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Re: Using C++ in GCC is OK


On Wed, 2010-06-02 at 19:50 +0200, Jakub Jelinek wrote:
> On Wed, Jun 02, 2010 at 06:17:25PM +0100, Richard Earnshaw wrote:
> > 
> > On Mon, 2010-05-31 at 10:02 -0700, Mark Mitchell wrote:
> > > I think virtual functions are on the edge; quite useful, but do result
> > > in the compiler adding a pointer to data objects and in uninlinable
> > > indirect calls at run-time.  Therefore, I would avoid them in the
> > > initial subset of C++ used in GCC.
> > 
> > We do, of course, have one very big 'virtual function' table in gcc --
> > namely the target hooks.  It would be a shame if that couldn't be made
> > into a proper class with virtual functions by some arbitrary rule --
> > it's a perfect example of when they should be considered.
> 
> And the advantage would be?
> 
> Currently targetm is a struct with function pointers, so calling
> targetm.hook_xyz ();
> means reading a pointer from &targetm + off, then calling it.
> If you make it a class with virtual functions and targetm
> would be an object of that class, then
> targetm.hook_xyz ();
> call means reading pointer from &targetm (the vtable pointer), followed
> by reading the function pointer from the virtual table, then calling it.
> One extra memory read for hook call.

A missing virtual hook would be a build failure, rather than a runtime
error.  So the advantage is easier maintenance.

R.


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