Re: [PATCH 2/2] gdb: add SYMBOL_LOOKUP_SCOPED_DEBUG_ENTER_EXIT

[Andrew Burgess <aburgess@redhat.com>]

Tom Tromey <tom@tromey.com> writes:

>>>>>> "Andrew" == Andrew Burgess via Gdb-patches <gdb-patches@sourceware.org> writes:
>
> Andrew> After the previous commit converted symbol lookup debug to use the new
> Andrew> debug scheme, this commit adds SYMBOL_LOOKUP_SCOPED_DEBUG_ENTER_EXIT.
>
> Andrew> The previous commit updated 'set debug symbol-lookup' to use the new
> Andrew> debug scheme, however SYMBOL_LOOKUP_SCOPED_DEBUG_ENTER_EXIT still does
> Andrew> not exist.  The reason for this is that 'set debug symbol-lookup'
> Andrew> controls an integer variable, rather than a bool, and, depending on
> Andrew> the value of this variable, different amounts of debug will be
> Andrew> produced.
>
> Andrew> Currently the *_SCOPED_DEBUG_ENTER_EXIT mechanism will only work for
> Andrew> control variables of type bool, this is because the underlying
> Andrew> scoped_debug_start_end class can only handle variables of type bool.
>
> Andrew> This commit templates scoped_debug_start_end so that the class can
> Andrew> accept either a 'bool &' or an invokable object, e.g. a lambda
> Andrew> function, or a function pointer.
>
> Andrew> The existing scoped_debug_start_end and scoped_debug_enter_exit macros
> Andrew> in common-debug.h are updated to support scoped_debug_enter_exit being
> Andrew> templated, however, nothing outside of common-debug.h needs to change.
>
> Andrew> I've then added SYMBOL_LOOKUP_SCOPED_DEBUG_ENTER_EXIT in symtab.h, and
> Andrew> added a couple of token uses in symtab.c.  I didn't want to add too
> Andrew> much in this first commit, this is really about updating
> Andrew> common-debug.h to support this new ability.
>
> This seems fine.  I suppose inlining will make it not cost too much... ?

I'm not particularly worried about the additional cost.  There's already
plenty of debug printing done in the symbol lookup path, and like you
say, with inlining, I would expect most of the debug checks to be folded
together.

If this does prove to be problematic, then I'm happy for some/all of
these to be pulled out later.  For me, the biggest win here, is having
the ability to add things like SYMBOL_LOOKUP_SCOPED_DEBUG_ENTER_EXIT in
GDB - even if I just add these temporarily while I'm debugging a
particular issue.

>
> Andrew> +#define SYMBOL_LOOKUP_SCOPED_DEBUG_ENTER_EXIT \
> Andrew> +  scoped_debug_enter_exit ([&] () -> bool { \
> Andrew> +    return symbol_lookup_debug > 0; \
> Andrew> +  }, "symbol-lookup")
>
> I wonder whether the [&] is needed here.  I couldn't see what it is
> capturing.

In the end I took Simon's suggestion and switched to using a global
function instead of the lambda, though scoped_debug_enter_exit will
still accept a lambda if that's ever wanted elsewhere.  The updated
patch is below.

I've now pushed both these patches.

Thanks,
Andrew

--

commit 2698da268bdd0b4a6815a15b41a42bac5f928ca7
Author: Andrew Burgess <aburgess@redhat.com>
Date:   Tue Dec 6 12:49:55 2022 +0000

    gdb: add SYMBOL_LOOKUP_SCOPED_DEBUG_ENTER_EXIT
    
    After the previous commit converted symbol-lookup debug to use the new
    debug scheme, this commit adds SYMBOL_LOOKUP_SCOPED_DEBUG_ENTER_EXIT.
    
    The previous commit didn't add SYMBOL_LOOKUP_SCOPED_DEBUG_ENTER_EXIT
    because symbol-lookup debug is controlled by an 'unsigned int' rather
    than a 'bool' control variable, we use the numeric value to offer
    different levels of verbosity for symbol-lookup debug.
    
    The *_SCOPED_DEBUG_ENTER_EXIT mechanism currently relies on capturing
    a reference to the bool control variable, and evaluating the variable
    both on entry, and at exit, this is done in the scoped_debug_start_end
    class (see gdbsupport/common-debug.h).
    
    This commit templates scoped_debug_start_end so that the class can
    accept either a 'bool &' or an invokable object, e.g. a lambda
    function, or a function pointer.
    
    The existing scoped_debug_start_end and scoped_debug_enter_exit macros
    in common-debug.h are updated to support scoped_debug_enter_exit being
    templated, however, nothing outside of common-debug.h needs to change.
    
    I've then added SYMBOL_LOOKUP_SCOPED_DEBUG_ENTER_EXIT in symtab.h, and
    added a couple of token uses in symtab.c.  I didn't want to add too
    much in this first commit, this is really about updating
    common-debug.h to support this new functionality.
    
