On Thu, Sep 10, 2020 at 09:26:28AM +0000, David Laight wrote: > From: Christophe Leroy > > Sent: 10 September 2020 09:14 > > > > Le 10/09/2020 à 10:04, David Laight a écrit : > > > From: Linus Torvalds > > >> Sent: 09 September 2020 22:34 > > >> On Wed, Sep 9, 2020 at 11:42 AM Segher Boessenkool > > >> <segher@xxxxxxxxxxxxxxxxxxx> wrote: > > >>> > > >>> It will not work like this in GCC, no. The LLVM people know about that. > > >>> I do not know why they insist on pushing this, being incompatible and > > >>> everything. > > >> > > >> Umm. Since they'd be the ones supporting this, *gcc* would be the > > >> incompatible one, not clang. > > > > > > I had an 'interesting' idea. > > > > > > Can you use a local asm register variable as an input and output to > > > an 'asm volatile goto' statement? > > > > > > Well you can - but is it guaranteed to work :-) > > > > > > > With gcc at least it should work according to > > https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html > > > > They even explicitely tell: "The only supported use for this feature is > > to specify registers for input and output operands when calling Extended > > asm " > > A quick test isn't good.... > > int bar(char *z) > { > __label__ label; > register int eax asm ("eax") = 6; > asm volatile goto (" mov $1, %%eax" ::: "eax" : label); > > label: > return eax; > } It is neither input nor output operand here! Only *then* is a local register asm guaranteed to be in the given reg: as input or output to an inline asm. Segher
