[ruby-core:54497] Re: [ruby-trunk - Bug #8299] Minor error in float parsing

On Apr 21, 2013, at 7:18 PM, naruse (Yui NARUSE) wrote:

> marcandre (Marc-Andre Lafortune) wrote:
>> For any float f, the two following conditions should hold:
>> (1) f.to_s.to_f == f        (round trips)
> 
> Yeah, unless you transfer the result of to_s to another environment.

Are you saying that round tripping is only valid on "the native architecture's double-precision floating point representation" (quote from Float's RDoc)?  That would make sense to me.

> x87 FPU stores and calculates floating numbers in 80bit on registers, even if it is stored in 64bit on memory.

Does Float on x87 systems use 80-bits to store its value?  IOW, is "the native architecture's double-precision floating point representation" 80 bits on x87?Asking (almost) the same thing a third way, what is "sizeof(double)" on a system with an x87 FPU?

> Therefore x87's result may differ from modern FPU's result.

I agree that the results of floating point operations on x87 may differ from the same operations on modern FPU's given the same input operands due to 80-bit intermediate values on x87, but does this affect the parsing of Float literals?

Do Float constants like EPSILON, MIN, and MAX differ between x87 systems and modern FPUs?

Does "x87" == "non-IEEE-754" and "modern FPU" == "IEEE-754"?

I find this topic fascinating and am just trying to understand the subtle semantics involved here!

Thanks,
Dave