Re: GCC asm register clobber

[DJ Delorie <dj at redhat dot com>]

> Thanks. In this case a specific register is required. Is it sufficient
> to write:
> 
> 	"=c" (out)     "c" (in)

No, the second one is always "0" (the number zero) to tell gcc that
the two parameters are the same register, not two values that must
(paradoxically) simultaneously be in the same register.

> If the 'out' parameter is not marked volatile, but is unused, what
> stops the compiler from optimizing it away, and having done so,
> concluding mistakenly that 'in' remains live in that register?

Hence the "0", which tells gcc how to correlate the parameters so it
knows what's really going on.

If gcc determines that it doesn't need the result of the asm, it
doesn't keep the asm.  You can make the asm volatile to prevent that:

  asm volatile (...)

This is why it's important to get the asm right, so that gcc knows
when it's *really* unneeded, not *accidentally* unneeded.