Re: [PATCH RFC 1/2] Remove "readnow" support

Simon Marchi
2026-02-12T19:43:24Z

On 2/12/26 8:29 AM, Andrew Burgess wrote:
> Eli Zaretskii <eliz@gnu.org> writes:
>
>>> From: Tom Tromey <tom@tromey.com>
>>> Date: Wed, 11 Feb 2026 11:49:09 -0700
>>> Cc: Tom Tromey <tom@tromey.com>
>>>
>>> I think the "readnow" feature should be removed.
>>>
>>> "readnow" exists basically to work around potential bugs in any
>>> "partial" reader. This used to work ok because gdb would scan all the
>>> expanded symtabs in addition to using the "quick" functions. This
>>> changed with the "search via psyms" series.
>>>
>>> So now, "readnow" is basically its own separate implementation. As
>>> such, I don't think it carries its weight. It is very slow and uses a
>>> lot of memory, and can have its own bugs requiring extra work on our
>>> part.
>>
>> Is there any other way for the user to force GDB to read all of the
>> symbols from all of the object files? If not, perhaps this
>> functionality still has its value? Maybe we should make it a "maint"
>> command instead?
>
> I second this. We do run into bugs with the indexes from time to time,
> and it's good to have some way to tell GDB to go read the full debug
> information so we can narrow the bug down. Plus it offers a work around
> in these cases.
>
> Is 'maint expand-symtabs' the equivalent? When someone uses -readnow
> should we maybe guide them towards the new way of doing things, even all
> we do is point towards a particular entry in the manual maybe?

From what I understand, "maint expand-symtabs" is not an exact
replacement for readnow. With the recent changes that Tom Tromey did
(always search via the "quick" functions), this is how it works when
searching for a symbol:

- the code invokes the quick_symbol_functions::search method
(cooked_index_functions::search for the DWARF index).
- if a compunit_symtab looks like it could contain said symbol, the
quick functions' search method calls a callback, passing the
compunit_symtab, and the core searches it
- if you "maint expand-symtabs" first, the search still goes through
cooked_index_functions::search. So if there is a bug in
cooked_index_functions::search, it won't bypass it. In other words,
which symtabs are currently expanded should not affect a search (this
was the point of the change, that was a source of bugs)

With readnow, the cooked index is not built. Instead, compunit_symtab
are built from the start, and readnow_functions is used instead of
cooked_index_functions. readnow_functions::search goes over the
all_units vector and searches all of them (I wonder if it could use
expanded_symbols_functions instead...). If there was a bug in
cooked_index_functions::search, then this would by pass it.

I don't know if it's really worth having a completely parallel code path
just in case there is a bug in the main path. That parallel code path
is a source of headaches of its own. That being said, if we could indeed
use expanded_symbols_functions, then keeping readnow wouldn't represent
much code I think.

Simon