PR python/20190 - compute TLS symbol without a frame

PR python/20190 - compute TLS symbol without a frame

PR python/20190 arose from an exception I noticed when trying to use
the Python unwinder for Spider Monkey in Firefox.

The problem is that the unwinder wants to examine the value of a
thread-local variable.  However, sympy_value rejects this because
symbol_read_needs_frame returns true for a TLS variable.

This problem arose once before, though in a different context:

https://sourceware.org/bugzilla/show_bug.cgi?id=11803

At the time Pedro and Daniel pointed out a simpler way to fix that bug
(see links in 20190 if you are interested); but for this new bug I
couldn't think of a similar fix and ended up implementing Daniel's
other suggestion:

https://sourceware.org/ml/gdb-patches/2010-07/msg00393.html

That is, this patch makes it possible to detect whether a symbol needs
a specific frame, or whether it just needs the inferior to have
registers.

Built and regtested on x86-64 Fedora 24.

2016-07-26  Tom Tromey  <tom@tromey.com>

	* symtab.c (register_symbol_computed_impl): Update.
	PR python/20190:
	* value.h (symbol_read_needs): Declare.
	(symbol_read_needs_frame): Add comment.
	* symtab.h (struct symbol_computed_ops) <read_variable>: Update
	comment.
	<get_symbol_read_needs>: Rename.  Change return type.
	* findvar.c (symbol_read_needs): New function.
	(symbol_read_needs_frame): Rewrite.
	(default_read_var_value): Use symbol_read_needs.
	* dwarf2loc.c (struct symbol_needs_baton): Rename.
	<needs>: Renamed from needs_frame.  Changed type.
	(needs_frame_read_addr_from_reg, symbol_needs_get_reg_value)
	(symbol_needs_read_mem, symbol_needs_frame_base)
	(symbol_needs_frame_cfa, symbol_needs_tls_address)
	(symbol_needs_dwarf_call): Rename.
	(needs_dwarf_reg_entry_value): Update.
	(symbol_needs_ctx_funcs, dwarf2_loc_desc_get_symbol_read_needs):
	Rename and update.
	(locexpr_get_symbol_read_needs, loclist_symbol_needs): Likewise.
	(dwarf2_locexpr_funcs, dwarf2_loclist_funcs): Update.
	* defs.h (enum symbol_needs_kind): New.

2016-07-26  Tom Tromey  <tom@tromey.com>

	PR python/20190:
	* gdb.threads/tls.exp (check_thread_local): Add python symbol
	test.