accuracy of floating point calculations: d vs cpp

[Guillaume Piolat]

On Monday, 22 July 2019 at 12:49:24 UTC, drug wrote:
> I have almost identical (I believe it at least) implementation 
> (D and C++) of the same algorithm that uses Kalman filtering. 
> These implementations though show different results (least 
> significant digits). Before I start investigating I would like 
> to ask if this issue (different results of floating points 
> calculation for D and C++) is well known? May be I can read 
> something about that in web? Does D implementation of floating 
> point types is different than the one of C++?
>
> Most of all I'm interesting in equal results to ease comparing 
> outputs of both implementations between each other. The 
> accuracy itself is enough in my case, but this difference is 
> annoying in some cases.

Typical floating point operations in single-precision like a 
simple (a * b) + c will provide a -140dB difference if order is 
changed. It's likely the order of operations is not the same in 
your program, so the least significant digit should be different.