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Re: [C++ patch] Set attributes for C++ runtime library calls


> On Thu, Aug 22, 2013 at 10:39 AM, Jan Hubicka <hubicka@ucw.cz> wrote:
> >> >
> >> > So the existing program needs to overwrite libsup++ symbol like we do with malloc?
> >> > Of course with user defineed malloc function we should not propagate the attribute,
> >> > but I think we could have it when we end up calling the runtime.
> >>
> >> I suspect the question is whether our current infrastructure permits
> >> to distinguish
> >> between declaration of 'operator new' we supply, and 'operator new' defined by
> >> user.  The way we currently arrange for user-defined 'operator new' to take
> >> over is that it is something that is done at linktime, (so is LTO
> >> prepared for this?)
> >
> > I think so (I am not expert though :))
> > The operator new we supply is called _Znwm and I want the particular decl with
> > assembler name _Znwm to have malloc attribute.  I get it right, it is the
> > declaration we build by
> >
> >     newtype = cp_build_type_attribute_variant (ptr_ftype_sizetype, newattrs);
> >     newtype = build_exception_variant (newtype, new_eh_spec);
> >     deltype = cp_build_type_attribute_variant (void_ftype_ptr, extvisattr);
> >     deltype = build_exception_variant (deltype, empty_except_spec);
> >
> >     push_cp_library_fn (NEW_EXPR, newtype);
> >     push_cp_library_fn (VEC_NEW_EXPR, newtype);
> >
> > User's new will be different declarations with different assembler name. It is
> > up to user to care about malloc attributes or whatever.
> 
> For functions that are replaceable, what counts is the provenance of the
> *definition*, not just of one its declarations.  The sequence of code that you
> show is mandated by C++.  Quoting C++11 3.7.4/2
> 
>    The library provides default definitions for the global allocation and
>    deallocation functions. Some global allocation and deallocation functions
>    are replaceable (18.6.1). A C++ program shall provide at most one
>    definition of a replaceable allocation or deallocation function. Any such
>    function definition replaces the default version provided in the
> library (17.6.4.6).
>    The following allocation and deallocation functions (18.6) are implicitly
>    declared in global scope in each translation unit of a program.

This is quite unfortunate.   Thre is nothing standard promise about return
value if this default new?  I.e. can I have something like

int a;
test()
{
  int *b=new (int);
}

with custom implementation of new that returns &a?

While compiling given object file we do not know if the user program happens to
define it somewhere else or not.  Even with LTO I do not think we have
mechanism to recognize statically or dynamically linked libsupc++ (we would
have to LTO _Zwnm that will need to stabilize LTO format to start with).

We probably can go with -fno-user-overwritten-new or something similar?

Honza


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