• Chris Katko (23/23) Sep 25 2021 Is there any automatic compiler-based or library methods for
• Elronnd (16/17) Sep 25 2021 The general case is obviously unsolvable. Trivial proof: float x
• Tejas (2/4) Sep 25 2021 Must we continue hurting Walter's feelings :(

Chris Katko <ckatko gmail.com> writes:

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;
	}
```

Elronnd <elronnd elronnd.net> writes:

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.

Tejas <notrealemail gmail.com> writes:

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 :(