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At <https://sourceware.org/ml/gdb-patches/2017-12/msg00298.html>, Joel wrote: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider the following code which first declares a tagged type (the equivalent of a class in Ada), and then a procedure which takes a pointer (access) to this type's 'Class. package Pck is type Top_T is tagged record N : Integer := 1; end record; procedure Inspect (Obj: access Top_T'Class); end Pck; Putting a breakpoint in that procedure and then running to it triggers an internal error: (gdb) break inspect (gdb) continue Breakpoint 1, pck.inspect (obj=0x63e010 /[...]/gdb/stack.c:621: internal-error: void print_frame_args(symbol*, frame_info*, int, ui_file*): Assertion `nsym != NULL' failed. What's special about this subprogram is that it takes an access to what we call a 'Class type, and for implementation reasons, the compiler adds an extra argument named "objL". If you are curious why, it allows the compiler for perform dynamic accessibility checks that are mandated by the language. If we look at the location where we get the internal error (in stack.c), we find that we are looping over the symbol of each parameter, and for each parameter, we do: /* We have to look up the symbol because arguments can have two entries (one a parameter, one a local) and the one we want is the local, which lookup_symbol will find for us. [...] nsym = lookup_symbol (SYMBOL_LINKAGE_NAME (sym), b, VAR_DOMAIN, NULL).symbol; gdb_assert (nsym != NULL); The lookup_symbol goes through the lookup structure, which means the symbol's linkage name ("objL") gets transformed into a lookup_name_info object (in block_lookup_symbol), before it gets fed to the block symbol dictionary iterators. This, in turn, triggers the symbol matching by comparing the "lookup" name which, for Ada, means among other things, lowercasing the given name to "objl". It is this transformation that causes the lookup find no matches, and therefore trip this assertion. Going back to the "offending" call to lookup_symbol in stack.c, what we are trying to do, here, is do a lookup by linkage name. So, I think what we mean to be doing is a completely literal symbol lookup, so maybe not even strcmp_iw, but actually just plain strcmp??? In the past, in practice, you could get that effect by doing a lookup using the C language. But that doesn't work, because we still end up somehow using Ada's lookup_name routine which transforms "objL". So, ideally, as I hinted before, I think what we need is a way to perform a literal lookup so that searches by linkage names like the above can be performed. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ This commit fixes the problem by implementing something similar to Joel's literal idea, but with some important differences. I considered adding a symbol_name_match_type::LINKAGE and supporting searching by linkage name for any language, but the problem with that is that the dictionaries only work with SYMBOL_SEARCH_NAME, because that's what is used for hashing. We'd need separate dictionaries for hashed linkage names. So with the current symbol tables infrastructure, it's not literal linkage names that we want to pass down, but instead literal _search_ names (SYMBOL_SEARCH_NAME, etc.). However, psymbols have no overload/function parameter info in C++, so a straight strcmp doesn't work properly for C++ name matching. So what we do is be a little less aggressive then and add a new symbol_name_match_type::SEARCH_SYMBOL instead that takes as input a non-user-input search symbol, and then we skip any decoding/demangling steps and make: - Ada treat that as a verbatim match, - other languages treat it as symbol_name_match_type::FULL. This also fixes the new '"maint check-psymtabs" for Ada' testcase for me (gdb.ada/maint_with_ada.exp). I've not removed the kfail yet because Joel still sees that testcase failing with this patch. That'll be fixed in follow up patches. gdb/ChangeLog: 2018-01-05 Pedro Alves <palves@redhat.com> PR gdb/22670 * ada-lang.c (literal_symbol_name_matcher): New function. (ada_get_symbol_name_matcher): Use it for symbol_name_match_type::SEARCH_NAME. * block.c (block_lookup_symbol): New parameter 'match_type'. Pass it down instead of assuming symbol_name_match_type::FULL. * block.h (block_lookup_symbol): New parameter 'match_type'. * c-valprint.c (print_unpacked_pointer): Use lookup_symbol_search_name instead of lookup_symbol. * compile/compile-object-load.c (get_out_value_type): Pass down symbol_name_match_type::SEARCH_NAME. * cp-namespace.c (cp_basic_lookup_symbol): Pass down symbol_name_match_type::FULL. * cp-support.c (cp_get_symbol_name_matcher): Handle symbol_name_match_type::SEARCH_NAME. * infrun.c (insert_exception_resume_breakpoint): Use lookup_symbol_search_name. * p-valprint.c (pascal_val_print): Use lookup_symbol_search_name. * psymtab.c (maintenance_check_psymtabs): Use symbol_name_match_type::SEARCH_NAME and SYMBOL_SEARCH_NAME. * stack.c (print_frame_args): Use lookup_symbol_search_name and SYMBOL_SEARCH_NAME. * symtab.c (lookup_local_symbol): Don't demangle the lookup name if symbol_name_match_type::SEARCH_NAME. (lookup_symbol_in_language): Pass down symbol_name_match_type::FULL. (lookup_symbol_search_name): New. (lookup_language_this): Pass down symbol_name_match_type::SEARCH_NAME. (lookup_symbol_aux, lookup_local_symbol): New parameter 'match_type'. Pass it down. * symtab.h (symbol_name_match_type::SEARCH_NAME): New enumerator. (lookup_symbol_search_name): New declaration. (lookup_symbol_in_block): New 'match_type' parameter. gdb/testsuite/ChangeLog: 2018-01-05 Joel Brobecker <brobecker@adacore.com> PR gdb/22670 * gdb.ada/access_tagged_param.exp: New file. * gdb.ada/access_tagged_param/foo.adb: New file. |
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README for GNU development tools This directory contains various GNU compilers, assemblers, linkers, debuggers, etc., plus their support routines, definitions, and documentation. If you are receiving this as part of a GDB release, see the file gdb/README. If with a binutils release, see binutils/README; if with a libg++ release, see libg++/README, etc. That'll give you info about this package -- supported targets, how to use it, how to report bugs, etc. It is now possible to automatically configure and build a variety of tools with one command. To build all of the tools contained herein, run the ``configure'' script here, e.g.: ./configure make To install them (by default in /usr/local/bin, /usr/local/lib, etc), then do: make install (If the configure script can't determine your type of computer, give it the name as an argument, for instance ``./configure sun4''. You can use the script ``config.sub'' to test whether a name is recognized; if it is, config.sub translates it to a triplet specifying CPU, vendor, and OS.) If you have more than one compiler on your system, it is often best to explicitly set CC in the environment before running configure, and to also set CC when running make. For example (assuming sh/bash/ksh): CC=gcc ./configure make A similar example using csh: setenv CC gcc ./configure make Much of the code and documentation enclosed is copyright by the Free Software Foundation, Inc. See the file COPYING or COPYING.LIB in the various directories, for a description of the GNU General Public License terms under which you can copy the files. REPORTING BUGS: Again, see gdb/README, binutils/README, etc., for info on where and how to report problems.