the global symbol table

[David Carlton <carlton at math dot stanford dot edu>]

I'm trying to get a handle on the global symbol table, and on what
would be a good way to reorganize its data in order to make
lookup_symbol (and related functions, e.g. search_symbol) faster and
to provide some sort of data structure that could also be used in
namespace searches.

I'm not sure that I have any really coherent questions, so I'm going
to free-associate for a while, and see what thoughts that jars loose
from other people's brains.  I've listed the main issues that I'm
worried about below.

As a reminder, here's what currently happens if you call lookup_symbol
and it doesn't find a local match somewhere:

* It looks up the global blocks in all the symtabs, and looks for them
  there.  If it finds it, it calls fixup_symbol_section (to get the
  bfd_section and section set correctly; this happens in the other
  cases below, too) and returns it.
* Then, #ifndef HPUXHPPA, it sometimes looks in the minimal symbol
  table.  If it finds something, it looks for it in the global and
  static blocks of the corresponding symtab.  (But why wouldn't it
  have already found the global match?  Hmm.)  Otherwise, it sometimes
  does some sort of mangled name lookup.
* Next, it looks in all the psymtabs.  If it finds a match, it
  converts the psymtab to a symtab, and returns the corresponding
  symbol there.
* Next, it looks in the static symbols for all of the symtabs.  (To
  what extent does this overlap with the lookup_minimal_symbol stuff
  earlier?)
* Then, it looks in the static symbols for all the psymtabs.
* And finally, #ifdef HPUXHPPA, it does the minimal symbol stuff that
  it otherwise did earlier.

This raises some issues:

* Do we _really_ want to search static symbols?  My guess is that the
  answer is 'yes', but it's not entirely clear to me that that's the
  case.  Certainly whenever we do a search that returns results that
  aren't strictly in line with what the language says, I get nervous.

* Even the simplest case of searching all the global blocks isn't
  implemented very well: surely we need some sort of fast lookup
  structure that won't require us to look at every single symtab.
  Assuming that we have some sort of expandable data structure in
  which lookups are fast, then exactly how should this work?  Should
  we keep replace the current global blocks by one big global table,
  should we have a global table that duplicates the information in the
  global blocks (so struct symbols corresponding to global symbols
  would be stored in two separate places), or should we have a global
  table that quickly maps names to symtabs, but have the actual
  symbols only stored in the global blocks of symtabs?  I'm currently
  leaning towards the second solution, with the possible longer-term
  goal of migrating towards the first solution, but it's not clear to
  me exactly what the consequences of these choices are.

* Which of the structs symbol, partial_symbol, and minimal_symbol can
  be unified?  I tend to think that partial_symbols should go away, to
  be replaced by special 'incomplete' sorts of struct symbols combined
  with code in lookup_symbol (or wherever) that says that, if you run
  into one of those kinds of incomplete symbols, read in the entire
  symbol table for that file and flesh out all of its partial symbols.
  (This could open a door to a general notion of incomplete symbols
  that could, depending on the debugging format, be completed in a
  more efficient manner than the current psymtab->symtab mechanisms.)
  I was hoping that minimal_symbols could also be turned into special
  kinds of symbols, but now I'm more dubious about that; if they have
  to remain separate, I should presumably tweak dictionaries to let
  them index any sort of struct general_symbol_info, and change the
  minimal symbol table in ways that correspond to the changes to the
  global symbol table.

* So just what's going on with the usage of minimal symbols in
  lookup_symbol?  Are there any situations where looking in the
  minimal symbol is actually helpful?  If so, do those only have to do
  with ickiness related to mangled names, or is there a more serious
  issue that I'm missing?

* One issue that I didn't comment on above is that lookup_symbol has a
  'symtab' argument that stores the symtab in which the symbol is
  found.  If we have to support that, that obviously affects the data
  structures involved.  But it seems to me that we _don't_ have to
  support that: I did a cursory look through GDB's sources and, as far
  as I can tell, that argument is only used by linespec.c's
  decode_line_1.  So it seems to me that we should remove that
  argument from lookup_symbol and provide some sort of alternate
  functionality to satisfy decode_line_1's needs.

David Carlton
carlton@math.stanford.edu