I am learning D.  I was working on Project Euler #199 (possible spoilers) and got some unexpected results.  It's probably a stupid mistake but I can't see it.

Here is an edited function from my program:

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.


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.

I am doing $dmd -run on the command line.  Working on a reasonably up to date Mac.

$ dmd --version
DMD64 D Compiler v2.083.1
Copyright (C) 1999-2018 by The D Language Foundation, All Rights Reserved written by Walter Bright

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...

On Monday, 7 January 2019 at 20:18:28 UTC, H. S. Teoh wrote:
> Either there's memory corruption somewhere, or there's a codegen bug in the compiler.

I think that must be it.  I ran it with ldc instead of dmd and it worked fine (solved the original problem! woohoo).

> Or the compiler somehow is  malfunctioning with -run.  Did
> you try compiling the program separately and running it?
> Does that make a difference?

I did try doing it without -run and it had the same issue.

> 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's the best way to get readable disassembly from dmd?  I tried godbolt but there was lots more stuff than I expected...

On Mon, Jan 07, 2019 at 09:42:42PM +0000, Joseph Malle via Digitalmars-d-learn wrote:
> On Monday, 7 January 2019 at 20:18:28 UTC, H. S. Teoh wrote:
> > Either there's memory corruption somewhere, or there's a codegen bug in the compiler.
> 
> I think that must be it.  I ran it with ldc instead of dmd and it worked fine (solved the original problem! woohoo).
> 
> > Or the compiler somehow is  malfunctioning with -run.  Did you try compiling the program separately and running it?  Does that make a difference?
> 
> I did try doing it without -run and it had the same issue.
> 
> > 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's the best way to get readable disassembly from dmd?  I tried godbolt but there was lots more stuff than I expected...

Hmm.  On Linux I'd use objdump:

	objdump -D myprogram | sed -ne'/radius/,/^$/p' > radius.disas

This extracts the assembly dump of the radius() function into the
file 'radius.disas'.

Not sure how to do this on a Mac.  (It should be possible to install objdump on Mac, but I don't own a Mac so I've no idea.)


T

-- 
Why do conspiracy theories always come from the same people??