# Re: [Simh] Looking for a milestone

```The discussions on this thread began with a question about the
accuracy of binary->decimal->binary conversions.  The key original
references are recorded in```
```
http://www.math.utah.edu/pub/tex/bib/fparith.bib
http://www.math.utah.edu/pub/tex/bib/fparith.html

in entries Goldberg:1967:BED, Matula:1968:BCT, and Matula:1968:C.
Those papers showed how many digits were needed for correct round-trip
conversions, but did not exhibit code to do so.

Some later papers had real source code, including Steele:1990:HPF,
Clinger:1990:HRF, Clinger:2004:RHR, Burger:1996:PFP, Knuth:1990:SPW,
and Abbott:1999:ASS.  The 20-year retrospectives in Clinger:2004:RHR
and Steele:2004:RHP sum up that earlier work, and may be the best
starting point to understand the problem.

It is decidedly nontrivial: the Abbott paper in the section
``Difficult numbers'' starting on page 739 discusses hard cases [where
the exact result in the output base is almost exactly halfway between
two machine numbers], and on page 740, they write ``The decimal size
may be unbounded, because there is a natural bound derived from the
exponent range, as was mentioned earlier. This bound is 126 digits for
single, 752 for double, and 11503 for extended precision.''

Entry Knuth:1990:SPW has the provocative title ``A Simple Program
Whose Proof Isn't'': it examines the conversions between fixed-binary
and decimal needed in the TeX typesetting system, a much simpler
problem whose proof eluded Knuth for several years until this paper.
A companion paper Gries:1990:BDO supplies an alternative proof of
Knuth's algorithm.

As to the first actual system to provide correct round-trip
conversions, the Abbott paper on the IBM mainframe (S/360 to zSeries
machines) describes the pressure to get it right the first time,
because of the longevity of that architecture, and the high cost of
repairing hardware implementations.

The 1990 Steele and Clinger references above supply software
implementations in Common Lisp exploiting the multiple-precision
arithmetic supported in that language.

Some current compilers, such as releases of gcc for the last several
years, use the GMP and MPFR multiple-precision arithmetic packages to
supply correct compile-time conversions.  Presumably, the revision
history of GNU glibc would reveal when similar actions were taken for
the strtod(), printf(), and scanf() families of the C and C++
programming languages. I have not personally investigated that point,
but perhaps other list members have, and can report their findings.

From scans of executables of assorted gcc versions, it appears that
GMP and MPFR came into use in gcc-family compilers about mid-2007.
The oldest gcc snapshot (of hundreds that I have installed) that
references both those libraries is gcc-4.3-20070720.  However, the
ChangeLog file in that release has mention on MPFR from 18-Nov-2006,
and an entry of 11-Jan-2007 that says "Add gmp and mpfr".

David M. Gay (then at AT&T Bell Labs renamed as Lucent Technologies)
released an accurate implementation of strtod() marked "Copyright (C)
1998-2001 by Lucent Technologies".  The oldest filedate in my personal
archives of versions of that software is 22-Jun-1998, with changes up
to 29-Nov-2007.

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