385 lines
17 KiB
Plaintext
385 lines
17 KiB
Plaintext
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$NetBSD: softfloat-source.txt,v 1.1 2000/06/06 08:15:10 bjh21 Exp $
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$FreeBSD$
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SoftFloat Release 2a Source Documentation
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John R. Hauser
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1998 December 14
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-------------------------------------------------------------------------------
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Introduction
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SoftFloat is a software implementation of floating-point that conforms to
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the IEC/IEEE Standard for Binary Floating-Point Arithmetic. SoftFloat can
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support four floating-point formats: single precision, double precision,
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extended double precision, and quadruple precision. All operations required
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by the IEEE Standard are implemented, except for conversions to and from
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decimal. SoftFloat is distributed in the form of C source code, so a
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C compiler is needed to compile the code. Support for the extended double-
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precision and quadruple-precision formats is dependent on the C compiler
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implementing a 64-bit integer type.
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This document gives information needed for compiling and/or porting
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SoftFloat.
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The source code for SoftFloat is intended to be relatively machine-
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independent and should be compilable using any ISO/ANSI C compiler. At the
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time of this writing, SoftFloat has been successfully compiled with the GNU
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C Compiler (`gcc') for several platforms.
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-------------------------------------------------------------------------------
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Limitations
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SoftFloat as written requires an ISO/ANSI-style C compiler. No attempt has
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been made to accomodate compilers that are not ISO-conformant. Older ``K&R-
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style'' compilers are not adequate for compiling SoftFloat. All testing I
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have done so far has been with the GNU C Compiler. Compilation with other
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compilers should be possible but has not been tested.
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The SoftFloat sources assume that source code file names can be longer than
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8 characters. In order to compile under an MS-DOS-type system, many of the
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source files will need to be renamed, and the source and makefiles edited
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appropriately. Once compiled, the SoftFloat binary does not depend on the
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existence of long file names.
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The underlying machine is assumed to be binary with a word size that is a
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power of 2. Bytes are 8 bits. Support for the extended double-precision
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and quadruple-precision formats depends on the C compiler implementing
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a 64-bit integer type. If the largest integer type supported by the
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C compiler is 32 bits, SoftFloat is limited to the single- and double-
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precision formats.
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-------------------------------------------------------------------------------
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Contents
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Introduction
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Limitations
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Contents
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Legal Notice
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SoftFloat Source Directory Structure
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SoftFloat Source Files
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processors/*.h
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softfloat/bits*/*/softfloat.h
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softfloat/bits*/*/milieu.h
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softfloat/bits*/*/softfloat-specialize
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softfloat/bits*/softfloat-macros
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softfloat/bits*/softfloat.c
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Steps to Creating a `softfloat.o'
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Making `softfloat.o' a Library
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Testing SoftFloat
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Timing SoftFloat
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Compiler Options and Efficiency
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Processor-Specific Optimization of `softfloat.c' Using `softfloat-macros'
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Contact Information
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-------------------------------------------------------------------------------
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Legal Notice
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SoftFloat was written by John R. Hauser. This work was made possible in
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part by the International Computer Science Institute, located at Suite 600,
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1947 Center Street, Berkeley, California 94704. Funding was partially
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provided by the National Science Foundation under grant MIP-9311980. The
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original version of this code was written as part of a project to build
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a fixed-point vector processor in collaboration with the University of
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California at Berkeley, overseen by Profs. Nelson Morgan and John Wawrzynek.
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THIS SOFTWARE IS DISTRIBUTED AS IS, FOR FREE. Although reasonable effort
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has been made to avoid it, THIS SOFTWARE MAY CONTAIN FAULTS THAT WILL AT
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TIMES RESULT IN INCORRECT BEHAVIOR. USE OF THIS SOFTWARE IS RESTRICTED TO
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PERSONS AND ORGANIZATIONS WHO CAN AND WILL TAKE FULL RESPONSIBILITY FOR ANY
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AND ALL LOSSES, COSTS, OR OTHER PROBLEMS ARISING FROM ITS USE.
