Lack of asm volatile qualifier (explicitly) again.

[Iain Buclaw]

On Tuesday, 28 July 2020 at 06:57:36 UTC, Cecil Ward wrote:
> I read recently that all asm in D is regarded as ‘volatile’ in 
> the GCC sense, which I take to mean that it is assume to 
> potentially have side effects, and so cannot be optimised away 
> to nothing by the compiler despite the lack of any outputs.
>
> I would like to be able to either use the asm volatile 
> qualifier now and have it do absolutely nothing or else have an 
> alternative way of expressing the licence for optimisation 
> allowed by the designer. If it is now way too late to declare 
> that suddenly the _lack_ of volatile means that the compiler 
> can go optimisation-crazy, then we need some alternative 
> ‘is_optimisable’ keyword.
>

Until recently the absence of the pure keyword implied volatile 
in gdc. I do plan to re-add it, either only in release mode, or 
when warnings are added for the following:

---
asm pure {
   "get flags" : "=r" (x);
}
assert(x == 1);

asm {
   "set flags" : : "r" (x + 1);
}

asm pure {
   "get flags" : "=r" (x);
}
assert(x == 2);
---

The second 'get flags' would be removed if optimizing as it is 
both identical to the first statement, and not clear to the 
compiler that there is a dependency between the setter and getter 
statements.

Just highlighting one example that might be surprising if you 
weren't thinking that optimizing mean that as well.