Re: [PATCH 1/3] gdb: store internalvars in an std::vector

[Lancelot SIX <lsix@lancelotsix.com>]

On Mon, Feb 13, 2023 at 11:21:37PM -0500, Simon Marchi via Gdb-patches wrote:
> Change the storage of internalvars to an std::vector of unique pointers
> to internalval.  This helps automate memory management, and will help
> keep internalvars sorted in a subsequent patch.
> 
> I initially tried to use an std::vector<internalval> initially, but some
> parts of the code need for the addresses of internalvars to be stable.
> 

Hi Simon,

As the end-goal is to have an order, did you consider using a container
which enforces it like a std::map?  For small number of objects (for
some definition of small…) vector is usually more efficient but as here
you hold pointers, I am not sure the vector brings a huge benefit.

You could use a std::map<std::string, internalvar> as the internalvar’s
address will remain stable.  No need to have a unique_ptr.

One downside of the std::map is that you might end up with the name
stored twice (once as key, and maybe still once in the internalvar
object).  std::set can also be used, but with other downsides.

The main difference with vector can come if we had 2 internal variables
with the same name.  The current code does not prevent this AFAICT, but
as one of the vars would effectively shadow the other, I am not sure
this is a case we want to support anyway.

Best,
Lancelot.

> Change-Id: I1fca7e7877cc984a3a3432c7639d45e68d437241
> ---
>  gdb/value.c | 32 +++++++++++++-------------------
>  1 file changed, 13 insertions(+), 19 deletions(-)
> 
> diff --git a/gdb/value.c b/gdb/value.c
> index 7873aeb9558e..e884913abe5a 100644
> --- a/gdb/value.c
> +++ b/gdb/value.c
> @@ -1829,7 +1829,6 @@ union internalvar_data
>  
>  struct internalvar
>  {
> -  struct internalvar *next;
>    char *name;
>  
>    /* We support various different kinds of content of an internal variable.
> @@ -1841,7 +1840,9 @@ struct internalvar
>    union internalvar_data u;
>  };
>  
> -static struct internalvar *internalvars;
> +using internalvar_up = std::unique_ptr<internalvar>;
> +
> +static std::vector<internalvar_up> internalvars;
>  
>  /* If the variable does not already exist create it and give it the
>     value given.  If no value is given then the default is zero.  */
> @@ -1891,11 +1892,9 @@ init_if_undefined_command (const char* args, int from_tty)
>  struct internalvar *
>  lookup_only_internalvar (const char *name)
>  {
> -  struct internalvar *var;
> -
> -  for (var = internalvars; var; var = var->next)
> +  for (internalvar_up &var : internalvars)
>      if (strcmp (var->name, name) == 0)
> -      return var;
> +      return var.get ();
>  
>    return NULL;
>  }
> @@ -1906,12 +1905,11 @@ lookup_only_internalvar (const char *name)
>  void
>  complete_internalvar (completion_tracker &tracker, const char *name)
>  {
> -  struct internalvar *var;
>    int len;
>  
>    len = strlen (name);
>  
> -  for (var = internalvars; var; var = var->next)
> +  for (internalvar_up &var : internalvars)
>      if (strncmp (var->name, name, len) == 0)
>        tracker.add_completion (make_unique_xstrdup (var->name));
>  }
> @@ -1922,12 +1920,12 @@ complete_internalvar (completion_tracker &tracker, const char *name)
>  struct internalvar *
>  create_internalvar (const char *name)
>  {
> -  struct internalvar *var = XNEW (struct internalvar);
> +  internalvars.emplace_back (new internalvar);
> +  internalvar *var = internalvars.back ().get ();
>  
>    var->name = xstrdup (name);
>    var->kind = INTERNALVAR_VOID;
> -  var->next = internalvars;
> -  internalvars = var;
> +
>    return var;
>  }
>  
> @@ -2412,8 +2410,6 @@ preserve_one_varobj (struct varobj *varobj, struct objfile *objfile,
>  void
>  preserve_values (struct objfile *objfile)
>  {
> -  struct internalvar *var;
> -
>    /* Create the hash table.  We allocate on the objfile's obstack, since
>       it is soon to be deleted.  */
>    htab_up copied_types = create_copied_types_hash ();
> @@ -2421,8 +2417,8 @@ preserve_values (struct objfile *objfile)
>    for (const value_ref_ptr &item : value_history)
>      item->preserve (objfile, copied_types.get ());
>  
> -  for (var = internalvars; var; var = var->next)
> -    preserve_one_internalvar (var, objfile, copied_types.get ());
> +  for (internalvar_up &var : internalvars)
> +    preserve_one_internalvar (var.get (), objfile, copied_types.get ());
>  
>    /* For the remaining varobj, check that none has type owned by OBJFILE.  */
>    all_root_varobjs ([&copied_types, objfile] (struct varobj *varobj)
> @@ -2438,14 +2434,12 @@ static void
>  show_convenience (const char *ignore, int from_tty)
>  {
>    struct gdbarch *gdbarch = get_current_arch ();
> -  struct internalvar *var;
>    int varseen = 0;
>    struct value_print_options opts;
>  
>    get_user_print_options (&opts);
> -  for (var = internalvars; var; var = var->next)
> +  for (internalvar_up &var : internalvars)
>      {
> -
>        if (!varseen)
>  	{
>  	  varseen = 1;
> @@ -2456,7 +2450,7 @@ show_convenience (const char *ignore, int from_tty)
>  	{
>  	  struct value *val;
>  
> -	  val = value_of_internalvar (gdbarch, var);
> +	  val = value_of_internalvar (gdbarch, var.get ());
>  	  value_print (val, gdb_stdout, &opts);
>  	}
>        catch (const gdb_exception_error &ex)
> -- 
> 2.39.1
>