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.)
