www.digitalmars.com         C & C++   DMDScript  

digitalmars.D - extended/real types again

I was reading the infamous JAVAHurt paper over,
http://www.cs.berkeley.edu/~wkahan/JAVAhurt.pdf

and found some words on it, describing the various
floating-point types that I was trying to talk about...


 From the JAVAhurt paper:
 Names for primitive floating-point types or for Borneo floating-point classes:
 
 float = 4-byte IEEE 754 Single with 24 sig. bits, usually hardware supported.
 double = 8-byte IEEE 754 Double with 53 sig. bits, usually hardware supported.
 long double = 10+-byte IEEE 754 Double Extended with at least 64 sig. bits etc.
 
 { longdouble(k) = k-byte IEEE 754 Double Extended ( for future use only with k
>> 10.)
   quadruple = long double(16) with 113 sig. bits rounded as IEEE 754/854
requires.
   DoubledDouble = 16-bytes with at least about 106 sig. bits perhaps rounded
perversely. }
 
 indigenous = the widest floating-point format supported in hardware at full
speed
   = long double = double extended on hardware that does it
   = double on computer hardware that does nothing wider.

This "indigenous" is the same as the "long double" in C and "real" in D. I was suggesting to re-introduce "extended" for the third type above... DoubledDouble would be the -mlong-double-128 option that IBM AIX has. See http://www.linuxbase.org/spec/ELF/ppc64/PPC-elf64abi-1.7.html#PREC has all the details on what Prof. Kahan calls a "perverse format" :-) Quadruple would be a future actual 128-bit type, that no-one has now. (and probably won't appear until we see 128-bit integer CPUs, or so?) Note: Maybe we want to reserve this as a new type, just like "cent" ? I thought that "real" was a perfect fit for an alias, since it's actually referring to one of two types - depending on hardware ? DoubledDouble is unusable in D due to lack of nan/inf and all traps, and Quadruple is not available yet. Leaving only Extended and Double. And, last but not least, the suggestion also "fixed" http://www.digitalmars.com/d/archives/28044.html : ... "sorry, but with these names for complex and imaginary numbers, you have created a joke among mathematicians for the years to come!" And then, a second concern, is the alignment size of such a "real"... There seems to be several possible values for this too, unfortunately. - Double => 8-byte / 64-bit alignment (natural) - old Windows "long double" => 10-byte / 80-bit alignment (unpadded) - X86 Linux "long double" => 12-byte / 96-bit alignment (word aligned) - X86_64 Linux "long double" => 16-byte / 128-bit alignment (Pentium+) So to support linking with C functions, all *four* types must be used ? Otherwise, a natural choice for native D alignment would be 128 bits ? --anders
Apr 03 2005