Hi!
On Fri, Mar 13, 2020 at 01:31:19PM +0000, Richard Sandiford wrote:
> This might not be what you mean, but for:
>
> int
> f1 (void)
> {
> int x = 1;
> asm volatile ("": "=m" (x));
> return x;
> }
>
> struct s { int i[5]; };
> struct s
> f2 (void)
> {
> struct s x = { 1, 2, 3, 4, 5 };
> asm volatile ("": "=m" (x));
> return x;
> }
>
> we do delete "x = 1" for f1. I think that's the expected behaviour.
> We don't yet delete the initialisation in f2, but I think in principle
> we could.
Right. And this is incorrect if the asm may throw.
> So the kind of contract I was advocating was:
>
> - the compiler can't make any assumptions about what the asm does
> or doesn't do to output operands when an exception is raised
>
> - the source code can't make any assumption about the values bound
> to output operands when an exception is raised
And the easiest (and only feasible?) way to do this is for the compiler
to automatically make an input for every output as well, imo.
Segher