    Within symtab.h I created a couple of global functions that can be
    used to query the status of the symbol_lookup_debug control variable,
    these functions are then used within the two existing macros:
    
      symbol_lookup_debug_printf
      symbol_lookup_debug_printf_v
    
    and also in the new SYMBOL_LOOKUP_SCOPED_DEBUG_ENTER_EXIT macro.

diff --git a/gdb/symtab.c b/gdb/symtab.c
index 14d81f5468d..a7a54159b6d 100644
--- a/gdb/symtab.c
+++ b/gdb/symtab.c
@@ -1950,6 +1950,8 @@ lookup_symbol_in_language (const char *name, const struct block *block,
 			   const domain_enum domain, enum language lang,
 			   struct field_of_this_result *is_a_field_of_this)
 {
+  SYMBOL_LOOKUP_SCOPED_DEBUG_ENTER_EXIT;
+
   demangle_result_storage storage;
   const char *modified_name = demangle_for_lookup (name, lang, storage);
 
@@ -2072,6 +2074,8 @@ lookup_symbol_aux (const char *name, symbol_name_match_type match_type,
 		   const domain_enum domain, enum language language,
 		   struct field_of_this_result *is_a_field_of_this)
 {
+  SYMBOL_LOOKUP_SCOPED_DEBUG_ENTER_EXIT;
+
   struct block_symbol result;
   const struct language_defn *langdef;
 
diff --git a/gdb/symtab.h b/gdb/symtab.h
index 6eca61a759a..d0fa3b11c77 100644
--- a/gdb/symtab.h
+++ b/gdb/symtab.h
@@ -2625,17 +2625,38 @@ extern unsigned int symtab_create_debug;
 
 extern unsigned int symbol_lookup_debug;
 
+/* Return true if symbol-lookup debug is turned on at all.  */
+
+static inline bool
+symbol_lookup_debug_enabled ()
+{
+  return symbol_lookup_debug > 0;
+}
+
+/* Return true if symbol-lookup debug is turned to verbose mode.  */
+
+static inline bool
+symbol_lookup_debug_enabled_v ()
+{
+  return symbol_lookup_debug > 1;
+}
+
 /* Print a "symbol-lookup" debug statement if symbol_lookup_debug is >= 1.  */
 
 #define symbol_lookup_debug_printf(fmt, ...) \
-  debug_prefixed_printf_cond (symbol_lookup_debug >= 1, "symbol-lookup", fmt, \
-			      ##__VA_ARGS__)
+  debug_prefixed_printf_cond (symbol_lookup_debug_enabled (),	\
+			      "symbol-lookup", fmt, ##__VA_ARGS__)
 
 /* Print a "symbol-lookup" debug statement if symbol_lookup_debug is >= 2.  */
 
 #define symbol_lookup_debug_printf_v(fmt, ...) \
-  debug_prefixed_printf_cond (symbol_lookup_debug >= 2, "symbol-lookup", fmt, \
-			      ##__VA_ARGS__)
+  debug_prefixed_printf_cond (symbol_lookup_debug_enabled_v (), \
+			      "symbol-lookup", fmt, ##__VA_ARGS__)
+
+/* Print "symbol-lookup" enter/exit debug statements.  */
+
+#define SYMBOL_LOOKUP_SCOPED_DEBUG_ENTER_EXIT \
+  scoped_debug_enter_exit (symbol_lookup_debug_enabled, "symbol-lookup")
 
 extern bool basenames_may_differ;
 
diff --git a/gdbsupport/common-debug.h b/gdbsupport/common-debug.h
index 00a1e1599bc..904c1a1ea24 100644
--- a/gdbsupport/common-debug.h
+++ b/gdbsupport/common-debug.h
@@ -88,6 +88,7 @@ extern int debug_print_depth;
    it on destruction, such that nested debug statements will be printed with
    an indent and appear "inside" this one.  */
 
+template<typename PT>
 struct scoped_debug_start_end
 {
   /* DEBUG_ENABLED is a reference to a variable that indicates whether debugging
@@ -95,35 +96,35 @@ struct scoped_debug_start_end
      separately at construction and destruction, such that the start statement
      could be printed but not the end statement, or vice-versa.
 
+     DEBUG_ENABLED should either be of type 'bool &' or should be a type
+     that can be invoked.
+
      MODULE and FUNC are forwarded to debug_prefixed_printf.
 
      START_PREFIX and END_PREFIX are the statements to print on construction and
      destruction, respectively.
 
