Restored Objective-C language support

  2016-09-13  Giah de Barag  <gdb@crelg.com>

Summary
-------

Patches are presented which restore objective-c language support.

These patches are relative to the head of the gdb-7.11-branch.

Impact
------

These changes impact only objective-c language mode.

Description
-----------

These patches enable GDB to (once again) evaluate objective-c
expressions:

(a) [object message:arg] messages,
(b) @"foo" NSStrings, and
(c) @selector(foo:bar:) selectors.
(d) "foo" c strings

These patches complete the previously-unfinished work of merging
objc-exp.y into c-exp.y.

Explanation of Changes
----------------------

Problem 1: Evaluating "foo" c-string literals fails in objective-c
           language mode.

    Description:

        (gdb) set language objective-c
        (gdb) p "hello"
        $1 = <error reading variable>

        Creating a basic C string literal does not work when
        language is objective-c.

    Solution:

        Change objective-c evaluate_subexp_standard to
        evaluate_subexp_c.

    Test:

        The current source language is "auto; currently objective-c".
        (gdb) p strlen("test")
        $1 = 4

Problem 2: Evaluating objc expressions fails in objective-c language
           mode. Examples:

           (a) [object message:arg] messages,
           (b) @"foo" NSStrings, and
           (c) @selector(foo:bar:) selectors.

    Description:

        (gdb) p [@"hello" performSelector:@selector(length)]

        Crashes and burns in many specactular ways, reported in
        bugs 20501 and 20503.

        /* file: c-exp.y */

        exp : NSSTRING
            { write_exp_elt_opcode (pstate, OP_OBJC_NSSTRING);
              write_exp_string (pstate, $1);
              write_exp_elt_opcode (pstate, OP_OBJC_NSSTRING); } ;

        exp : SELECTOR
            { write_exp_elt_opcode (pstate, OP_OBJC_SELECTOR);
              write_exp_string (pstate, $1);
              write_exp_elt_opcode (pstate, OP_OBJC_SELECTOR); } ;

        Communication between scanner and parser is broken; when the
        program attempts to reduce these productions, $1 is null.

    Solution:

        Establish communication between the scanner and the parser by
        creating a global variable, last_parsed_token, to hold the last
        scanned token (struct typed_stoken) for string and for selector.

        The existing @selector() scanner was weak. A new, robust
        @selector() scanner was written and inserted to improve quality.

        The string scanner puts a pointer to the last scanned token into
        last_parsed_token (in its choice of object names, gdb sometimes
        conflates the meaning of scanning and parsing, which we went along
        with). The newly-created selector scanner also does the same.

    Test:

        (gdb) p [@"hello" performSelector:@selector(length)]
        $2 = 5
        (gdb)

        Here is another test, more complicated and pedantic, that
        invokes all the features that were broken, creating an
        NSString on the fly and sending it a a message which turns it
        into an NSArray, then sending that NSArray a message which
        sends a message to all its objects and creates another NSArray
        out of the results.

        (gdb) po [[@"(london, paris, rome, berlin)" propertyList]  \
              performSelector:@selector(mappedArrayUsingSelector:) \
                   withObject:@selector(capitalizedString)]
        (London, Paris, Rome, Berlin)
        (gdb)

        This single invocation tests:
        ----------------------------
        * the “po” command
        * creating a literal NSString @"foo" (which responds to messages)
        * sending a message [obj message:arg] to the result of a message
        * referencing a selector @selector(foo) (two different ones in
          one invocation)

Bugs Fixed
----------

These patches fix all bugs reported in:

    https://sourceware.org/bugzilla/show_bug.cgi?id=20501
    https://sourceware.org/bugzilla/show_bug.cgi?id=20503

By the way, this bug is obsolete and should be updated:

    https://sourceware.org/bugzilla/show_bug.cgi?id=11925

Note to GDB Maintainers
-----------------------

Can you adapt these patches to all relevant branches, or do you need me to
provide patches relative to other branches?

I can supply a recipe to build GNUstep if you want to test this patch
yourself.

Also, this is the first time I am doing something like this, so if I am
neglecting any rule of communication of this list, please inform me, and I
will correct it.

Patches
-------

Here are the patches relative to gdb-7.11-branch. Do you need them
relative to the other branches?

----- CUT HERE -----

Restored Objective-C language support

Commit Message

Comments

Patch