Unexpected results with doubles

[H. S. Teoh]

On Mon, Jan 07, 2019 at 07:57:14PM +0000, Joseph Malle via Digitalmars-d-learn wrote:
[...]
> auto radius(const double r1, const double r2, const double r3) {
>   auto const k1 = 1/r1;
>   auto const k2 = 1/r2;
>   auto const k3 = 1/r3;
>   writeln();
>   writeln("1   ", [k1, k2, k3]);
>   writeln("2   ", [k1 * k2, k2 * k3, k3 * k1]);
>   writeln("3   ", [k1 * k2 + k2 * k3 + k3 * k1]);
>   assert(!isNaN(k1 * k2 + k2 * k3 + k3 * k1));
>   writeln("4   ", [sqrt(k1 * k2 + k2 * k3 + k3 * k1)]);
>   assert(!isNaN(sqrt(k1 * k2 + k2 * k3 + k3 * k1)));
>   auto rv = 1 / (k1 + k2 + k3 + 2.0 * sqrt(k1 * k2 + k2 * k3 + k3 * k1));
>   assert(!isNaN(rv));
>   writeln("radius ", [r1, r2, r3], " => ", rv);
>   writeln();
>   return rv;
> }
> 
> Here is some output:
> 
> 1   [41.7846, 6.4641, 6.4641]
> 2   [270.1, 41.7846, 270.1]
> 3   [581.985]
> 4   [24.1244]
> radius [0.0239323, 0.154701, 0.154701] => 0.00971237
> 
> 
> 1   [41.7846, 6.4641, 6.4641]
> 2   [270.1, 41.7846, 270.1]
> 3   [581.985]
> 4   [nan]
> radius [0.0239323, 0.154701, 0.154701] => 0.00971237
> 
> 
> 1   [41.7846, 6.4641, 6.4641]
> 2   [270.1, 41.7846, 270.1]
> 3   [581.985]
> 4   [nan]
> radius [0.0239323, 0.154701, 0.154701] => 0.00971237
> 
> The "4   [nan]" is unexpected.  Each time they have the same input/same
> output.  But sometimes the 4th line is nan and sometimes it's not.  The
> asserts never fail.  I've seen this unexpected nan a few times with other
> inputs for this function.

Either there's memory corruption somewhere, or there's a codegen bug in
the compiler.  Or the compiler somehow is malfunctioning with -run.  Did
you try compiling the program separately and running it?  Does that make
a difference?


> If I change it to:
> 
> auto x = sqrt(k1 * k2 + k2 * k3 + k3 * k1);
> writeln("4   ", [x]);
> assert(!isNaN(x));
> 
> Then the assert fails.  I checked if the assert fails before the
> writeln too (as a sanity check) and yes, x is always NaN it seems.
[...]

The way to dig into the cause is to disassemble the radius() function
and post the disassembly here.  Then we can take a look to find out
what's going on.

What are the values of k1, k2, k3?


T

-- 
I think the conspiracy theorists are out to get us...