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Martin J. O'Riordan via llvm-dev

2015-Nov-09 21:34 UTC

[llvm-dev] [cfe-dev] [RFC] attribute((internal_linkage))

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

Evgenii Stepanov via llvm-dev

2015-Nov-09 21:58 UTC

head link

[llvm-dev] [cfe-dev] [RFC] attribute((internal_linkage))

In fact, during the code review we've decided against allowing this
attribute on namespaces: http://reviews.llvm.org/D13925

The intended use of this attribute in libc++ is on functions that
programs are not allowed to take address of, like all methods of
standard library classes. Visibility("hidden") attribute (which we are
replacing with internal_linkage) violates ODR the same way.

Also, this attribute (for us) is not about locality or performance.
Its primary purpose is to reliably exclude a class method from the
export table. This is the way libc++ manages ABI stability: all class
methods that are not part of the library ABI are defined in the
headers with a set of attributes that ensure that they become internal
to the translation unit. Always_inline does that in almost all cases,
but almost is not good enough. Internal_linkage is a straighforward
way to achieve the same.


On Mon, Nov 9, 2015 at 1:34 PM, Martin J. O'Riordan
<martin.oriordan at movidius.com> wrote:> 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
>

Martin J. O'Riordan via llvm-dev

2015-Nov-10 10:54 UTC

head link

[llvm-dev] [cfe-dev] [RFC] attribute((internal_linkage))

Thanks Evgenii,

I hadn't realised that you had decided not to apply the attribute to
namespaces, this certainly eases my mind about the issue.

And thanks also for explaining more clearly the rationale behind its application
to symbols that are not intended to be exported.

	MartinO

-----Original Message-----
From: Evgenii Stepanov [mailto:eugeni.stepanov at gmail.com] 
Sent: 09 November 2015 21:59
To: Martin J. O'Riordan
Cc: Robinson, Paul; Chris Lattner; Steven Stewart-Gallus; llvm-dev
Subject: Re: [llvm-dev] [cfe-dev] [RFC] __attribute__((internal_linkage))

In fact, during the code review we've decided against allowing this
attribute on namespaces: http://reviews.llvm.org/D13925

The intended use of this attribute in libc++ is on functions that programs are
not allowed to take address of, like all methods of standard library classes.
Visibility("hidden") attribute (which we are replacing with
internal_linkage) violates ODR the same way.

Also, this attribute (for us) is not about locality or performance.
Its primary purpose is to reliably exclude a class method from the export table.
This is the way libc++ manages ABI stability: all class methods that are not
part of the library ABI are defined in the headers with a set of attributes that
ensure that they become internal to the translation unit. Always_inline does
that in almost all cases, but almost is not good enough. Internal_linkage is a
straighforward way to achieve the same.


On Mon, Nov 9, 2015 at 1:34 PM, Martin J. O'Riordan <martin.oriordan at
movidius.com> wrote:> 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
>

Akira Hatanaka via llvm-dev

2016-Jul-13 01:11 UTC

head link

[llvm-dev] [cfe-dev] [RFC] attribute((internal_linkage))

Hi Evgenii,

I was wondering what the status is of your work to attach
"internal_linkage" to methods of standard library classes in libc++.
The
following piece of code doesn't link because a symbol (string::empty) is
undefined and it sounds like your work might fix the linkage error I'm
seeing.

$ cat test1.cpp
#include <string>
#include <functional>

int main() {
  using namespace std::placeholders;
  auto b = std::bind(&std::string::empty, _1);
}

On Mon, Nov 9, 2015 at 1:58 PM, Evgenii Stepanov via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> In fact, during the code review we've decided against allowing this
> attribute on namespaces: http://reviews.llvEm.org/D13925
> <http://reviews.llvm.org/D13925>
>
> The intended use of this attribute in libc++ is on functions that
> programs are not allowed to take address of, like all methods of
> standard library classes. Visibility("hidden") attribute (which
we are
> replacing with internal_linkage) violates ODR the same way.
>
> Also, this attribute (for us) is not about locality or performance.
> Its primary purpose is to reliably exclude a class method from the
> export table. This is the way libc++ manages ABI stability: all class
> methods that are not part of the library ABI are defined in the
> headers with a set of attributes that ensure that they become internal
> to the translation unit. Always_inline does that in almost all cases,
> but almost is not good enough. Internal_linkage is a straighforward
> way to achieve the same.
>
>
> On Mon, Nov 9, 2015 at 1:34 PM, Martin J. O'Riordan
> <martin.oriordan at movidius.com> wrote:
> > 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
> >
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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