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

	Jakub


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