From @pjacklam

Speed up Math​::BigFloat -> blog().

- Intermediate computations in blog() increases the number of digits
  significantly in some cases. Reduce the number of digits by rounding.
  However, keep some extra digits for remaining intermediate computations
  before the final rounding.

- This patch fixes CPAN RT #43924.

From @pjacklam

From a6be9fcc4d8599540a1ec04fd04f8d9d0672cad0 Mon Sep 17 00:00:00 2001
From: Peter John Acklam <pjacklam@online.no>
Date: Fri, 24 Apr 2015 15:13:33 +0200
Subject: [PATCH] Speed up Math::BigFloat -> blog().

- Intermediate computations in blog() increases the number of digits
  significantly in some cases. Reduce the number of digits by rounding.
  However, keep some extra digits for remaining intermediate computations
  before the final rounding.

- This patch fixes CPAN RT #43924.
---
 dist/Math-BigInt/lib/Math/BigFloat.pm | 1 +
 1 file changed, 1 insertion(+)

diff --git a/dist/Math-BigInt/lib/Math/BigFloat.pm b/dist/Math-BigInt/lib/Math/BigFloat.pm
index 1c31496..3c51914 100644
--- a/dist/Math-BigInt/lib/Math/BigFloat.pm
+++ b/dist/Math-BigInt/lib/Math/BigFloat.pm
@@ -1476,6 +1476,7 @@ sub _log_10
     {
     $twos++; $x->bdiv($two,$scale+4);		# keep all digits
     }
+  $x->bround($scale+4);
   # $twos > 0 => did mul 2, < 0 => did div 2 (but we never did both)
   # So calculate correction factor based on ln(2):
   if ($twos != 0)
-- 
2.1.4

From @kentfredric

On 25 April 2015 at 01​:16, Peter J. Acklam via RT <perlbug-followup@​perl.org

wrote​:

# New Ticket Created by (Peter J. Acklam)
# Please include the string​: [perl #124382]
# in the subject line of all future correspondence about this issue.
# <URL​: https://rt-archive.perl.org/perl5/Ticket/Display.html?id=124382 >

Speed up Math​::BigFloat -> blog().

- Intermediate computations in blog() increases the number of digits
significantly in some cases. Reduce the number of digits by rounding.
However, keep some extra digits for remaining intermediate computations
before the final rounding.

- This patch fixes CPAN RT #43924.

From a6be9fcc4d8599540a1ec04fd04f8d9d0672cad0 Mon Sep 17 00​:00​:00 2001
From​: Peter John Acklam <pjacklam@​online.no>
Date​: Fri, 24 Apr 2015 15​:13​:33 +0200
Subject​: [PATCH] Speed up Math​::BigFloat -> blog().

- Intermediate computations in blog() increases the number of digits
significantly in some cases. Reduce the number of digits by rounding.
However, keep some extra digits for remaining intermediate computations
before the final rounding.

- This patch fixes CPAN RT #43924.
---
dist/Math-BigInt/lib/Math/BigFloat.pm | 1 +
1 file changed, 1 insertion(+)

diff --git a/dist/Math-BigInt/lib/Math/BigFloat.pm
b/dist/Math-BigInt/lib/Math/BigFloat.pm
index 1c31496..3c51914 100644
--- a/dist/Math-BigInt/lib/Math/BigFloat.pm
+++ b/dist/Math-BigInt/lib/Math/BigFloat.pm
@​@​ -1476,6 +1476,7 @​@​ sub _log_10
{
$twos++; $x->bdiv($two,$scale+4); # keep all digits
}
+ $x->bround($scale+4);
# $twos > 0 => did mul 2, < 0 => did div 2 (but we never did both)
# So calculate correction factor based on ln(2)​:
if ($twos != 0)
--
2.1.4

Q) Does this affect the overall precision of the floating point math?

I am ignorant of the internals, but that sort of concern seems to be the
sort of thing lay-folk who are simply using the library would care about,
and so I'm just looking for some kind of assurance the rounding is not
*premature* rounding.

--
Kent

*KENTNL* - https://metacpan.org/author/KENTNL

The RT System itself - Status changed from 'new' to 'open'

From @pjacklam

Q) Does this affect the overall precision of the floating point math?

I am ignorant of the internals, but that sort of concern seems to be the
sort of thing lay-folk who are simply using the library would care about,
and so I'm just looking for some kind of assurance the rounding is not
*premature* rounding.

The code already rounds to $scale+4 digits, in other cases which involve more intermediate computation, so using four extra digits in this case, should not make things any worse than they already are. I am working on a test program to check blog() more throughly.

From @tonycoz

On Fri Apr 24 06​:16​:29 2015, pjacklam wrote​:

Speed up Math​::BigFloat -> blog().

- Intermediate computations in blog() increases the number of digits
significantly in some cases. Reduce the number of digits by rounding.
However, keep some extra digits for remaining intermediate computations
before the final rounding.

- This patch fixes CPAN RT #43924.

Thanks, applied to blead as 0d2463e.

Tony

@tonycoz - Status changed from 'open' to 'resolved'

@tonycoz - Status changed from 'resolved' to 'pending release'

From @khwilliamson

Thank you for submitting this report. You have helped make Perl better.
 
With the release of Perl 5.24.0 on May 9, 2016, this and 149 other issues have been resolved.

Perl 5.24.0 may be downloaded via https://metacpan.org/release/RJBS/perl-5.24.0

@khwilliamson - Status changed from 'pending release' to 'resolved'