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List: cfe-dev
Subject: Re: [cfe-dev] Adding support for multiple non-virtual inheritance for -cxx-abi microsoft
From: Richard Smith <richard () metafoo ! co ! uk>
Date: 2013-04-09 0:47:15
Message-ID: CAOfiQqnBrWnZOH9VVE1W1irMpn+f-8JxhdgpRosk_yBwxv62uA () mail ! gmail ! com
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On Mon, Apr 8, 2013 at 5:27 PM, John McCall <rjmccall@apple.com> wrote:
> On Apr 8, 2013, at 5:24 PM, Richard Smith <richard@metafoo.co.uk> wrote:
>
> On Mon, Apr 8, 2013 at 11:33 AM, John McCall <rjmccall@apple.com> wrote:
>
>> On Apr 8, 2013, at 11:08 AM, Reid Kleckner <rnk@google.com> wrote:
>> > On Mon, Apr 8, 2013 at 2:02 PM, Timur Iskhodzhanov <timurrrr@google.com>
>> wrote:
>> >> 2013/4/8 John McCall <rjmccall@apple.com>:
>> >>> I also find it curious that MSVC uses a thunk for member pointers,
>> since
>> >>> the required this-adjustment is already plainly expressible in the
>> member
>> >>> pointer value.
>> >> Me too actually.
>> >> Reid, wdyt?
>> >
>> > I think it allows them to avoid the union between non-virtual methods
>> > and virtual methods. Seems a bit cleaner and more obvious to me, but
>> > it has tradeoffs in terms of code size at the call site and the number
>> > of conditional vs. indirect branches that you have to do:
>> > indirect to thunk and indirect through vtable, vs conditional between
>> > two indirect calls
>>
>> Oh, does MSVC not do the union thing? They always make a thunk to do
>> the virtual call?
>>
>> If so, this "thunk" is potentially quite a bit more than just a thunk —
>> it may
>> actually have ABI pointer-equality requirements on it for e.g. member
>> pointer equality tests.
>>
>
> I don't believe there's any need for that -- comparisons on pointers to
> virtual member functions produce unspecified results (see [expr.eq]p2,
> second-last sentence).
>
>
> Oh, interesting. I guess Itanium made it work because it fell out easily
> enough to do so.
>
Even on Itanium, it (arguably) doesn't always work:
struct A { virtual void f() = 0; };
struct B { virtual void f() = 0; };
struct C : A, B { virtual void f(); };
void (C::*af)() = &A::f;
void (C::*bf)() = &B::f;
int main() { return af == bf; } // returns 0
[Attachment #5 (text/html)]
<div dir="ltr">On Mon, Apr 8, 2013 at 5:27 PM, John McCall <span dir="ltr"><<a \
href="mailto:rjmccall@apple.com" target="_blank">rjmccall@apple.com</a>></span> \
wrote:<br><div class="gmail_extra"><div class="gmail_quote"> <blockquote \
class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div \
style="word-wrap:break-word"><div><div class="h5"><div> <div>On Apr 8, 2013, at 5:24 \
PM, Richard Smith <<a href="mailto:richard@metafoo.co.uk" \
target="_blank">richard@metafoo.co.uk</a>> wrote:</div><blockquote \
type="cite"><div dir="ltr">On Mon, Apr 8, 2013 at 11:33 AM, John McCall <span \
dir="ltr"><<a href="mailto:rjmccall@apple.com" \
target="_blank">rjmccall@apple.com</a>></span> wrote:<br> <div \
class="gmail_extra"><div class="gmail_quote"> <blockquote class="gmail_quote" \
style="margin:0px 0px 0px \
0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div>On \
Apr 8, 2013, at 11:08 AM, Reid Kleckner <<a href="mailto:rnk@google.com" \
target="_blank">rnk@google.com</a>> wrote:<br>
> On Mon, Apr 8, 2013 at 2:02 PM, Timur Iskhodzhanov <<a \
href="mailto:timurrrr@google.com" target="_blank">timurrrr@google.com</a>> \
wrote:<br> >> 2013/4/8 John McCall <<a href="mailto:rjmccall@apple.com" \
target="_blank">rjmccall@apple.com</a>>:<br> >>> I also find it curious \
that MSVC uses a thunk for member pointers, since<br> >>> the required \
this-adjustment is already plainly expressible in the member<br> >>> pointer \
value.<br> >> Me too actually.<br>
>> Reid, wdyt?<br>
><br>
> I think it allows them to avoid the union between non-virtual methods<br>
> and virtual methods. Seems a bit cleaner and more obvious to me, but<br>
> it has tradeoffs in terms of code size at the call site and the number<br>
> of conditional vs. indirect branches that you have to do:<br>
> indirect to thunk and indirect through vtable, vs conditional between<br>
> two indirect calls<br>
<br>
</div>Oh, does MSVC not do the union thing? They always make a thunk to do<br>
the virtual call?<br>
<br>
If so, this "thunk" is potentially quite a bit more than just a thunk — it \
may<br> actually have ABI pointer-equality requirements on it for e.g. member<br>
pointer equality tests.<br></blockquote><div> </div><div>I don't believe \
there's any need for that -- comparisons on pointers to virtual member functions \
produce unspecified results (see [expr.eq]p2, second-last sentence).</div>
</div></div></div>
</blockquote></div><br></div></div><div>Oh, interesting. I guess Itanium made it \
work because it fell out easily enough to do \
so.</div></div></blockquote><div><br></div><div style>Even on Itanium, it (arguably) \
doesn't always work:</div> <div style><div><br></div><div>struct A { virtual void \
f() = 0; };</div><div>struct B { virtual void f() = 0; };</div><div>struct C : A, B { \
virtual void f(); };</div><div>void (C::*af)() = &A::f;</div><div>void (C::*bf)() \
= &B::f;</div> <div style>int main() { return af == bf; } // returns \
0</div></div></div></div></div>
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