floating point conversion

[Famous via Digitalmars-d-learn]

On Sunday, 1 June 2014 at 15:31:53 UTC, Martin Krejcirik wrote:
> On 1.6.2014 16:42, Famous wrote:
>> from string should be the same, exacly, always. Casting a 
>> float to
>> double should be deterministic as well.
>
> void main()
> {
>     float a = 1.234f;
>     double b = 1.234;
>     assert (a == cast(float) b); // fails in DMD x86, works in 
> GDC, LDC
> }
>
> Maybe enhancement request ?

This is different. The decimal 1.234 cannot be exacly represented 
as radix-2 floating-point number. In your example above, a and b 
are not equal. They are both approximations to 1.234 but the 
value of b is closer. Then casting b to float might or might not 
result in the same value as that of a. This is what bearophile 
and Qox referred to.

Since every float can be exacly converted to double the following 
situation is different:

float a = 1.234f;
double b = a;
assert(a == b);  // always succeeds in a sane environment

When taking into account constant folding, well, decide on your 
own, whether the following behaviour is sane:

void main()
{
   const float a = 1.234f;
   float b = 1.234f;

   assert(a == 1.234); // ok
   assert(b == 1.234); // fails
}