$fp2 =  -8.2727285363069939e-293;;
printf "% 25.17g\n", $fp2;;
 -7.9999999999999948e-293     ### WTF? ###

[download]


Hadn't noticed this before - a perl that produces that result is simply brainfucked.
I can reproduce that garbage on a perl-5.18.0 that I built with my Platform_SDK compiler on Windows 7: 
C:\>perl -le "printf '%.16e', -8.2727285363069939e-293;"
-7.9999999999999948e-293

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Yet, according to the very same perl, the 2 values are entirely different: 
C:\>perl -le "print scalar reverse unpack 'h*', pack 'd<', -7.99999999
+99999948e-293;"
83465a792c83e3b6

C:\>perl -le "print scalar reverse unpack 'h*', pack 'd<', -8.27272853
+63069939e-293;"
83498bf832dfdfab

[download]
UIM, this is not perl's fault. (But I am mistaken - see UPDATE and UPDATE2.) Perls built (from the same source) with gcc on the same machine do not suffer this problem.
I would not rely on the accuracy of *any* floating point result that such a perl produced.

UPDATE: After some double-checking I have found that the mingw-w64 gcc-4.7.x x64 compilers built perl-5.18.x with exactly the same problem as the Platform_SDK compiler build.
The problem went away with perl-5.20.0 onwards, but I don't know if that was because of changes to the perl source. I used later versions of gcc to build 5.20.0 onwards - and maybe that's what fixed the issue.

UPDATE2: The problem with 5.18.0 is simply that perl assigns the wrong value to the NV - instead of assigning the hex format 834a6aec8f941351, it assigns 83498bf832dfdfab, which by my calculation is off by 245,141,108,700,070 ULPs. (Such inaccuracies are usually quite small, and I was thrown by the large size of this one.)

Cheers,
Rob

In reply to Re: Determining the minimum representable increment/decrement possible? by syphilis
in thread Determining the minimum representable increment/decrement possible? by BrowserUk

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