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; } 0000000000000040 <bar>: 40: b8 01 00 00 00 mov $0x1,%eax 45: b8 06 00 00 00 mov $0x6,%eax 4a: c3 retq although adding: asm volatile ("" : "+r" (eax)); either side of the 'asm volatile goto' does fix it. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
