Hi, On Mon, 29 Feb 2016, Bernd Schmidt wrote:
> On 02/29/2016 06:07 PM, Michael Matz wrote: > > > %rbx would have to be implicitly used/clobbered by the asm. In addition > > it would have to be used by all function entries and exits (so that a > > function body where the global reg var is merely visible but not used > > doesn't accidentally clobber that register) and of course by calls. > > Nearly all this exists as of today. From df-scan.c: Okay, that looks good, I agree (modulo the asms). > > FWIW: signal handlers need no consideration (if they were allowed to > > inspect/alter global reg vars we would have lost and no improvement on > > fixed_regs[] could be done). They are explicitely documented to not be > > able to access global reg vars. (They already can't accidentally clobber > > the register in question because signal handlers don't do that) > > Oh, they can't modify the register in question because the OS would > restore it? Yep. > Ok so maybe reopen and apply my patch for gcc-7, with a tweak for asms. That seems workable. At least I can't imagine other implicit uses of such registers. Ciao, Michael.
