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Re: Using C++ in GCC is OK
On Mon, May 31, 2010 at 11:54 AM, Jakub Jelinek <jakub@redhat.com> wrote:
> On Mon, May 31, 2010 at 09:44:08AM -0700, Mark Mitchell wrote:
>> >> I just really hope we will have strict criteria that any transition will
>> >> not make compiler slower and will not increase compiler build time.
>> >
>> > Nor will grow the memory footprint, at least of the important data
>> > structures, or increase maintanance costs by making the code less readable,
>> > etc.
>>
>> There is no reason that use of C++ should increase the memory footprint
>> of the compiler, make the compiler slower, or make the code less
>> readable. ?Poor use of C++ might lead to those things; good use will
>> not. ?That is why we need coding standards and patch review.
>
> E.g. when you start using virtual methods, suddenly you need a vtable
> pointer in the object and thus the object grew by 8 bytes.
aren't we already doing this with the various hooks we have?
We do not need to generate RTTI if all we are interested in is vcalls.
> In many cases it would be really addition of that pointer, dropping an 8/16
> bit code would slow things down too much (using virtual method to get you
> say enum tree_code from a tree would be way too slow, similarly for rtti).
Currently, we go through lot of manual checking with globals tables where
the vcall mechanism already ensures that.
I would suggest that when we are worrying about this kind of this, we also
look at the alternatives we currently have and how they compare.
>
> Similarly if the compiler massively starts using virtual methods everywhere,
> there will be slow downs caused by the increased number of harder to predict
> indirect calls.
that is why reviewers will use their best judgements, in particular
to decide where
a virtual function is preferable to a huge switch, current practice,
which will generate more data than a vtable.
>
> ? ? ? ?Jakub
>