From: Christophe Leroy

Sent: 10 September 2020 09:14

Le 10/09/2020 à 10:04, David Laight a écrit :

quoted

From: Linus Torvalds

quoted

Sent: 09 September 2020 22:34
On Wed, Sep 9, 2020 at 11:42 AM Segher Boessenkool
[off-list ref] wrote:

quoted

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

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