Re: automatic NaN propogation detection?

[Tejas via Digitalmars-d-learn]

On Saturday, 25 September 2021 at 08:18:49 UTC, Elronnd wrote:
(Not saying @live isn't useless, just that this doesn't 
indicate that.)


Must we continue hurting Walter's feelings :(

Re: automatic NaN propogation detection?

[Elronnd via Digitalmars-d-learn]

On Saturday, 25 September 2021 at 07:53:11 UTC, Chris Katko wrote:

Is that some sort of "NP complete" can't-fix issue or something?


The general case is obviously unsolvable.  Trivial proof: float x 
= nan; if (undecidable) use x.  I'm sure your imagination can 
supply more realistic cases (but I promise they really do come 
up!)


However that doesn't mean we can't do flow analysis 
conservatively.  Similar caveats apply to @live, but that does 
not mean it is useless.  (Not saying @live isn't useless, just 
that this doesn't indicate that.)  The bigger problem is that it 
really is hard to tell if you 'meant' to use a nan somewhere or 
not.


And if you tried to apply such a semantic analysis pass to 
existing code, you would find it riddled with false positives, 
where a float was default-initialized to nan and the compiler was 
unable to convince itself the variable was overridden before 
being used in all paths.

automatic NaN propogation detection?

[Chris Katko via Digitalmars-d-learn]

Is there any automatic compiler-based or library methods for 
detecting NaNs? I mean, if the compiler is outputting code that 
it knows is going to be set in memory to NaN, why isn't it giving 
me at least a compiler warning? Is that some sort of "NP 
complete" can't-fix issue or something?


I mean, I can pass NaN to std.math.round() and it doesn't fire 
off an exception or anything. It compiles fine even though it's 
impossible-as-compiled to be correct. (Unless my absurd intention 
was to find the rounded value of NaN.) Instead, I'm stuck finding 
out where the NaN started, from a trail of destruction of values 
destroyed by NaN propogation. Why not stop it at its source?


Even dscanner won't flag this code!

```d
import std.stdio;
import std.math;

int main()
{
float x;
writeln(x);
writeln(round(x));
return 0;
}
```