llvm dev - Nov 2015 - [cfe-dev] [RFC] __attribute__((internal_linkage))

> -----Original Message-----
> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of
Chris
> Lattner via llvm-dev
> Sent: Saturday, October 31, 2015 2:58 PM
> To: sstewartgallus00 at mylangara.bc.ca
> Cc: LLVM Developers
> Subject: Re: [llvm-dev] [cfe-dev] [RFC] __attribute__((internal_linkage))
> 
> Hi Stewart,
> 
> I saw this get brought up at the LLVM devmtg in the context of improving
> the stack size of libc++ frames.
> 
> Have you guys considered a different approach to solving this problem?  In
> the case of libc++, the _LIBCPP_INLINE_VISIBILITY macro currently expands
> out to "__attribute__ ((__always_inline__))”.  It seems reasonable to
me
> to have it also add the “nodebug” attribute as well.  This should allow
> the allocas generated by inlining lots of frames to be promoted to
> registers (because there is no debug info to pessimize).
Are you suggesting that 'nodebug' should affect code generation?
It most definitely should not...
--paulr
> 
> This would dramatically shrink the stack frames of code using libc++ at -
> O0, and would also make it go a lot faster.
> 
> -Chris
> 
> 
> > On Oct 29, 2015, at 6:35 AM, Martin J. O'Riordan via llvm-dev
<llvm-
> dev at lists.llvm.org> wrote:
> >
> > I haven't been able to figure out from this thread why this
attribute is
> necessary at all.
> >
> > Anonymous or unnamed namespaces were added to C++ for this very
purpose,
> but the ISO C++ Standard does not discuss "linkage" per-se
because it is
> outside the scope of the language specification.  But it does describes it
> in terms of having a hidden name which is "unique" to the
translation
> unit, and under the "as if" rule, internal linkage is a common
convention
> for achieving this.
> >
> > This is just Standardese for dealing with what compiler implementers
> typically handle as "internal linkage" anyway; at the same time,
the use
> of 'static' for this purpose was deprecated.  This is specifically
stated
> in C++98 section 7.3.1.1 and unnamed namespaces are still similarly
> defined in C++17 (Working Paper) although the specific reference to the
> deprecation of 'static' for this purpose is now gone.
> >
> > The closest the Standard gets to talking about linkage is with Linkage
> Specifications, and even here it tries to avoid to avoid treading on the
> definitions things like internal versus external linkage.
> >
> > So I am curious, what does this proposed attribute on namespaces
achieve
> that cannot already be achieved with an anonymous or unnamed namespace?
> >
> > Thanks,
> >
> > 	Martin O'Riordan - Movidius Ltd.
> >
> > -----Original Message-----
> > From: cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org] On Behalf Of
> Evgenii Stepanov via cfe-dev
> > Sent: 23 October 2015 22:41
> > To: Steven Stewart-Gallus <sstewartgallus00 at mylangara.bc.ca>
> > Cc: Clang Dev <cfe-dev at lists.llvm.org>
> > Subject: Re: [cfe-dev] [RFC] __attribute__((internal_linkage))
> >
> > Sounds right. The proposed attribute is a rough equivalent of
> >
> > static void foo::do_method(void)
> > {
> >  // implementation here
> > }
> >
> > where "static" does not mean a static class member, but
makes the
> declaration local to the translation unit. C-style "static".
> >
> > The patch in http://reviews.llvm.org/D13925 (waiting for review, btw!)
> supports this attribute on classes and namespaces, with this syntax:
> >
> > namespace __attribute__((internal_linkage)) { }
> >
> >
> > On Fri, Oct 23, 2015 at 9:03 AM, Steven Stewart-Gallus via cfe-dev
<cfe-
> dev at lists.llvm.org> wrote:
> >> Hello,
> >>
> >> Can I ask for some clarification?
> >>
> >> Apparently, C++ doesn't allow to static class methods?
> >>
> >> While in C one might write inside a header file:
> >>
> >> static inline void mylib_foo_do_method(struct mylib_foo *foo) {
> >>   // implementation here
> >> }
> >>
> >> In C++ the typical style would be to write something like:
> >>
> >> namespace mylib {
> >>
> >> void foo::do_method(void)
> >> {
> >>  // implementation here
> >> }
> >> }
> >>
> >> Unfortunately, the C++ case then implies some linkage behaviour
that
> >> some might not want.
> >>
> >> Apparently, one can't do things like:
> >>
> >> namespace mylib {
> >>
> >> static void foo::do_method(void)
> >> {
> >>  // implementation here
> >> }
> >> }
> >>
> >> Or,
> >>
> >> namespace {
> >> namespace mylib {
> >>
> >> static void foo::do_method(void)
> >> {
> >>  // implementation here
> >> }
> >> }
> >> }
> >>
> >> Also, if you wanted to an attribute to a whole namespace you
should do
> >> something like the following I think:
> >>
> >> namespace mylib {
> >> [[clang::internal_linkage]];
> >>
> >> static void foo::do_method(void)
> >> {
> >>  // implementation here
> >> }
> >> }
> >>
> >>
> >> Thank you,
> >> Steven Stewart-Gallus
> >> _______________________________________________
> >> cfe-dev mailing list
> >> cfe-dev at lists.llvm.org
> >> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> > _______________________________________________
> > cfe-dev mailing list
> > cfe-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

