[MPlayer-dev-eng] Asm parameter constraints (was: Improved remove-logo filter)

[Trent Piepho]

On Sun, 5 Nov 2006, Loren Merritt wrote:
> On Sat, 4 Nov 2006, Trent Piepho wrote:
> > On Fri, 3 Nov 2006, Uoti Urpala wrote:
> >
> >> Michael's original comment which you disagreed with said that if you do
> >> use [out]"=m"(a):[in]"m"(a) then [out] and [in] could be the same memory
> >> location or different ones, and [out] might overlap some other input
> >> parameter. I don't see anything in your latest reply which would show
> >> that gcc guarantees the opposite will always be true.
> >
> > You are saying that if you write [out]"=m"(a):[in]"m"(b) then out and in
> > could overlap?  This is what you mean when you say, "[out] might overlap
> > some other input parameter?"
> >
> > If that is the case, what would you write so that out and in can't overlap?
> > Or do you think there is no way to keep [out]"=m"(a) from overlapping some
> > other input paramters, and it is thus impossible to have a "=m" parameter
> > that is used before all inputs are consumed?
>
> [out]"=&m"(a)
>
> earlyclobber ("&") is specifally for that situation.

void foo(void) { int a, b; asm("# %0":"=&m"(a):"g"(b)); }

test.c: In function 'foo':
test.c:1: error: '&' constraint used with no register class
test.c:1: error: '&' constraint used with no register class

My question was mearly rhetorical, as I already knew (unlike those trying
to "correct" me) that '&' only applies to registers and can not be used
with a constraint that does not allow a register, such as 'm'.

So either I have been right all along, and a memory operand to an asm can't
overlap another unrelated memory operand, or it is impossible to write to a
memory operand before all the inputs are consumed.  I wonder when mplayer and
lavc will be audited to remove all the asm code that writes to memory and
then expects an input to still exist.