From: David Laight

Sent: 10 September 2020 10:26

...

quoted

quoted

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.

Actually this is pretty sound:
	__label__ label;
	register int eax asm ("eax");
	// Ensure eax can't be reloaded from anywhere
	// In particular it can't be reloaded after the asm goto line
	asm volatile ("" : "=r" (eax));
	// Provided gcc doesn't save eax here...
	asm volatile goto ("xxxxx" ::: "eax" : label);
	// ... and reload the saved value here.
	// The input value here will be that modified by the 'asm goto'.
	// Since this modifies eax it can't be moved before the 'asm goto'.
	asm volatile ("" : "+r" (eax));
	// So here eax must contain the value set by the "xxxxx" instructions.

    David

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