Sorry, I totally missed updates to this thread.

Anonymous namespace does not work for libc++ because we need to hide
some class members, and export the rest. Namespace would hide all of
them. AFAIK, "static" is un-deprecated in C++11 for multiple reasons,
including this one.

I agree that nodebug should not affect codegen. It would also kill
debug info (ex. all debug locations in libc++ headers).


On Mon, Nov 2, 2015 at 11:57 AM, Robinson, Paul via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
>
>> -----Original Message-----
>> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of
Chris
>> Lattner via llvm-dev
>> Sent: Saturday, October 31, 2015 2:58 PM
>> To: sstewartgallus00 at mylangara.bc.ca
>> Cc: LLVM Developers
>> Subject: Re: [llvm-dev] [cfe-dev] [RFC]
__attribute__((internal_linkage))
>>
>> Hi Stewart,
>>
>> I saw this get brought up at the LLVM devmtg in the context of
improving
>> the stack size of libc++ frames.
>>
>> Have you guys considered a different approach to solving this problem? 
In
>> the case of libc++, the _LIBCPP_INLINE_VISIBILITY macro currently
expands
>> out to "__attribute__ ((__always_inline__))”.  It seems reasonable
to me
>> to have it also add the “nodebug” attribute as well.  This should allow
>> the allocas generated by inlining lots of frames to be promoted to
>> registers (because there is no debug info to pessimize).
>
> Are you suggesting that 'nodebug' should affect code generation?
> It most definitely should not...
> --paulr
>
>>
>> This would dramatically shrink the stack frames of code using libc++ at
-
>> O0, and would also make it go a lot faster.
>>
>> -Chris
>>
>>
>> > On Oct 29, 2015, at 6:35 AM, Martin J. O'Riordan via llvm-dev
<llvm-
>> dev at lists.llvm.org> wrote:
>> >
>> > I haven't been able to figure out from this thread why this
attribute is
>> necessary at all.
>> >
>> > Anonymous or unnamed namespaces were added to C++ for this very
purpose,
>> but the ISO C++ Standard does not discuss "linkage" per-se
because it is
>> outside the scope of the language specification.  But it does describes
it
>> in terms of having a hidden name which is "unique" to the
translation
>> unit, and under the "as if" rule, internal linkage is a
common convention
>> for achieving this.
>> >
>> > This is just Standardese for dealing with what compiler
implementers
>> typically handle as "internal linkage" anyway; at the same
time, the use
>> of 'static' for this purpose was deprecated.  This is
specifically stated
>> in C++98 section 7.3.1.1 and unnamed namespaces are still similarly
>> defined in C++17 (Working Paper) although the specific reference to the
>> deprecation of 'static' for this purpose is now gone.
>> >
>> > The closest the Standard gets to talking about linkage is with
Linkage
>> Specifications, and even here it tries to avoid to avoid treading on
the
>> definitions things like internal versus external linkage.
>> >
>> > So I am curious, what does this proposed attribute on namespaces
achieve
>> that cannot already be achieved with an anonymous or unnamed namespace?
>> >
>> > Thanks,
>> >
>> >     Martin O'Riordan - Movidius Ltd.
>> >
>> > -----Original Message-----
>> > From: cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org] On Behalf
Of
>> Evgenii Stepanov via cfe-dev
