lkml.org 
[lkml]   [2020]   [Sep]   [10]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: remove the last set_fs() in common code, and remove it for x86 and powerpc v3
    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@kernel.crashing.org> 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

    \
     
     \ /
      Last update: 2020-09-10 17:55    [W:4.870 / U:0.740 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site