On Wednesday, May 4, 2022 at 10:28:31 AM UTC-7, Thomas Koenig wrote:

(snip)

> There is no "exactly x=0.1" in binary floating point (and I'm not
> sure that even a single Fortran compiler supports decimal floating
> point, despite radix being present in selected_real_kind), which
> is why I chose that particular number, to add a bit of difficulty.

I now have an IBM Power7 machines, which (like Power6) has decimal
floating point. I haven't gotten to install an OS yet, with AIX and Linux
being two choices, and don't know which compilers have support for it.

On 5/4/22 11:33 PM, gah4 wrote:
> On Wednesday, May 4, 2022 at 10:28:31 AM UTC-7, Thomas Koenig wrote:
>
> (snip)
>
>> There is no "exactly x=0.1" in binary floating point (and I'm not
>> sure that even a single Fortran compiler supports decimal floating
>> point, despite radix being present in selected_real_kind), which
>> is why I chose that particular number, to add a bit of difficulty.
>
> I now have an IBM Power7 machines, which (like Power6) has decimal
> floating point. I haven't gotten to install an OS yet, with AIX and Linux
> being two choices, and don't know which compilers have support for it.

This issue has absolutely nothing to do with decimal arithmetic and
whether a particular processor supports it or not.

In the original code, there was an if statement that compared to the
literal value 0.1. That literal value is translated to floating point,
and in floating point, it has a definite value. That numerical value
might be different between the two, but that doesn't matter. Just like
it doesn't matter if the value is represented in 32-bit floating point
or 46-bit floating point or 64-bit floating point, however it is
translated, it has a value. What matters is that some value exists. Then
expressions such as "x > 0.1" have a meaning given the values of x and
the values of 0.1. Those values can compare to be equal, regardless of
whether binary or floating point arithmetic is being done.

Or stated a different way, when the programmer writes "x > 0.1", he is
telling the processor to compare the value of x to another value that it
can represent, he is not telling the processor to compare x to some
hypothetical value that it might not be able represent.

As for fortran support of decimal floating point, I am curious how this
is done too. Presumably there are some KIND values for those REALs? What
happens with mixed-kind arithmetic? Is one kind converted to the other,
or is there hardware support for the mixed-kind operations?

$.02 -Ron Shepard