>> > Sent: 23 October 2015 22:41
>> > To: Steven Stewart-Gallus <sstewartgallus00 at
mylangara.bc.ca>
>> > Cc: Clang Dev <cfe-dev at lists.llvm.org>
>> > Subject: Re: [cfe-dev] [RFC] __attribute__((internal_linkage))
>> >
>> > Sounds right. The proposed attribute is a rough equivalent of
>> >
>> > static void foo::do_method(void)
>> > {
>> >  // implementation here
>> > }
>> >
>> > where "static" does not mean a static class member, but
makes the
>> declaration local to the translation unit. C-style "static".
>> >
>> > The patch in http://reviews.llvm.org/D13925 (waiting for review,
btw!)
>> supports this attribute on classes and namespaces, with this syntax:
>> >
>> > namespace __attribute__((internal_linkage)) { }
>> >
>> >
>> > On Fri, Oct 23, 2015 at 9:03 AM, Steven Stewart-Gallus via cfe-dev
<cfe-
>> dev at lists.llvm.org> wrote:
>> >> Hello,
>> >>
>> >> Can I ask for some clarification?
>> >>
>> >> Apparently, C++ doesn't allow to static class methods?
>> >>
>> >> While in C one might write inside a header file:
>> >>
>> >> static inline void mylib_foo_do_method(struct mylib_foo *foo)
{
>> >>   // implementation here
>> >> }
>> >>
>> >> In C++ the typical style would be to write something like:
>> >>
>> >> namespace mylib {
>> >>
>> >> void foo::do_method(void)
>> >> {
>> >>  // implementation here
>> >> }
>> >> }
>> >>
>> >> Unfortunately, the C++ case then implies some linkage
behaviour that
>> >> some might not want.
>> >>
>> >> Apparently, one can't do things like:
>> >>
>> >> namespace mylib {
>> >>
>> >> static void foo::do_method(void)
>> >> {
>> >>  // implementation here
>> >> }
>> >> }
>> >>
>> >> Or,
>> >>
>> >> namespace {
>> >> namespace mylib {
>> >>
>> >> static void foo::do_method(void)
>> >> {
>> >>  // implementation here
>> >> }
>> >> }
>> >> }
>> >>
>> >> Also, if you wanted to an attribute to a whole namespace you
should do
>> >> something like the following I think:
>> >>
>> >> namespace mylib {
>> >> [[clang::internal_linkage]];
>> >>
>> >> static void foo::do_method(void)
>> >> {
>> >>  // implementation here
>> >> }
>> >> }
>> >>
>> >>
>> >> Thank you,
>> >> Steven Stewart-Gallus
>> >> _______________________________________________
>> >> cfe-dev mailing list
>> >> cfe-dev at lists.llvm.org
>> >> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>> > _______________________________________________
>> > cfe-dev mailing list
>> > cfe-dev at lists.llvm.org
>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>> >
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > llvm-dev at lists.llvm.org
>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

With respect only to '__attribute__((internal_linkage))', not
'nodebug' and other parts of this topic; does hiding "some"
members and not others not introduce a violation of the ODR because some members
of the class as it appears in one translation unit are not the same actual
definitions as the apparently "same" members of the class in another
translation unit?  In particular the ODR requirement that would ensure that the
address of a method was the same regardless of where the address was taken,
including 'static' methods (no 'this' pointer)?  Or hidden
things like the guard variables for local static objects, default argument
initialisers (which are instanced), etc.?

