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Re: EGCS: pointer to member functions.
- To: Alexandre Oliva <oliva at dcc dot unicamp dot br>
- Subject: Re: EGCS: pointer to member functions.
- From: Jason Merrill <jason at cygnus dot com>
- Date: 23 Jun 1999 00:46:27 -0700
- Cc: mrs at wrs dot com (Mike Stump), brendan at dgs dot monash dot edu dot au, egcs at egcs dot cygnus dot com, martin at mira dot isdn dot cs dot tu-berlin dot de, egcs-patches at egcs dot cygnus dot com
- References: <199906171530.IAA14508@kankakee.wrs.com> <oriu8h1gr1.fsf@saci.lsd.dcc.unicamp.br> <u9pv2or9pf.fsf@yorick.cygnus.com> <or909b4f6h.fsf@saci.lsd.dcc.unicamp.br>
>>>>> Alexandre Oliva <oliva@dcc.unicamp.br> writes:
> On Jun 22, 1999, Jason Merrill <jason@cygnus.com> wrote:
>> I'm really don't think this change is a good idea; going through a thunk
>> costs more than a simple test. With branch prediction, the cost of the
>> test is about zero, while the thunk adds an extra jump.
> But the thunk would only be used for
> pointer-to-virtual-member-function. With the current code, users are
> paying the price of an additional test and branch even when they only
> use pointer-to-non-virtual-member-function.
If the class has virtual functions, yes.
> Moreover, even if the test cost is close to zero, it would still have
> to branch, and, given that the `thunk' I have in mind is as simple as
> a `trampoline', I don't see how it could be less efficient than the
> current approach. Only if branches with good prediction were much
> cheaper than a 100% certain jump; is this the case?
So I'm told. I must admit that I don't really understand the situation
myself.
Jason