "gdbsupport/traits.h"
+#include "gdbsupport/enum-flags.h"
+
+struct agent_expr;
+struct axs_value;
+
+namespace expr
+{
+
+/* The check_objfile overloads are used to check whether a particular
+   component of some operation references an objfile.  The passed-in
+   objfile will never be a debug objfile.  */
+
+/* See if EXP_OBJFILE matches OBJFILE.  */
+static inline bool
+check_objfile (struct objfile *exp_objfile, struct objfile *objfile)
+{
+  if (exp_objfile->separate_debug_objfile_backlink)
+    exp_objfile = exp_objfile->separate_debug_objfile_backlink;
+  return exp_objfile == objfile;
+}
+
+static inline bool 
+check_objfile (struct type *type, struct objfile *objfile)
+{
+  struct objfile *ty_objfile = type->objfile_owner ();
+  if (ty_objfile != nullptr)
+    return check_objfile (ty_objfile, objfile);
+  return false;
+}
+
+static inline bool 
+check_objfile (struct symbol *sym, struct objfile *objfile)
+{
+  return check_objfile (symbol_objfile (sym), objfile);
+}
+
+static inline bool 
+check_objfile (const struct block *block, struct objfile *objfile)
+{
+  return check_objfile (block_objfile (block), objfile);
+}
+
+static inline bool
+check_objfile (minimal_symbol *minsym, struct objfile *objfile)
+{
+  /* This may seem strange but minsyms are only used with an objfile
+     as well.  */
+  return false;
+}
+
+static inline bool
+check_objfile (internalvar *ivar, struct objfile *objfile)
+{
+  return false;
+}
+
+static inline bool
+check_objfile (const std::string &str, struct objfile *objfile)
+{
+  return false;
+}
+
+static inline bool 
+check_objfile (const operation_up &op, struct objfile *objfile)
+{
+  return op->uses_objfile (objfile);
+}
+
+static inline bool
+check_objfile (enum exp_opcode val, struct objfile *objfile)
+{
+  return false;
+}
+
+static inline bool
+check_objfile (ULONGEST val, struct objfile *objfile)
+{
+  return false;
+}
+
+template<typename T>
+static inline bool
+check_objfile (enum_flags<T> val, struct objfile *objfile)
+{
+  return false;
+}
+
+template<typename T>
+static inline bool 
+check_objfile (const std::vector<T> &collection, struct objfile *objfile)
+{
+  for (const auto &item : collection)
+    {
+      if (check_objfile (item, objfile))
+	return true;
+    }
+  return false;
+}
+
+template<typename S, typename T>
+static inline bool 
+check_objfile (const std::pair<S, T> &item, struct objfile *objfile)
+{
+  return (check_objfile (item.first, objfile)
+	  || check_objfile (item.second, objfile));
+}
+
+/* Base class for most concrete operations.  This class holds data,
+   specified via template parameters, and supplies generic
+   implementations of the 'dump' and 'uses_objfile' methods.  */
+template<typename... Arg>
+class tuple_holding_operation : public operation
+{
+public:
+
+  explicit tuple_holding_operation (Arg... args)
+    : m_storage (std::forward<Arg> (args)...)
+  {
+  }
+
+  DISABLE_COPY_AND_ASSIGN (tuple_holding_operation);
+
+  bool uses_objfile (struct objfile *objfile) const override
+  {
+    return do_check_objfile<0, Arg...> (objfile, m_storage);
+  }
+
+  void dump (struct ui_file *stream, int depth) const override
+  {
+    do_dump<0, Arg...> (stream, depth, m_storage);
+  }
+
+protected:
+
+  /* Storage for the data.  */
+  std::tuple<Arg...> m_storage;
+
+private:
+
+  /* do_dump does the work of dumping the data.  */
+  template<int I, typename... T>
+  typename std::enable_if<I == sizeof... (T), void>::type
+  do_dump (struct ui_file *stream, int depth, const std::tuple<T...> &value)
+    const
+  {
+  }
+
+  template<int I, typename... T>
+  typename std::enable_if<I < sizeof... (T), void>::type
+  do_dump (struct ui_file *stream, int depth, const std::tuple<T...> &value)
+    const
+  {
+    do_dump<I + 1, T...> (stream, depth, value);
+  }
+
+  /* do_check_objfile does the work of checking whether this object
+     refers to OBJFILE.  */
+  template<int I, typename... T>
+  typename std::enable_if<I == sizeof... (T), bool>::type
+  do_check_objfile (struct objfile *objfile, const std::tuple<T...> &value)
+    const
+  {
+    return false;
+  }
+
+  template<int I, typename... T>
+  typename std::enable_if<I < sizeof... (T), bool>::type
+  do_check_objfile (struct objfile *objfile, const std::tuple<T...> &value)
+    const
+  {
+    if (check_objfile (std::get<I> (value), objfile))
+      return true;
+    return do_check_objfile<I + 1, T...> (objfile, value);
+  }
+};
+
+/* The check_constant overloads are used to decide whether a given
+   concrete operation is a constant.  This is done by checking the
+   operands.  */
+
+static inline bool
+check_constant (const operation_up &item)
+{
+  return item->constant_p ();
+}
+
+static inline bool
+check_constant (struct minimal_symbol *msym)
+{
+  return false;
+}
+
+static inline bool
+check_constant (struct type *type)
+{
+  return true;
+}
+
+static inline bool
+check_constant (const struct block *block)
+{
+  return true;
+}
+
+static inline bool
+check_constant (const std::string &str)
+{
+  return true;
+}
+
+static inline bool
+check_constant (struct objfile *objfile)
+{
+  return true;
+}
+
+static inline bool
+check_constant (ULONGEST cst)
+{
+  return true;
+}
+
+static inline bool
+check_constant (struct symbol *sym)
+{
+  enum address_class sc = SYMBOL_CLASS (sym);
+  return (sc == LOC_BLOCK
+	  || sc == LOC_CONST
+	  || sc == LOC_CONST_BYTES
+	  || sc == LOC_LABEL);
+}
+
+template<typename T>
+static inline bool
+check_constant (const std::vector<T> &collection)
+{
+  for (const auto &item : collection)
+    if (!check_constant (item))
+      return false;
+  return true;
+}
+
+template<typename S, typename T>
+static inline bool
+check_constant (const std::pair<S, T> &item)
+{
+  return check_constant (item.first) && check_constant (item.second);
+}
+
+/* Base class for concrete operations.  This class supplies an
+   implementation of 'constant_p' that works by checking the
+   operands.  */
+template<typename... Arg>
+class maybe_constant_operation
+  : public tuple_holding_operation<Arg...>
+{
+public:
+
+  using tuple_holding_operation<Arg...>::tuple_holding_operation;
+
+  bool constant_p () const override
+  {
+    return do_check_constant<0, Arg...> (this->m_storage);
+  }
+
+private:
+
+  template<int I, typename... T>
+  typename std::enable_if<I == sizeof... (T), bool>::type
+  do_check_constant (const std::tuple<T...> &value) const
+  {
+    return true;
+  }
+
+  template<int I, typename... T>
+  typename std::enable_if<I < sizeof... (T), bool>::type
+  do_check_constant (const std::tuple<T...> &value) const
+  {
+    if (!check_constant (std::get<I> (value)))
+      return false;
+    return do_check_constant<I + 1, T...> (value);
+  }
+};
+
+} /* namespace expr */
+
+#endif /* EXPOP_H */

