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.

1 files changed, 21 insertions, 10 deletions

diff --git a/gdb/findvar.c b/gdb/findvar.c
index c64ec06..6e28a29 100644
--- a/gdb/findvar.c
+++ b/gdb/findvar.c
@@ -337,14 +337,13 @@ address_to_signed_pointer (struct gdbarch *gdbarch, struct type *type,
   store_signed_integer (buf, TYPE_LENGTH (type), byte_order, addr);
 }
 
-/* Will calling read_var_value or locate_var_value on SYM end
-   up caring what frame it is being evaluated relative to?  SYM must
-   be non-NULL.  */
-int
-symbol_read_needs_frame (struct symbol *sym)
+/* See value.h.  */
+
+enum symbol_needs_kind
+symbol_read_needs (struct symbol *sym)
 {
   if (SYMBOL_COMPUTED_OPS (sym) != NULL)
-    return SYMBOL_COMPUTED_OPS (sym)->read_needs_frame (sym);
+    return SYMBOL_COMPUTED_OPS (sym)->get_symbol_read_needs (sym);
 
   switch (SYMBOL_CLASS (sym))
     {
@@ -358,7 +357,7 @@ symbol_read_needs_frame (struct symbol *sym)
     case LOC_REF_ARG:
     case LOC_REGPARM_ADDR:
     case LOC_LOCAL:
-      return 1;
+      return SYMBOL_NEEDS_FRAME;
 
     case LOC_UNDEF:
     case LOC_CONST:
@@ -374,9 +373,17 @@ symbol_read_needs_frame (struct symbol *sym)
     case LOC_CONST_BYTES:
     case LOC_UNRESOLVED:
     case LOC_OPTIMIZED_OUT:
-      return 0;
+      return SYMBOL_NEEDS_NONE;
     }
-  return 1;
+  return SYMBOL_NEEDS_FRAME;
+}
+
+/* See value.h.  */
+
+int
+symbol_read_needs_frame (struct symbol *sym)
+{
+  return symbol_read_needs (sym) == SYMBOL_NEEDS_FRAME;
 }
 
 /* Private data to be used with minsym_lookup_iterator_cb.  */
@@ -575,6 +582,7 @@ default_read_var_value (struct symbol *var, const struct block *var_block,
   struct value *v;
   struct type *type = SYMBOL_TYPE (var);
   CORE_ADDR addr;
+  enum symbol_needs_kind sym_need;
 
   /* Call check_typedef on our type to make sure that, if TYPE is
      a TYPE_CODE_TYPEDEF, its length is set to the length of the target type
@@ -583,8 +591,11 @@ default_read_var_value (struct symbol *var, const struct block *var_block,
      set the returned value type description correctly.  */
   check_typedef (type);
 
-  if (symbol_read_needs_frame (var))
+  sym_need = symbol_read_needs (var);
+  if (sym_need == SYMBOL_NEEDS_FRAME)
     gdb_assert (frame != NULL);
+  else if (sym_need == SYMBOL_NEEDS_REGISTERS && !target_has_registers)
+    error (_("Cannot read `%s' without registers"), SYMBOL_PRINT_NAME (var));
 
   if (frame != NULL)
     frame = get_hosting_frame (var, var_block, frame);