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-------------------------------------------------------------------------------
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SoftFloat Source Directory Structure
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Because SoftFloat is targeted to multiple platforms, its source code
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is slightly scattered between target-specific and target-independent
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directories and files. The directory structure is as follows:
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processors
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softfloat
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bits64
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templates
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386-Win32-gcc
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SPARC-Solaris-gcc
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bits32
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templates
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386-Win32-gcc
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SPARC-Solaris-gcc
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The two topmost directories and their contents are:
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softfloat - Most of the source code needed for SoftFloat.
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processors - Target-specific header files that are not specific to
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SoftFloat.
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The `softfloat' directory is further split into two parts:
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bits64 - SoftFloat implementation using 64-bit integers.
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bits32 - SoftFloat implementation using only 32-bit integers.
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Within these directories are subdirectories for each of the targeted
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platforms. The SoftFloat source code is distributed with targets
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`386-Win32-gcc' and `SPARC-Solaris-gcc' (and perhaps others) already
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prepared for both the 32-bit and 64-bit implementations. Source files that
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are not within these target-specific subdirectories are intended to be
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target-independent.
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The naming convention used for the target-specific directories is
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`<processor>-<executable-type>-<compiler>'. The names of the supplied
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target directories should be interpreted as follows:
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<processor>:
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386 - Intel 386-compatible processor.
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SPARC - SPARC processor (as used by Sun machines).
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<executable-type>:
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Win32 - Microsoft Win32 executable.
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Solaris - Sun Solaris executable.
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<compiler>:
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gcc - GNU C Compiler.
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You do not need to maintain this convention if you do not want to.
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Alongside the supplied target-specific directories is a `templates'
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directory containing a set of ``generic'' target-specific source files. A
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new target directory can be created by copying the `templates' directory and
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editing the files inside. (Complete instructions for porting SoftFloat to a
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new target are in the section _Steps_to_Creating_a_`softfloat.o'_.) Note
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that the `templates' directory will not work as a target directory without
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some editing. To avoid confusion, it would be wise to refrain from editing
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the files inside `templates' directly.
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-------------------------------------------------------------------------------
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SoftFloat Source Files
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The purpose of each source file is described below. In the following,
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the `*' symbol is used in place of the name of a specific target, such as
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`386-Win32-gcc' or `SPARC-Solaris-gcc', or in place of some other text, as
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in `bits*' for either `bits32' or `bits64'.
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- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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processors/*.h
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The target-specific `processors' header file defines integer types
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of various sizes, and also defines certain C preprocessor macros that
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characterize the target. The two examples supplied are `386-gcc.h' and
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`SPARC-gcc.h'. The naming convention used for processor header files is
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`<processor>-<compiler>.h'.
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If 64-bit integers are supported by the compiler, the macro name `BITS64'
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should be defined here along with the corresponding 64-bit integer
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types. In addition, the function-like macro `LIT64' must be defined for
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constructing 64-bit integer literals (constants). The `LIT64' macro is used
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consistently in the SoftFloat code to annotate 64-bit literals.
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If `BITS64' is not defined, only the 32-bit version of SoftFloat can be
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compiled. If `BITS64' _is_ defined, either can be compiled.
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If an inlining attribute (such as an `inline' keyword) is provided by the
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compiler, the macro `INLINE' should be defined to the appropriate keyword.
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If not, `INLINE' can be set to the keyword `static'. The `INLINE' macro
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appears in the SoftFloat source code before every function that should
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be inlined by the compiler. SoftFloat depends on inlining to obtain
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good speed. Even if inlining cannot be forced with a language keyword,
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the compiler may still be able to perform inlining on its own as an
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optimization. If a command-line option is needed to convince the compiler
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to perform this optimization, this should be assured in the makefile. (See
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the section _Compiler_Options_and_Efficiency_ below.)