I must admit that having spent years trying to lock down the definition of the
ODR, that this seems to fly in the opposite direction of the intent of that
rule.

I can (sort of) see why for particular reasons of locality you might want
multiple actual definitions, even though they might exhibit the same behaviour,
but maybe the answer for locality optimisation would be for the linker
technology to evolve so that multiple copies of the same definition (sic) could
be made; perhaps even at the Basic Block level [Block Level Linking].  But this
too would lie under the auspices of the "as if" rule and respect the
ODR.

There is something I am missing in this that is not making me at all comfortable
about the use of this attribute for namespaces (apart from the reopening issues
raised by others), and in particular the application to parts of a class but not
the whole class.  I just feel that this is leading to semantic trouble.

Thanks,

	MartinO

-----Original Message-----
From: Evgenii Stepanov [mailto:eugeni.stepanov at gmail.com] 
Sent: 09 November 2015 18:49
To: Robinson, Paul <Paul_Robinson at playstation.sony.com>
Cc: Chris Lattner <clattner at apple.com>; sstewartgallus00 at
mylangara.bc.ca; llvm-dev at lists.llvm.org; Martin J. O'Riordan
<martin.oriordan at movidius.com>
Subject: Re: [llvm-dev] [cfe-dev] [RFC] __attribute__((internal_linkage))

Sorry, I totally missed updates to this thread.

Anonymous namespace does not work for libc++ because we need to hide some class
members, and export the rest. Namespace would hide all of them. AFAIK,
"static" is un-deprecated in C++11 for multiple reasons, including
this one.

I agree that nodebug should not affect codegen. It would also kill debug info
(ex. all debug locations in libc++ headers).


