Releases 3 through 3c of Berkeley SoftFloat contain bugs in the
square root functions that may be of concern for some uses.
Those bugs are believed to be repaired in Release 3d, as explained
Berkeley SoftFloat is a free, high-quality software implementation of binary
floating-point that conforms to the IEEE Standard for Floating-Point
SoftFloat is completely faithful to the IEEE Standard, while at the same time
being relatively fast.
All functions dictated by the original 1985 version of the standard are
supported except for conversions to and from decimal.
The latest release of SoftFloat implements five floating-point formats:
16-bit half-precision, 32-bit single-precision,
64-bit double-precision, 80-bit
double-extended-precision, and 128-bit quadruple-precision.
All required rounding modes, exception flags, and special values are supported.
Fused multiply-add is also implemented for all formats except
SoftFloat is distributed in the form of ISO/ANSI C source code and should be
compilable with almost any ISO-compliant C compiler.
Using the GNU C Compiler (
gcc), the package has been compiled and
tested for several platforms.
Target-specific code is provided for various Intel x86 processors.
Other machines can be targeted using these as examples.
Since Release 3, SoftFloat has depended on the existence of a
64-bit integer type in C.
If the C compiler used to compile SoftFloat does not support
64-bit integers, it is still possible to use the older
Release 2c to implement the two most common formats,
32-bit single-precision and 64-bit double-precision,
but not the other formats.
Release 3 was a complete rewrite of SoftFloat, funded by the
University of California, Berkeley.
Compared to earlier releases, Release 3 added functions for
converting to and from unsigned integers, functions for fused multiply-add, and
better algorithms for division, remainder, and square root.
Unlike past versions, Release 3 and later have a U.C. Berkeley
open-source license (specified in the documentation).
The latest version of SoftFloat is Release 3d (2017 August).
This version fixes bugs that were found in the square root functions for the
64-bit, 80-bit, and 128-bit
(Thanks to Alexei Sibidanov at the University of Victoria for reporting an
The bugs affected all prior Release-3 versions of SoftFloat.
For 64-bit double-precision, the flaw in the square root function
was of very minor impact, causing a 1-ulp error
(1 unit in the last place) a few times out of a billion.
The bugs in the 80-bit and 128-bit square root
functions were more serious.
Although incorrect results again occurred only a few times out of a billion,
when they did occur a large portion of the less-significant bits could be
Release 3d makes no changes to the SoftFloat library interface
compared to the previous Release 3c.
Since the original Release 3, the main changes to the interface
have been that Release 3b added support for the
16-bit half-precision format, and Release 3c added
optional support for a rarely used rounding mode, round to odd, also
known as jamming.
More information about Berkeley Softfloat Release 3d is in the
following files from the SoftFloat package:
Berkeley SoftFloat Release 3d: Library Interface.
This document includes a section on the many differences between
Release 3 and the earlier Release 2.
Berkeley SoftFloat Release 3d: Source Documentation.
History of Berkeley SoftFloat, to Release 3d.
The following archive contains all source code and documentation for
The older Releases 3a through 3c (with known bugs) are still available here:
For those who may need it, SoftFloat versions preceding Release 3
have been updated to Release 2c (2015 January).
The only changes in this release compared to earlier releases 2a
and 2b are these:
If you have been successfully using Release 2a or 2b
on a 32-bit processor (or compiled as though for a
32-bit processor), you probably do not need to download this
Some bugs are fixed that affected correct compilation for some
64-bit processors only.
The documentation has been improved in minor ways.
The restrictions on legal use have been further clarified (not applicable to
Release 3 or later).
Once compiled, SoftFloat can be tested using the
program of Berkeley TestFloat:
Berkeley TestFloat is a small collection of
programs for testing that an implementation of binary floating-point conforms
to the IEEE Standard for Floating-Point Arithmetic.
The following table shows the speeds of some of SoftFloat's functions on
machines of different strengths.
For this table, SoftFloat was compiled using the GNU C Compiler with only
ordinary optimization enabled (
Speeds are reported in Mop/s (millions of floating-point operations per
Function times are affected by operand values and cache effects, among other
factors, so your results may vary.
1-GHz ARM Cortex-A8 (32-bit),
32-kB I/D caches, Linux, GCC 4.6.3
2.2-GHz Intel Xeon E5-2430 (64-bit),
Linux, GCC 4.8.5
Credit and contacts
Berkeley SoftFloat was written by me,
John R. Hauser.
Funding for the development of SoftFloat Release 3 and later was
provided indirectly by portions of grants to U.C. Berkeley from Microsoft,
Intel, DARPA, Nokia, NVIDIA, Oracle, Samsung, and Google, and by a portion of a
U.C. Discovery Grant.
The SoftFloat documentation has more details.
Bugs in SoftFloat and other comments can be reported to me at
2017 August 10