On Monday, 1 October 2012 at 04:10:25 UTC, Ivan Agafonov wrote:
On Sunday, 30 September 2012 at 06:20:56 UTC, jerro wrote:
The second one uses fstp twice, then fld twice. I don't know,
maybe
this could be a bug.
You're right the lack of one fst/fld in the first case is a
bug. x87 floating point registers are 80 bit. This:
fstp dword [ebp-0CH]
Converts the value in ST0 to single precision float and stores
it to memory (and pops ST0). When it is later loaded with fld,
it is not the same as before storing since some precision is
lost (because the D code compares floats and not reals, this
is the correct behavior). In the first example, this storing
and loading only happens for the first function call. For the
second call the value is returned in ST0 and stays in x87
registers until it is compared with fucompp so it is not
truncated as the result of the first function call was. That's
why the compared values are not equal.
Can you or anyone report this bug? I don't know how to do this.
I'm not really sure that this is a bug anymore. Apparently c++
does this like this too
(http://www.parashift.com/c++-faq/floating-point-arith2.html). I
do think it would be more useful if the result of floating point
comparison would be defined in a cases like this, but for
floating point operation in general it makes sense to keep
intermediate results in registers. So I don't know whether
introducing a special case for this is worth it.
If you care about this, you should probably make a thread on
digitalmars.D, where more people will see it and comment on it.