gdb/expression.h

@@ -89,6 +89,105 @@ enum noside
 				   does in many situations.  */
   };
 
+struct expression;
+struct agent_expr;
+struct axs_value;
+struct type;
+struct ui_file;
+
+namespace expr
+{
+
+class operation;
+typedef std::unique_ptr<operation> operation_up;
+
+/* Base class for an operation.  An operation is a single component of
+   an expression.  */
+
+class operation
+{
+protected:
+
+  operation () = default;
+  DISABLE_COPY_AND_ASSIGN (operation);
+
+public:
+
+  virtual ~operation () = default;
+
+  /* Evaluate this operation.  */
+  virtual value *evaluate (struct type *expect_type,
+			   struct expression *exp,
+			   enum noside noside) = 0;
+
+  /* Evaluate this operation in a context where C-like coercion is
+     needed.  */
+  virtual value *evaluate_with_coercion (struct expression *exp,
+					 enum noside noside)
+  {
+    return evaluate (nullptr, exp, noside);
+  }
+
+  /* Evaluate this expression in the context of a cast to
+     EXPECT_TYPE.  */
+  virtual value *evaluate_for_cast (struct type *expect_type,
+				    struct expression *exp,
+				    enum noside noside);
+
+  /* Evaluate this expression in the context of a sizeof
+     operation.  */
+  virtual value *evaluate_for_sizeof (struct expression *exp,
+				      enum noside noside);
+
+  /* Evaluate this expression in the context of an address-of
+     operation.  Must return the address.  */
+  virtual value *evaluate_for_address (struct expression *exp,
+				       enum noside noside);
+
+  /* True if this is a constant expression.  */
+  virtual bool constant_p () const
+  { return false; }
+
+  /* Return true if this operation uses OBJFILE (and will become
+     dangling when OBJFILE is unloaded), otherwise return false.
+     OBJFILE must not be a separate debug info file.  */
+  virtual bool uses_objfile (struct objfile *objfile) const
+  { return false; }
+
+  /* Generate agent expression bytecodes for this operation.  */
+  void generate_ax (struct expression *exp, struct agent_expr *ax,
+		    struct axs_value *value,
+		    struct type *cast_type = nullptr);
+
+  /* Return the opcode that is implemented by this operation.  */
+  virtual enum exp_opcode opcode () const = 0;
+
+  /* Print this operation to STREAM.  */
+  virtual void dump (struct ui_file *stream, int depth) const = 0;
+
+protected:
+
+  /* Called by generate_ax to do the work for this particular
+     operation.  */
+  virtual void do_generate_ax (struct expression *exp,
+			       struct agent_expr *ax,
+			       struct axs_value *value,
+			       struct type *cast_type)
+  {
+    error (_("Cannot translate to agent expression"));
+  }
+};
+
+/* A helper function for creating an operation_up, given a type.  */
+template<typename T, typename... Arg>
+operation_up
+make_operation (Arg... args)
+{
+  return operation_up (new T (std::forward<Arg> (args)...));
+}
+
+}
+
 union exp_element
   {
     enum exp_opcode opcode;