Re: [RFC] Proposal for a new DWARF name index section

[Jan Kratochvil <jan.kratochvil@redhat.com>]

On Mon, 10 Aug 2009 19:36:14 +0200, Tom Tromey wrote:
> but the reason to include this information in the index has to do with
> setting breakpoints, not with expression evaluation.
> 
> I don't think breakpoint setting should necessarily follow language
> rules.

OK, thanks for the clarification, forgot etc.

Still when thinking about it:
* I do not find the symbols reading much slow myself (working _on_ small GDB).
* People complaining it is slow usually use IDEs which use rather file:line
  based breakpoints, don't they?  (As it was discussed on RH IRC today.)
  = Assuming the C++ people do not put breakpoints on static out-of-scope
    functions by name.

For the latter case I agree a fix is needed but an index of static names will
not help with it.


> It is not uncommon for a program to have a uniquely-named static
> function.  It seems friendly to users to let them type "break func" in
> any context.

(One needs to think about same-name functions both static and global in
different files but sure it is unrelated to the new index.)


> Anyway, that is my logic.  Which part of this do you disagree with?
> Or, am I missing something else?

We have concluded the currently missing information is for:
* static functions (are they really needed for the file:line IDE usecases?)
* inlined functions which have no concrete out-of-line instance
  (the same file:line IDE usecase question)

IMO not for:
* static non-function symbols are deprecated (backward GDB compatibility only)


> There does not seem to be a big downside to introducing a new section
> that does exactly what we want.  It is automatically backward
> compatible.  It is (I believe) not difficult to implement.  And,
> finally, we can make it reliable by fiat.

While it is an improvement with existing .debug_pubnames, .debug_pubtypes and
.debug_aranges one can:

* Lookup everything in current CU which can is fully read-in from .debug_info.
* Always lookup global symbols from other CUs through the DWARF indexes.
* Fallback to the full read-in only for:
  * static functions in out of the language (compiler) scope
  * inlined functions which have no concrete out-of-line instance
  * reference to a non-existing symbol

archer-tromey-delayed-symfile could be probably more improved by properly
following the indexes.  While I did fix a regression I broke a performance by
my patch before, it could be probably patched better:
	[delayed-symfile] [commit] Fix a regression on forgotten delayed read of a type info.
	http://sourceware.org/ml/archer/2009-q1/msg00232.html


As a summary GDB could already give (with proper non-existing patches) in the
common usecases acceptable performance even based just on the existing DWARF
indexes, couldn't it?  I did not think so before this mail thread.



Thanks,
Jan