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readme

      1 
      2 	  *********************************
      3  	  * Announcing FDLIBM Version 5.3 *
      4 	  *********************************
      5 ============================================================
      6 			FDLIBM
      7 ============================================================
      8 	developed at Sun Microsystems, Inc. 
      9 
     10 What's new in FDLIBM 5.3?
     11 
     12 CONFIGURE
     13 	To build FDLIBM, edit the supplied Makefile or create
     14 	a local Makefile by running "sh configure" 
     15 	using the supplied configure script contributed by Nelson Beebe
     16 
     17 BUGS FIXED
     18 
     19     1. e_pow.c incorrect results when 
     20 	x is very close to -1.0 and y is very large, e.g.
     21   	pow(-1.0000000000000002e+00,4.5035996273704970e+15) = 0
     22   	pow(-9.9999999999999978e-01,4.5035996273704970e+15) = 0
     23 	Correct results are close to -e and -1/e.
     24 
     25     2. k_tan.c error was > 1 ulp target for FDLIBM
     26 	5.2: Worst error at least 1.45 ulp at
     27 	tan(1.7765241907548024E+269) = 1.7733884462610958E+16
     28 	5.3: Worst error 0.96 ulp
     29 
     30 NOT FIXED YET
     31 
     32     3. Compiler failure on non-standard code
     33 	Statements like
     34 	            *(1+(int*)&t1) = 0;
     35 	are not standard C and cause some optimizing compilers (e.g. GCC)
     36 	to generate bad code under optimization.    These cases
     37 	are to be addressed in the next release.
     38 	
     39 FDLIBM (Freely Distributable LIBM) is a C math library 
     40 for machines that support IEEE 754 floating-point arithmetic. 
     41 In this release, only double precision is supported.
     42 
     43 FDLIBM is intended to provide a reasonably portable (see 
     44 assumptions below), reference quality (below one ulp for
     45 major functions like sin,cos,exp,log) math library 
     46 (libm.a).  For a copy of FDLIBM, please see
     47 	http://www.netlib.org/fdlibm/
     48 or
     49 	http://www.validlab.com/software/
     50 
     51 --------------
     52 1. ASSUMPTIONS
     53 --------------
     54 FDLIBM (double precision version) assumes:
     55  a.  IEEE 754 style (if not precise compliance) arithmetic;
     56  b.  32 bit 2's complement integer arithmetic;
     57  c.  Each double precision floating-point number must be in IEEE 754 
     58      double format, and that each number can be retrieved as two 32-bit 
     59      integers through the using of pointer bashing as in the example 
     60      below:
     61 
     62      Example: let y = 2.0
     63 	double fp number y: 	2.0
     64 	IEEE double format:	0x4000000000000000
     65 
     66 	Referencing y as two integers:
     67 	*(int*)&y,*(1+(int*)&y) =	{0x40000000,0x0} (on sparc)
     68 					{0x0,0x40000000} (on 386)
     69 
     70 	Note: Four macros are defined in fdlibm.h to handle this kind of
     71 	retrieving:
     72 
     73 	__HI(x)		the high part of a double x 
     74 			(sign,exponent,the first 21 significant bits)
     75 	__LO(x)		the least 32 significant bits of x
     76 	__HIp(x)	same as __HI except that the argument is a pointer
     77 			to a double
     78 	__LOp(x)	same as __LO except that the argument is a pointer
     79 			to a double
     80 	
     81 	To ensure obtaining correct ordering, one must define  __LITTLE_ENDIAN
     82 	during compilation for little endian machine (like 386,486). The 
     83 	default is big endian.
     84 
     85 	If the behavior of pointer bashing is undefined, one may hack on the 
     86 	macro in fdlibm.h.
     87 	
     88   d. IEEE exceptions may trigger "signals" as is common in Unix
     89      implementations. 
     90 
     91 -------------------
     92 2. EXCEPTION CASES
     93 -------------------
     94 All exception cases in the FDLIBM functions will be mapped
     95 to one of the following four exceptions:
     96 
     97    +-huge*huge, +-tiny*tiny,    +-1.0/0.0,	+-0.0/0.0
     98     (overflow)	(underflow)  (divided-by-zero) 	(invalid)
     99 
    100 For example, ieee_log(0) is a singularity and is thus mapped to 
    101 	-1.0/0.0 = -infinity.
    102 That is, FDLIBM's log will compute -one/zero and return the
    103 computed value.  On an IEEE machine, this will trigger the 
    104 divided-by-zero exception and a negative infinity is returned by 
    105 default.
    106 
    107 Similarly, ieee_exp(-huge) will be mapped to tiny*tiny to generate
    108 an underflow signal. 
    109 
    110 
    111 --------------------------------
    112 3. STANDARD CONFORMANCE WRAPPER 
    113 --------------------------------
    114 The default FDLIBM functions (compiled with -D_IEEE_LIBM flag)  
    115 are in "IEEE spirit" (i.e., return the most reasonable result in 
    116 floating-point arithmetic). If one wants FDLIBM to comply with
    117 standards like SVID, X/OPEN, or POSIX/ANSI, then one can 
    118 create a multi-standard compliant FDLIBM. In this case, each
    119 function in FDLIBM is actually a standard compliant wrapper
    120 function.  