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- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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softfloat/bits*/*/softfloat.h
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The target-specific `softfloat.h' header file defines the SoftFloat
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interface as seen by clients.
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Unlike the actual function definitions in `softfloat.c', the declarations
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in `softfloat.h' do not use any of the types defined by the `processors'
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header file. This is done so that clients will not have to include the
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`processors' header file in order to use SoftFloat. Nevertheless, the
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target-specific declarations in `softfloat.h' must match what `softfloat.c'
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expects. For example, if `int32' is defined as `int' in the `processors'
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header file, then in `softfloat.h' the output of `float32_to_int32' should
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be stated as `int', although in `softfloat.c' it is given in target-
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independent form as `int32'.
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For the `bits64' implementation of SoftFloat, the macro names `FLOATX80' and
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`FLOAT128' must be defined in order for the extended double-precision and
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quadruple-precision formats to be enabled in the code. Conversely, either
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or both of the extended formats can be disabled by simply removing the
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`#define' of the respective macro. When an extended format is not enabled,
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none of the functions that either input or output the format are defined,
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and no space is taken up in `softfloat.o' by such functions. There is no
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provision for disabling the usual single- and double-precision formats.
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- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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softfloat/bits*/*/milieu.h
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The target-specific `milieu.h' header file provides declarations that are
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needed to compile SoftFloat. In addition, deviations from ISO/ANSI C by
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the compiler (such as names not properly declared in system header files)
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are corrected in this header if possible.
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- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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softfloat/bits*/*/softfloat-specialize
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This target-specific C source fragment defines:
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-- whether tininess for underflow is detected before or after rounding by
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default;
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-- what (if anything) special happens when exceptions are raised;
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-- how signaling NaNs are distinguished from quiet NaNs;
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-- the default generated quiet NaNs; and
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-- how NaNs are propagated from function inputs to output.
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These details are not decided by the IEC/IEEE Standard. This fragment is
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included verbatim within `softfloat.c' when SoftFloat is compiled.
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- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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softfloat/bits*/softfloat-macros
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This target-independent C source fragment defines a number of arithmetic
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functions used as primitives within the `softfloat.c' source. Most of the
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functions defined here are intended to be inlined for efficiency. This
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fragment is included verbatim within `softfloat.c' when SoftFloat is
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compiled.
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Target-specific variations on this file are possible. See the section
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_Processor-Specific_Optimization_of_`softfloat.c'_Using_`softfloat-macros'_
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below.
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- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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softfloat/bits*/softfloat.c
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The target-independent `softfloat.c' source file contains the body of the
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SoftFloat implementation.
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- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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The inclusion of the files above within each other (using `#include') can be
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shown graphically as follows:
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softfloat/bits*/softfloat.c
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softfloat/bits*/*/milieu.h
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processors/*.h
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softfloat/bits*/*/softfloat.h
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softfloat/bits*/*/softfloat-specialize
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softfloat/bits*/softfloat-macros
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Note in particular that `softfloat.c' does not include the `processors'
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header file directly. Rather, `softfloat.c' includes the target-specific
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`milieu.h' header file, which in turn includes the processor header file.
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-------------------------------------------------------------------------------
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Steps to Creating a `softfloat.o'
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Porting and/or compiling SoftFloat involves the following steps:
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1. If one does not already exist, create an appropriate `.h' file in the
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`processors' directory.
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2. If `BITS64' is defined in the `processors' header file, choose whether
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to compile the 32-bit or 64-bit implementation of SoftFloat. If
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`BITS64' is not defined, your only choice is the 32-bit implementation.
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The remaining steps occur within either the `bits32' or `bits64'
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subdirectories.
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3. If one does not already exist, create an appropriate target-specific
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subdirectory by copying the given `templates' directory.
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4. In the target-specific subdirectory, edit the files `softfloat-specialize'
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and `softfloat.h' to define the desired exception handling functions
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and mode control values. In the `softfloat.h' header file, ensure also
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that all declarations give the proper target-specific type (such as
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`int' or `long') corresponding to the target-independent type used in
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`softfloat.c' (such as `int32'). None of the type names declared in the
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`processors' header file should appear in `softfloat.h'.