On Mon, Nov 2, 2015 at 11:57 AM, Robinson, Paul via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
>
>
>> -----Original Message-----
>> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of
>> Chris Lattner via llvm-dev
>> Sent: Saturday, October 31, 2015 2:58 PM
>> To: sstewartgallus00 at mylangara.bc.ca
>> Cc: LLVM Developers
>> Subject: Re: [llvm-dev] [cfe-dev] [RFC] 
>> __attribute__((internal_linkage))
>>
>> Hi Stewart,
>>
>> I saw this get brought up at the LLVM devmtg in the context of 
>> improving the stack size of libc++ frames.
>>
>> Have you guys considered a different approach to solving this 
>> problem?  In the case of libc++, the _LIBCPP_INLINE_VISIBILITY macro 
>> currently expands out to "__attribute__ ((__always_inline__))”. 
It
>> seems reasonable to me to have it also add the “nodebug” attribute as 
>> well.  This should allow the allocas generated by inlining lots of 
>> frames to be promoted to registers (because there is no debug info to
pessimize).
>
> Are you suggesting that 'nodebug' should affect code generation?
> It most definitely should not...
> --paulr
>
>>
>> This would dramatically shrink the stack frames of code using libc++ 
>> at - O0, and would also make it go a lot faster.
>>
>> -Chris
>>
>>
>> > On Oct 29, 2015, at 6:35 AM, Martin J. O'Riordan via llvm-dev 
>> > <llvm-
>> dev at lists.llvm.org> wrote:
>> >
>> > I haven't been able to figure out from this thread why this 
>> > attribute is
>> necessary at all.
>> >
>> > Anonymous or unnamed namespaces were added to C++ for this very 
>> > purpose,
>> but the ISO C++ Standard does not discuss "linkage" per-se
because it
>> is outside the scope of the language specification.  But it does 
>> describes it in terms of having a hidden name which is
"unique" to
>> the translation unit, and under the "as if" rule, internal
linkage is
>> a common convention for achieving this.
>> >
>> > This is just Standardese for dealing with what compiler 
>> > implementers
>> typically handle as "internal linkage" anyway; at the same
time, the
>> use of 'static' for this purpose was deprecated.  This is 
>> specifically stated in C++98 section 7.3.1.1 and unnamed namespaces 
>> are still similarly defined in C++17 (Working Paper) although the 
>> specific reference to the deprecation of 'static' for this
purpose is now gone.
>> >
>> > The closest the Standard gets to talking about linkage is with 
>> > Linkage
>> Specifications, and even here it tries to avoid to avoid treading on 
>> the definitions things like internal versus external linkage.
>> >
>> > So I am curious, what does this proposed attribute on namespaces 
>> > achieve
>> that cannot already be achieved with an anonymous or unnamed namespace?
>> >
>> > Thanks,
>> >
>> >     Martin O'Riordan - Movidius Ltd.
>> >
>> > -----Original Message-----
>> > From: cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org] On Behalf
Of
>> Evgenii Stepanov via cfe-dev
>> > Sent: 23 October 2015 22:41
>> > To: Steven Stewart-Gallus <sstewartgallus00 at
mylangara.bc.ca>
>> > Cc: Clang Dev <cfe-dev at lists.llvm.org>
>> > Subject: Re: [cfe-dev] [RFC] __attribute__((internal_linkage))
>> >
>> > Sounds right. The proposed attribute is a rough equivalent of
>> >
>> > static void foo::do_method(void)
>> > {
>> >  // implementation here
>> > }
>> >
>> > where "static" does not mean a static class member, but
makes the
>> declaration local to the translation unit. C-style "static".
>> >
>> > The patch in http://reviews.llvm.org/D13925 (waiting for review, 
>> > btw!)
>> supports this attribute on classes and namespaces, with this syntax:
>> >
>> > namespace __attribute__((internal_linkage)) { }
>> >
>> >
>> > On Fri, Oct 23, 2015 at 9:03 AM, Steven Stewart-Gallus via cfe-dev
>> > <cfe-
>> dev at lists.llvm.org> wrote:
>> >> Hello,
>> >>
>> >> Can I ask for some clarification?
>> >>
>> >> Apparently, C++ doesn't allow to static class methods?
>> >>
>> >> While in C one might write inside a header file:
>> >>
>> >> static inline void mylib_foo_do_method(struct mylib_foo *foo)
{
>> >>   // implementation here
>> >> }
>> >>
>> >> In C++ the typical style would be to write something like:
>> >>
>> >> namespace mylib {
>> >>
>> >> void foo::do_method(void)
>> >> {
>> >>  // implementation here
>> >> }
>> >> }
>> >>
>> >> Unfortunately, the C++ case then implies some linkage
behaviour
>> >> that some might not want.
>> >>
>> >> Apparently, one can't do things like:
>> >>
>> >> namespace mylib {
>> >>
>> >> static void foo::do_method(void)
>> >> {
>> >>  // implementation here
>> >> }
>> >> }
>> >>
>> >> Or,
>> >>
>> >> namespace {
>> >> namespace mylib {
>> >>
>> >> static void foo::do_method(void)
>> >> {
>> >>  // implementation here
>> >> }
>> >> }
>> >> }
>> >>
>> >> Also, if you wanted to an attribute to a whole namespace you 
>> >> should do something like the following I think:
>> >>
>> >> namespace mylib {
>> >> [[clang::internal_linkage]];
>> >>
>> >> static void foo::do_method(void)
>> >> {
>> >>  // implementation here
>> >> }
>> >> }
>> >>
>> >>
>> >> Thank you,
>> >> Steven Stewart-Gallus
>> >> _______________________________________________
>> >> cfe-dev mailing list
>> >> cfe-dev at lists.llvm.org
>> >> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>> > _______________________________________________
>> > cfe-dev mailing list
>> > cfe-dev at lists.llvm.org
>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>> >
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > llvm-dev at lists.llvm.org
>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev