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

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

On Friday, November 24 2017, Pedro Alves wrote:

> On 11/24/2017 04:54 AM, Sergio Durigan Junior wrote:
>> 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.
>
> No, you can't return early the very first thing, because
>  --readnever is supposed to skip _debug_ info, not ELF/minimal symbols...
> So the "return early" would have to be _after_ the
> elf_read_minimal_symbols call:

Hm, OK, it makes sense and I confess I thought "why is Pedro mentioning
elf_symfile_read if the feature is about skipping DWARF, not ELF?".

>  static void
>  elf_symfile_read (struct objfile *objfile, symfile_add_flags symfile_flags)
>  {
>    bfd *abfd = objfile->obfd;
>    struct elfinfo ei;
>
>    memset ((char *) &ei, 0, sizeof (ei));
>    bfd_map_over_sections (abfd, elf_locate_sections, (void *) & ei);
>  
>    elf_read_minimal_symbols (objfile, symfile_flags, &ei);
>
> I don't know whether we can reorder that.  Maybe we can.
>
> When I looked at this quickly yesterday, I saw that elf_location_sections
> is what finds the stabs and mdebug sections:
>
> static void
> elf_locate_sections (bfd *ignore_abfd, asection *sectp, void *eip)
> {
>   struct elfinfo *ei;
>
>   ei = (struct elfinfo *) eip;
>   if (strcmp (sectp->name, ".stab") == 0)
>     {
>       ei->stabsect = sectp;
>     }
>   else if (strcmp (sectp->name, ".mdebug") == 0)
>     {
>       ei->mdebugsect = sectp;
>     }
> }
>
> and it seemed to be that skipping the section location would make
> the parts of  elf_symfile_read that actually read the symbols
> be no-ops, because the stabsect/mdebusect pointers would be NULL.
>
> But if returning early or something else works, that's fine.

OK, thanks for clarifying.

>> As for the *_build_psymtabs functions, I am doing exactly that: if
>> objfile->flags contains OBJF_READNEVER, then just return and do nothing.
>
> Sure, that should work too.  It's just the difference between
> skipping checking whether debug info is available (skipping before
> calling into those), vs letting gdb do the work to figure out whether
> debug info is available, but then ignore it.
> The grep for "*_build_psymtabs" was intended as a pointer to find
> what the relevant code is, including to look at the code that
> is calling those functions, see if there's something to be done there.

I think it makes more sense, logically speaking, to not mess with
elf_symfile_read and instead modify the *_build_psymtabs functions.

Thanks,

-- 
Sergio
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