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

Mon Nov 2 11:57:20 PST 2015


> -----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
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> >
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