Re: Switching to C++ by default in 4.8

[Xinliang David Li <davidxl@google.com>]

On Tue, Apr 10, 2012 at 1:46 AM, Jakub Jelinek <jakub@redhat.com> wrote:
> On Mon, Apr 09, 2012 at 04:34:32PM -0700, Xinliang David Li wrote:
>> Class hierarchy is one such feature that is useful. Assuming we have
>> two hierarchies for gcc: one for values rooted at ValExp, and one for
>> gimple stmts rooted at GimpInst.
>>
>> 1) For IR browsing,
>>    *) all the macro accessors can be eliminated -- a big plus for debugging;
>
> Not that clear, if all the macros are replaced by tons of inline functions,
> the debugging experience can be actually significantly worse.  Already some the
> inline functions used like tree_operand_length used by TREE_OPERAND_LENGTH
> macro are extremely annoying from debugging POV.

What is the root cause of the annoyance? Mixing macros and inline
functions does not sound good, but using deeply nested macros do not
seem to help the debugging situation either.

>
>>    *) gcc implementation has lots of hard coded TREE_OPERAND (exp, nn)
>>
>>      e.g.
>>             exp->as_component_ref().get_field() ..
>>             exp->as_mem_access().get_base() ...
>>             exp->as_mem_acesss().get_address() --> produces the
>> address expression for memory access
>>             exp->as_mem_access().get_alias_handle ()
>>
>>             gimple_inst->serialize (&fixup_list) --> a virtual
>> function overriden by actual instruction types that knows its byte
>> code format.
>
> That silently assumes we want to change basic GIMPLE/tree data structures
> to virtual classes, which is a significant change that has a significant
> cost as well.  E.g. all such changed data structures grow by a virtual
> pointer field.  Those data structures are heavily optimized for memory
> footprint.

Not really -- not having flat interfaces as we have today does not
mean virtual functions have to be used -- this is just implementation
detail.

> Not to mention it is very questionable if the above stuff is more readable
> than what we currently have.

The above is just quickly cooked up examples. A carefully designed C++
based API can be self documenting and make the client code very
readable. It is hard to believe that there is no room for improvement
in GCC.

thanks,

David

>
>        Jakub