[Bug default/29811] extern array declaration following by its definition reports different sizes in DWARF vs CTF

["guillermo.e.martinez at oracle dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=29811

--- Comment #4 from Guillermo E. Martinez <guillermo.e.martinez at oracle dot com> ---
(In reply to dodji from comment #3)
> "guillermo.e.martinez at oracle dot com via Libabigail"
> <libabigail@sourceware.org> a écrit:
> 
> > Thanks for this great explanation!
> 
> You are welcome!
> 
> [...]
> 
> 
> >> As you can see there, that DIE has no "size" attribute.  That is in line
> >> with the type of is_basic_table, as declared in the C source code, which
> >> is "Array of unknown size".
> >> 
> >
> > So, Is it a limitation of DWARF info?
> 
> I wouldn't say it's a limitation.  Rather, I'd say that it's a feature.
> In that case DWARF keeps the information about the exact type of the
> variable as written in the source code.  That type really is "array of
> unknown size".
> 
> The actual size taken by the variable in memory can be derived from the
> initializer.  Not from the type definition.  So I think that DWARF is
> correct here.
> 
> [...]
> 
> >
> > Right, just that it is limited to _symbol_information not saying much about of
> > type symbol, it is most evident when we use abidiff changing the array size,
> > using CTF and DWARF front-end (e.g I changed the array's elements to two):
> 
> If I understand correctly, you changed the size of the array in the
> initializer, right?
> 

Yes, correct.

> >
> > $ abidiff test-01.o test-02.o
> >
> > Functions changes summary: 0 Removed, 0 Changed, 0 Added function               
> >   Variables changes summary: 0 Removed, 1 Changed, 0 Added variable             
> >
> >   1 Changed variable:                                                           
> >
> >     [C] 'unsigned int is_basic_table[]' was changed at test03.c:1:1:            
> >       size of symbol changed from 4 to 8                                        
> 
> I would guess so.  So both variables have the same type (array of
> uninitialized size), but their initializer have different size.  See,
> that information is kept all along.
> 
> If on the other hand, you say that the type size should be set to 1,
> then we wouldn't know the difference between:
> 
>     unsigned is_basic_table[] = {0};
> 
> and
> 
>     unsigned is_basic_table[1];
> 
> Even if in the end, the ABI ends up being the same, there is a subtle
> difference there that would be lost.  And I think that it's important to
> keep information for users at this point.
> 
> Is it possible to have that information from CTF?  I mean, is it
> possible to have CTF tell us that is_basic_table is an array of unknown
> size?
> 

hmmm sorry for my ignorance, but is_basic_table is an array of unknown size?
As far as I know 'extern' reserved word is telling to compiler that symbol used
in the current translation unit is _declared_ in another one, and is this that
we are interested right?

So, the following declaration produces both CTF and DWARF subrange nodes at
'infinite':

$ echo 'unsigned int is_basic_table[];' | gcc -g -gctf -c -o ~/c/test03.o -x c
-                                                                               
  <stdin>:1:14: warning: array ‘is_basic_table’ assumed to have one element     

$ abidw ~/c/test03.o

<abi-corpus version='2.1' path='/home/byby/c/test03.o'
architecture='elf-amd-x86_64'>
    <elf-variable-symbols>
      <elf-symbol name='is_basic_table' size='4' type='object-type'
binding='global-binding' visibility='default-visibility' is-defined='yes'/>
    </elf-variable-symbols>
    <abi-instr address-size='64' path='&lt;stdin&gt;'
comp-dir-path='/home/byby/oracle/src/libabigail-new' language='LANG_C11'>
      <type-decl name='unsigned int' size-in-bits='32' id='type-id-1'/>
      <array-type-def dimensions='1' type-id='type-id-1'
size-in-bits='infinite' id='type-id-2'>
        <subrange length='infinite' id='type-id-3'/>
      </array-type-def>
      <var-decl name='is_basic_table' type-id='type-id-2'
mangled-name='is_basic_table' visibility='default'
filepath='/home/byby/oracle/src/libabigail-new/&lt;stdin&gt;' line='1'
column='1' elf-symbol-id='is_basic_table'/>
    </abi-instr>
</abi-corpus>

$ abidw --ctf ~/c/test03.o                                                      

<abi-corpus version='2.1' path='/home/byby/c/test03.o'
architecture='elf-amd-x86_64'>
    <elf-variable-symbols>
      <elf-symbol name='is_basic_table' size='4' type='object-type'
binding='global-binding' visibility='default-visibility' is-defined='yes'/>
    </elf-variable-symbols>
    <abi-instr address-size='64' language='LANG_C'>
      <type-decl name='unsigned int' size-in-bits='32' id='type-id-1'/>
      <array-type-def dimensions='1' type-id='type-id-1'
size-in-bits='infinite' id='type-id-2'>
        <subrange length='infinite' type-id='type-id-3' id='type-id-4'/>
      </array-type-def>
      <var-decl name='is_basic_table' type-id='type-id-2'
mangled-name='is_basic_table' visibility='default'
elf-symbol-id='is_basic_table'/>
      <type-decl name='void' id='type-id-3'/>
    </abi-instr>
</abi-corpus>


> >
> > $ abidiff --ctf test-01.o test-02.o
> >
> > Functions changes summary: 0 Removed, 0 Changed, 0 Added function               
> >   Variables changes summary: 0 Removed, 1 Changed, 0 Added variable             
> >
> >   1 Changed variable:                                                           
> >
> >     [C] 'unsigned int is_basic_table[1]' was changed to 'unsigned int
> > is_basic_table[2]':                                                             
> >       size of symbol changed from 4 to 8                                        
> >       type of variable changed:                                                 
> >         type name changed from 'unsigned int[1]' to 'unsigned int[2]'           
> >         array type size changed from 32 to 64                                   
> >         array type subrange 1 changed length from 1 to 2
> 
> Here, it looks like abidiff is reporting about the difference between:
> 
> unsigned is_basic_table[1];
> 
> and
> unsigned is_basic_table[2];
> 
> The information about the fact that we are actually looking at
> 
> unsigned is_basic_table[], initialized with two different initializers
> is lost.
> 
> So, it seems to me that DWARF is more fined grained here.
> 
> It would be nice to have that same level of finesse from CTF, if
> possible.  Is that possible?
> 
> [...]
> 
> 
> >> Do you know what happens if you set the alignment to zero in the CTF
> >> front-end?
> >> 
> >
> > Yes, I changed the alignment value in CTF front-end in commit: 8b832a9edfa,
> > and I tested with latest commit until now: 4cf2ef8f9.
> >
> > $ abidiff abi-ctf.xml abi-dwarf.xml
> >
> > Functions changes summary: 0 Removed, 0 Changed, 0 Added function               
> >   Variables changes summary: 0 Removed, 1 Changed, 0 Added variable             
> >
> >   1 Changed variable:                                                           
> >
> >     [C] 'unsigned int is_basic_table[1]' was changed to 'unsigned int
> > is_basic_table[]' at test03.c:1:1:                                              
> >       type of variable changed:                                                 
> >         type name changed from 'unsigned int[1]' to 'unsigned int[]'            
> >         array type size changed from 32 to infinity                             
> >         array type subrange 1 changed length from 1 to infinity   
> 
> OK, fair enough.
> 
> I think that here, we should teach libabigail's comparison engine to
> take into account the fact that although the array type size is unknown,
> its symbol size is known and hasn't changed.  Thus, this is not an ABI
> change.
> 
> This is on me.

Kind regards,
guillermo

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