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5. In the target-specific subdirectory, edit the files `milieu.h' and
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`Makefile' to reflect the current environment.
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6. In the target-specific subdirectory, execute `make'.
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For the targets that are supplied, if the expected compiler is available
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(usually `gcc'), it should only be necessary to execute `make' in the
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target-specific subdirectory.
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-------------------------------------------------------------------------------
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Making `softfloat.o' a Library
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SoftFloat is not made into a software library by the supplied makefile.
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If desired, `softfloat.o' can easily be put into its own library (in Unix,
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`softfloat.a') using the usual system tool (in Unix, `ar').
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-------------------------------------------------------------------------------
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Testing SoftFloat
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SoftFloat can be tested using the `testsoftfloat' program by the same
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author. The `testsoftfloat' program is part of the TestFloat package
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available at the Web page `http://HTTP.CS.Berkeley.EDU/~jhauser/arithmetic/
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TestFloat.html'.
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-------------------------------------------------------------------------------
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Timing SoftFloat
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A program called `timesoftfloat' for timing the SoftFloat functions is
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included with the SoftFloat source code. Compiling `timesoftfloat' should
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pose no difficulties once `softfloat.o' exists. The supplied makefile
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will create a `timesoftfloat' executable by default after generating
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`softfloat.o'. See `timesoftfloat.txt' for documentation about using
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`timesoftfloat'.
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-------------------------------------------------------------------------------
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Compiler Options and Efficiency
|
||
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|
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In order to get good speed with SoftFloat, it is important that the compiler
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|
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inline the routines that have been marked `INLINE' in the code. Even if
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|
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inlining cannot be forced by an appropriate definition of the `INLINE'
|
||
|
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macro, the compiler may still be able to perform inlining on its own as
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|
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an optimization. In that case, the makefile should be edited to give the
|
||
|
|
compiler whatever option is required to cause it to inline small functions.
|
||
|
|
|
||
|
|
The ability of the processor to do fast shifts has been assumed. Efficiency
|
||
|
|
will not be as good on processors for which this is not the case (such as
|
||
|
|
the original Motorola 68000 or Intel 8086 processors).
|
||
|
|
|
||
|
|
|
||
|
|
-------------------------------------------------------------------------------
|
||
|
|
Processor-Specific Optimization of `softfloat.c' Using `softfloat-macros'
|
||
|
|
|
||
|
|
The `softfloat-macros' source fragment defines arithmetic functions used
|
||
|
|
as primitives by `softfloat.c'. This file has been written in a target-
|
||
|
|
independent form. For a given target, it may be possible to improve on
|
||
|
|
these functions using target-specific and/or non-ISO-C features (such
|
||
|
|
as `asm' statements). For example, one of the ``macro'' functions takes
|
||
|
|
two word-size integers and returns their full product in two words.
|
||
|
|
This operation can be done directly in hardware on many processors; but
|
||
|
|
because it is not available through standard C, the function defined in
|
||
|
|
`softfloat-macros' uses four multiplies to achieve the same result.
|
||
|
|
|
||
|
|
To address these shortcomings, a customized version of `softfloat-macros'
|
||
|
|
can be created in any of the target-specific subdirectories. A simple
|
||
|
|
modification to the target's makefile should be sufficient to ensure that
|
||
|
|
the custom version is used instead of the generic one.
|
||
|
|
|
||
|
|
|
||
|
|
-------------------------------------------------------------------------------
|
||
|
|
Contact Information
|
||
|
|
|
||
|
|
At the time of this writing, the most up-to-date information about
|
||
|
|
SoftFloat and the latest release can be found at the Web page `http://
|
||
|
|
HTTP.CS.Berkeley.EDU/~jhauser/arithmetic/SoftFloat.html'.
|
||
|
|
|
||
|
|
|