Re: basic asm and memory clobbers

[Richard Henderson <rth at redhat dot com>]

On 11/20/2015 04:20 PM, Segher Boessenkool wrote:
> On Fri, Nov 20, 2015 at 02:05:01PM +0100, 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.
> >
> > 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.
>
> Should  asm("bla");  then be an extended asm with no input, no outputs,
> no (non-automatic) clobbers?  That would be the most straightforward and
> logical semantics, but will it break user code?

I'm suggesting that we don't accept that at all inside a function.  One must 
audit the source and make a conscious decision to write asm("bla" : ); instead.

Accepting basic asm outside of a function is perfectly ok, since that's just a 
mechanism by which one can inject complete assembly routines into a C 
translation unit.


r~