Re: Recent libstdc++ regression on i686-linux: abi/cxx_runtime_only_linkage.cc

["Peter Dimov" <pdimov at mmltd dot net>]

Mark Mitchell:
> Richard Henderson wrote:
> > H.J. Lu wrote:
> > > Can we declare that Linux/ia32 generates i486 insn by default?
> >
> > We the gcc team?  I'm not sure.  For now I'll say no.
> >
> > We an individual linux distributor?  Certainly.
> > In fact I would be surprised if i586 wasn't a
> > decent minimum these days.
>
> I agree.  We the GCC team have to accept that some CPUs may not have the
> ability to do this.  That might be old x86 CPUs; it might also be
> brand-new embedded CPUs.

Setting the default to -march=i486 will still let people who target i386 to 
use -march=i386.

The problem, from the point of view of a library such as boost::shared_ptr, 
is that there is no way to distinguish between user A, who just types g++ 
foo.cpp and expects to get a program that works well on a typical machine, 
and user B, who types g++ -march=i386 foo.cpp, with the explicit intent to 
run the result on a 386.

Since A users outnumber B users, boost::shared_ptr assumes A and uses 486 
atomic instructions even though __i486__ is not defined.

Has the default been 486, I'd be able to recognize user B's intent and not 
use 486 instructions.

(Not that I've ever received a bug report about shared_ptr not working on 
386.)