      If the FMT format string is non-nullptr, then a `: ` is appended to the
-     messages, followed by the rendering of that format string.  The format
-     string is rendered during construction and is re-used as is for the
-     message on exit.  */
+     messages, followed by the rendering of that format string with ARGS.
+     The format string is rendered during construction and is re-used as is
+     for the message on exit.  */
 
-  scoped_debug_start_end (bool &debug_enabled, const char *module,
+  scoped_debug_start_end (PT &debug_enabled, const char *module,
 			  const char *func, const char *start_prefix,
-			  const char *end_prefix, const char *fmt, ...)
-    ATTRIBUTE_NULL_PRINTF (7, 8)
+			  const char *end_prefix, const char *fmt,
+			  va_list args)
+    ATTRIBUTE_NULL_PRINTF (7, 0)
     : m_debug_enabled (debug_enabled),
       m_module (module),
       m_func (func),
       m_end_prefix (end_prefix),
       m_with_format (fmt != nullptr)
   {
-    if (m_debug_enabled)
+    if (is_debug_enabled ())
       {
 	if (fmt != nullptr)
 	  {
-	    va_list args;
-	    va_start (args, fmt);
 	    m_msg = string_vprintf (fmt, args);
-	    va_end (args);
-
 	    debug_prefixed_printf (m_module, m_func, "%s: %s",
 				   start_prefix, m_msg->c_str ());
 	  }
@@ -137,6 +138,8 @@ struct scoped_debug_start_end
 
   DISABLE_COPY_AND_ASSIGN (scoped_debug_start_end);
 
+  scoped_debug_start_end (scoped_debug_start_end &&other) = default;
+
   ~scoped_debug_start_end ()
   {
     if (m_must_decrement_print_depth)
@@ -145,7 +148,7 @@ struct scoped_debug_start_end
 	--debug_print_depth;
       }
 
-    if (m_debug_enabled)
+    if (is_debug_enabled ())
       {
 	if (m_with_format)
 	  {
@@ -167,7 +170,16 @@ struct scoped_debug_start_end
   }
 
 private:
-  bool &m_debug_enabled;
+
+  /* This function is specialized based on the type PT.  Returns true if
+     M_DEBUG_ENABLED indicates this debug setting is enabled, otherwise,
+     return false.  */
+  bool is_debug_enabled () const;
+
+  /* Reference to the debug setting, or a callback that can read the debug
+     setting.  Access the value of this by calling IS_DEBUG_ENABLED.  */
+  PT &m_debug_enabled;
+
   const char *m_module;
   const char *m_func;
   const char *m_end_prefix;
@@ -184,18 +196,60 @@ struct scoped_debug_start_end
   bool m_must_decrement_print_depth = false;
 };
 
+/* Implementation of is_debug_enabled when PT is an invokable type.  */
+
+template<typename PT>
+inline bool
+scoped_debug_start_end<PT>::is_debug_enabled () const
+{
+  return m_debug_enabled ();
+}
+
+/* Implementation of is_debug_enabled when PT is 'bool &'.  */
+
+template<>
+inline bool
+scoped_debug_start_end<bool &>::is_debug_enabled () const
+{
+  return m_debug_enabled;
+}
+
+/* Wrapper around the scoped_debug_start_end constructor to allow the
+   caller to create an object using 'auto' type, the actual type will be
+   based on the type of the PRED argument.  All arguments are forwarded to
+   the scoped_debug_start_end constructor.  */
+
+template<typename PT>
+static inline scoped_debug_start_end<PT &> ATTRIBUTE_NULL_PRINTF (6, 7)
+make_scoped_debug_start_end (PT &&pred, const char *module, const char *func,
+			     const char *start_prefix,
+			     const char *end_prefix, const char *fmt, ...)
+{
+  va_list args;
+  va_start (args, fmt);
+  auto res = scoped_debug_start_end<PT &> (pred, module, func, start_prefix,
+					   end_prefix, fmt, args);
+  va_end (args);
+
+  return res;
+}
+
 /* Helper to define a module-specific start/end debug macro.  */
 
-#define scoped_debug_start_end(debug_enabled, module, fmt, ...) \
-  scoped_debug_start_end CONCAT(scoped_debug_start_end, __LINE__) \
-    (debug_enabled, module, __func__, "start", "end", fmt, ##__VA_ARGS__)
+#define scoped_debug_start_end(debug_enabled, module, fmt, ...)		\
+  auto CONCAT(scoped_debug_start_end, __LINE__)				\
+    = make_scoped_debug_start_end (debug_enabled, module, 	\
+				   __func__, "start", "end",	\
+				   fmt, ##__VA_ARGS__)
 
 /* Helper to define a module-specific enter/exit debug macro.  This is a special
    case of `scoped_debug_start_end` where the start and end messages are "enter"
    and "exit", to denote entry and exit of a function.  */
 
-#define scoped_debug_enter_exit(debug_enabled, module) \
-  scoped_debug_start_end CONCAT(scoped_debug_start_end, __LINE__) \
-    (debug_enabled, module, __func__, "enter", "exit", nullptr)
+#define scoped_debug_enter_exit(debug_enabled, module)	\
+  auto CONCAT(scoped_debug_start_end, __LINE__)				\
+    = make_scoped_debug_start_end (debug_enabled, module, 	\
+				   __func__, "enter", "exit",	\
+				   nullptr)
 
 #endif /* COMMON_COMMON_DEBUG_H */