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Re^7: Defining an XS symbol in the Makefile.PL

by syphilis (Bishop)
on Aug 19, 2019 at 01:40 UTC ( #11104675=note: print w/replies, xml ) Need Help??


in reply to Re^6: Defining an XS symbol in the Makefile.PL
in thread Defining an XS symbol in the Makefile.PL

Also, I simply copied your probe logic, but I must ask if you are sure that that is the right way to do it.

Thanks for querying - I've just now realized that I've forgotten about the DoubleDouble type of long double. (Given the amount of time I've spent puzzling over that beast, I'm actually quite appalled that I could do that !!)
It's a case that technically should be given special handling. At the moment, the DoubleDoubles will end up in the elsif{} block, demanding a precision of "%.36" which is sufficient for the vast majority of cases ... but not all :-(
The DoubleDouble is an nvtype that is rarely encountered and I'm tempted to leave the code as it is and just change the comment from:
# IEEE long double or __float128 to # IEEE long double or __float128 or DoubleDouble
I'll have to do some testing on my DoubleDouble builds and look at the options.

On recent perls the approach that I took could be replaced by examining $Config{nvtype} in conjunction with $Config{longdblkind} (when nvtype is long double).
However, for perls prior to 5.22, $Config{longdblkind} is unavailable, so we need to use another method.
Besides, using nvtype and longdblkind looks even messier than what I've done - though the logic is perhaps more transparent.

The 'long double' is the type that makes things awkward - it can be either 64, 80, or 128 bits, and on 64-bit builds of perl the 80-bit, 128-bit and DoubleDouble long doubles all typically report a $Config{nvsize} value of 16, thus severely limiting the usefulness of that Config value.

Cheers,
Rob

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Re^8: Defining an XS symbol in the Makefile.PL
by jcb (Hermit) on Aug 19, 2019 at 02:40 UTC

    Of course a 64-bit Perl will report both 80-bit and 128-bit floats as 16 bytes: the alignment rules require that much storage since both types span two 64-bit words. Do the different long double types have differing ranges? Could you test progressively larger numbers and arrive at an answer that way?

    Perhaps something like: (obviously untested and the numbers are bogus)

    my @bigfloats = ([6, '1.234e56', '1e50'], [12, '2.3456e789', '1e777'], + # "lorem ipsum ..." [18, '3.45678e9012', '1e8994'], [36, '4.567890e12345' +, '1e12329']); # Each of the above arrays has (0) the number of decimal digits of pre +cision needed, # (1) a number near the large end of the +range for a type, # (2) a number near epsilon for the sampl +e number (1) my $prec = 0; foreach my $step (@bigfloats) { $prec = $step->[0]; my $num = (0+$step->[1]); my $sum = ($num + $step->[2]); last unless $num eq $step->[1] and $sum > $num; } $defines .= ' -DLU_NV_PREC='.$prec;

    Then in XS: (here is the macro trick I mentioned earlier in "(cpp)Stringification")

    ... #define STR_(s) #s #define DSTR_(s) STR_(s) ... #ifndef LU_NV_PREC #error "LU_NV_PREC is not set" #endif #define MY_FORMAT "%." DSTR_(LU_NV_PREC) "e" ...
      Do the different long double types have differing ranges?

      Yes, but we don't need to know anything about the range for the task at hand.
      We just need to know the maximum number of bits of precision for each type, and the number of decimal digits required to accurately handle each of those maximum precisions.
      That is:
      53 bits needs 17 decimal digits
      64 bits needs 21 decimal digits
      113 bits needs 36 decimal digits

      The DoubleDouble can actually accommodate some (not all) 2098-bit values - which would require 633 decimal digits, but I'm still pondering what should be done about that.
      I doubt that"%.632" NVgf will produce reliable results anyway.
      Unpacking the bytes of the NV is probably cheaper and quicker than obtaining a numeric value, so maybe that's a better path to take for *all* NVs - not just the DoubleDouble.

      Cheers,
      Rob

        You seem to misunderstand. The test code I offered above takes advantage of varying ranges to distinguish between known types. The code tests range by doing a string->FP->string round-trip and comparing the result to the original string. It also directly tests available precision by adding a (smaller) value and testing if the sum is larger than the initial value. The highest of these tests that passes defines the number of digits of precision to use.

        It also looks like you may have found a better way to do this that does not require determining the FP type in advance?

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