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Re: Switching to C++ by default in 4.8

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.

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


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