Re: 'src/abg-dwarf-reader.cc:compare_dies_string_attribute_value' optimization

[Dodji Seketeli <dodji@seketeli.org>]

Hello Thomas and Giuliano,

[...]


Thomas Schwinge <thomas@codesourcery.com> a écrit:

[...]

>> OK.  Then:
>>
>>     (gdb) print l_attr.valp && r_attr.valp
>>     $8 = true
>>     (gdb) print *l_attr.valp == *r_attr.valp
>>     $9 = true
>>
>> This now does 'return true;'!
>>
>> Given 'unsigned char *valp', what this only checks is that one byte at
>> 'l_attr.valp' equals one byte at 'r_attr.valp' -- which evidently isn't
>> sufficient to demonstrate that 'l_attr' and 'r_attr' do match.  Therefore
>> it seems that this optimization is not correct?
>>

Giuliano Procida <gprocida@google.com> a écrit:

> Looks like you've found one bug. It has compared addresses and moved on to
> test data.
>
> It could do strcmp at this point, assuming these things are null-terminated
> strings, which is the case for DW_FORM_strp but is not for other things.
>
> But there is another bug. I've instrumented the code and found cases (in
> the test suite) where the fast path returns false, but the slow path
> returns true. Both sides are DW_FORM_strx1. The data are not strings but
> indexes and that the same string appears at different indexes.

I agree.  This is a bug.

I tried to fix and indeed, hell did break lose as I realized there were
several other issues across the pipeline (including several issues that
I have long wanted to address) that needed so that this one fits in
correctly.

I came up with a patch set that lies in the branch 'fix-dwarf-str-cmp',
at
https://sourceware.org/git/?p=libabigail.git;a=shortlog;h=refs/heads/fix-dwarf-str-cmp.

The tip of that patch set addresses this issue specifically:
https://sourceware.org/git/?p=libabigail.git;a=commit;h=3d277a9cc05873cf4aeb97273d585e0b07af917d.

But for that fix to be applied, we need the whole patch set that fixes
several other issues left and right.

Giuliano & Thomas, could you please test that branch on your specific
environments?  If it works OK, then I can consider posting the patch-set
properly to the list.  If it doesn't, at least I'd wouldn't have broken
the world without notice ;-)

Thanks.

-- 
		Dodji