digitalmars.D.learn - Unexpected results with doubles

• Joseph Malle (54/54) Jan 07 I am learning D. I was working on Project Euler #199 (possible
• H. S. Teoh (14/66) Jan 07 Either there's memory corruption somewhere, or there's a codegen bug in
• Joseph Malle (6/14) Jan 07 I think that must be it. I ran it with ldc instead of dmd and it
• H. S. Teoh (10/29) Jan 07 Hmm. On Linux I'd use objdump:
Joseph Malle <malle umich.edu> writes:
```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
```
Jan 07
"H. S. Teoh" <hsteoh quickfur.ath.cx> writes:
```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...
```
Jan 07
Joseph Malle <malle umich.edu> writes:
```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...
```
Jan 07
"H. S. Teoh" <hsteoh quickfur.ath.cx> writes:
```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:

This extracts the assembly dump of the radius() function into the