On Mon, Apr 02, 2012 at 04:07:59PM +0200, Michael Matz wrote:
> On Fri, 30 Mar 2012, Jan Hubicka wrote:
>
> > > Motion across hardreg sets/uses are not restricted. And I would not
> > > expect
> > > an optimizing compiler to do that (it's your own fault to use hardregs in
> > > complex C code).
> >
> > Well, the syscall sequence is an example of somehting that should be
> > inlined into arbitrary code w/o potential risk of ICEs.
>
> The syscall sequence doesn't consist of multiple independend instructions,
> but rather is one asm with the right constraints. So gcc has no chance to
> move something between the register setup (which only is explicit after
> reload) and the syscall instruction. If syscall sequences were
> implemented like this:
>
> inline int syscall1(int number, long arg1) {
> register int ax __asm__("eax");
> register long di __asm__("rdi");
> ax = number;
> di = arg1;
> __asm__ volatile ("syscall");
> }
>
> _then_ we would probably get miscompilations here and there.
That is actually what is used on most targets, including x86_64 (i?86 is an
exception here, because it has single hard register constraints for most of
the interesting registers).
Jakub
