On 11/20/2015 06:05 AM, Richard Henderson wrote:
I'd be perfectly happy to deprecate and later completely remove basic
asm within functions.
Because IMO it's essentially useless. It has no inputs, no outputs, and
no way to tell the compiler what machine state has been changed. We can
say that "it clobbers everything", but that's not actually useful, and
quite difficult as you're finding out.
And even more difficult to document exactly what happens, largely
because the implementation in GCC isn't consistent across passes and
there's differences in behaviour that are dependent on the target
implementation as well.
Unfortunately there's not a single place where we can ensure consistent
behaviour, so each pass has had to handle ASMs independently, and I'm
pretty sure they all get it wrong to varying degrees.
It seems to me that it would be better to remove the feature, forcing
what must be an extremely small number of users to audit and update to
extended asm.
That might be a little drastic. Though if we want to go this direction,
the first step is to deprecate for a major release cycle. It would be
interesting to see how much stuff would complain/break.
jeff