    121 
    122 File organization:
    123     1. For FDLIBM's kernel (internal) function,
    124 		File name	Entry point
    125 		---------------------------
    126 		k_sin.c		__kernel_sin
    127 		k_tan.c		__kernel_tan
    128 		---------------------------
    129     2. For functions that have no standards conflict 
    130 		File name	Entry point
    131 		---------------------------
    132 		s_sin.c		sin
    133 		s_erf.c		erf
    134 		---------------------------
    135     3. Ieee754 core functions
    136 		File name	Entry point
    137 		---------------------------
    138 		e_exp.c		__ieee754_exp
    139 		e_sinh.c	__ieee754_sinh
    140 		---------------------------
    141     4. Wrapper functions
    142 		File name	Entry point
    143 		---------------------------
    144 		w_exp.c		exp
    145 		w_sinh.c	sinh
    146 		---------------------------
    147 
    148 Wrapper functions will twist the result of the ieee754 
    149 function to comply to the standard specified by the value 
    150 of _LIB_VERSION 
    151     if _LIB_VERSION = _IEEE_, return the ieee754 result;
    152     if _LIB_VERSION = _SVID_, return SVID result;
    153     if _LIB_VERSION = _XOPEN_, return XOPEN result;
    154     if _LIB_VERSION = _POSIX_, return POSIX/ANSI result.
    155 (These are macros, see fdlibm.h for their definition.)
    156 
    157 
    158 --------------------------------
    159 4. HOW TO CREATE FDLIBM's libm.a
    160 --------------------------------
    161 There are two types of libm.a. One is IEEE only, and the other is
    162 multi-standard compliant (supports IEEE,XOPEN,POSIX/ANSI,SVID).
    163 
    164 To create the IEEE only libm.a, use 
    165 	    make "CFLAGS = -D_IEEE_LIBM"	 
    166 This will create an IEEE libm.a, which is smaller in size, and 
    167 somewhat faster.
    168 
    169 To create a multi-standard compliant libm, use
    170     make "CFLAGS = -D_IEEE_MODE"   --- multi-standard fdlibm: default
    171 					 to IEEE
    172     make "CFLAGS = -D_XOPEN_MODE"  --- multi-standard fdlibm: default
    173 					 to X/OPEN
    174     make "CFLAGS = -D_POSIX_MODE"  --- multi-standard fdlibm: default
    175 					 to POSIX/ANSI
    176     make "CFLAGS = -D_SVID3_MODE"  --- multi-standard fdlibm: default
    177 					 to SVID
    178 
    179 
    180 Here is how one makes a SVID compliant libm.
    181     Make the library by
    182 		make "CFLAGS = -D_SVID3_MODE".
    183     The libm.a of FDLIBM will be multi-standard compliant and 
    184     _LIB_VERSION is initialized to the value _SVID_ . 
    185 
    186     example1:
    187     ---------
    188 	    main()
    189 	    {
    190 		double ieee_y0();
    191 		printf("y0(1e300) = %1.20e\n",y0(1e300));
    192 		exit(0);
    193 	    }
    194 
    195     % cc example1.c libm.a
    196     % a.out
    197     y0: TLOSS error
    198     ieee_y0(1e300) = 0.00000000000000000000e+00
    199 
    200 
    201 It is possible to change the default standard in multi-standard 
    202 fdlibm. Here is an example of how to do it:
    203     example2:
    204     ---------
    205 	#include "fdlibm.h"	/* must include FDLIBM's fdlibm.h */
    206 	main()
    207 	{
    208 		double ieee_y0();
    209 		_LIB_VERSION =  _IEEE_;
    210 		printf("IEEE: ieee_y0(1e300) = %1.20e\n",y0(1e300));
    211 		_LIB_VERSION = _XOPEN_;
    212 		printf("XOPEN ieee_y0(1e300) = %1.20e\n",y0(1e300));
    213 		_LIB_VERSION = _POSIX_;
    214 		printf("POSIX ieee_y0(1e300) = %1.20e\n",y0(1e300));
    215 		_LIB_VERSION = _SVID_;
    216 		printf("SVID  ieee_y0(1e300) = %1.20e\n",y0(1e300));
    217 		exit(0);
    218 	}
    219 
    220     % cc example2.c libm.a
    221     % a.out
    222       IEEE: ieee_y0(1e300) = -1.36813604503424810557e-151
    223       XOPEN ieee_y0(1e300) = 0.00000000000000000000e+00
    224       POSIX ieee_y0(1e300) = 0.00000000000000000000e+00
    225       y0: TLOSS error
    226       SVID  ieee_y0(1e300) = 0.00000000000000000000e+00
    227 
    228 Note:	Here _LIB_VERSION is a global variable. If global variables 
    229 	are forbidden, then one should modify fdlibm.h to change
    230 	_LIB_VERSION to be a global constant. In this case, one
    231 	may not change the value of _LIB_VERSION as in example2.
    232 
    233 ---------------------------
    234 5. NOTES ON PORTING FDLIBM
    235 ---------------------------
    236 	Care must be taken when installing FDLIBM over existing
    237 	libm.a.
    238 	All co-existing function prototypes must agree, otherwise
    239 	users will encounter mysterious failures.
    240 
    241 	So far, the only known likely conflict is the declaration 
    242 	of the IEEE recommended function scalb:
    243 
    244 		double ieee_scalb(double,double)	(1)	SVID3 defined
    245 		double ieee_scalb(double,int)	(2)	IBM,DEC,...
    246 
    247 	FDLIBM follows Sun definition and use (1) as default. 
    248 	If one's existing libm.a uses (2), then one may raise
    249 	the flags _SCALB_INT during the compilation of FDLIBM
    250 	to get the correct function prototype.
    251 	(E.g., make "CFLAGS = -D_IEEE_LIBM -D_SCALB_INT".)
    252 	NOTE that if -D_SCALB_INT is raised, it won't be SVID3
    253 	conformant.
    254 
    255 --------------
    256 6. PROBLEMS ?
    257 --------------
    258 Please send comments and bug reports to the electronic mail address
    259 suggested by: 
    260 		fdlibm-comments AT sun.com
    261 
    262