Segher Boessenkool <seg...@kernel.crashing.org> writes:
> 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.
Well...
>
>> 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
...with this interpretation, the deletions above would be correct even
if the asm throws.
> 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.
Modifying the asm like that feels a bit dangerous, And the other problem
still exists: he compiler might assume that the output isn't modified
unless the asm completes normally.
Thanks,
Richard
