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.

1 files changed, 36 insertions, 0 deletions

diff --git a/gdb/eval.c b/gdb/eval.c
index 8636bcf..8adbcef 100644
--- a/gdb/eval.c
+++ b/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.  */