Re: [PATCH v2] Add support for the --readnever command-line option (DWARF only)

[Sergio Durigan Junior <sergiodj at redhat dot com>]

On Thursday, November 23 2017, Pedro Alves wrote:

> On 11/23/2017 05:21 PM, Sergio Durigan Junior wrote:
>
>>>> +This option is currently limited to debug information in DWARF format.
>>>> +For all other format, this option has no effect.
>>>
>>> How hard would it be to just make it work?  There's only stabs and mdebug
>>> left, I think?  There should be a single a function somewhere that we can
>>> add an early return.  And then we don't need to document this limitation...
>>>
>>> For example, in elf_symfile_read, we could just skip the elf_locate_sections
>>> call.  In coffread.c we could skip reading stabs right after 
>>>   bfd_map_over_sections (abfd, coff_locate_sections....);
>>>
>>> Looking for:
>>>
>>>  $ grep -h "^[a-z]*_build_psymtabs" gdb/
>>>  coffstab_build_psymtabs (struct objfile *objfile,
>>>  elfstab_build_psymtabs (struct objfile *objfile, asection *stabsect,
>>>  stabsect_build_psymtabs (struct objfile *objfile, char *stab_name,
>>>  mdebug_build_psymtabs (minimal_symbol_reader &reader,
>>>  elfmdebug_build_psymtabs (struct objfile *objfile,
>>>
>>> finds all the relevant places.
>>>
>>> Maybe it wouldn't be that hard to make this be an objfile flag
>>> afterall (like OBJF_READNOW is).  That'd make it possible
>>> to add the location "-readnever" counterpart switch to add-symbol-file
>>> too, BTW:
>
> I meant "logical" instead of "location".  I was staring at
> gdb/location.c at that time.  :-P
>
>>>
>>>  symfile.c:        if (strcmp (arg, "-readnow") == 0)
>>>  symfile.c:        else if (strcmp (arg, "-readnow") == 0)
>> 
>> Hm, I'll look into this.  Just to make it clear: the idea is to have
>> both a --readnever global option and also a OBJF_READNEVER specific to
>> each objfile?
>
> Sure, the idea is to do something similar to what's done for --readnow.

Sorry, but I guess I need a few more details on this.

The way I understand the code at elf_symfile_read, the very first thing
to do would be to check if OBJF_READNEVER is set and return early if it
is.  But it seems that you're proposing something a bit different when
you say that we should "... just skip the elf_locate_sections call."  It
doesn't seem to me that is worth continuing on that function if
OBJF_READNEVER is present.

As for the *_build_psymtabs functions, I am doing exactly that: if
objfile->flags contains OBJF_READNEVER, then just return and do nothing.

The patch is almost ready for resubmission (well, I still need to figure
out how to test the --readnow && --readnever scenario), but I want to
make sure I got this part right.

Thanks,

-- 
Sergio
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