262 lines
8.3 KiB
Plaintext
262 lines
8.3 KiB
Plaintext
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*********************************
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* Announcing FDLIBM Version 5.3 *
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*********************************
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============================================================
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FDLIBM
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============================================================
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developed at Sun Microsystems, Inc.
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What's new in FDLIBM 5.3?
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CONFIGURE
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To build FDLIBM, edit the supplied Makefile or create
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a local Makefile by running "sh configure"
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using the supplied configure script contributed by Nelson Beebe
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BUGS FIXED
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1. e_pow.c incorrect results when
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x is very close to -1.0 and y is very large, e.g.
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pow(-1.0000000000000002e+00,4.5035996273704970e+15) = 0
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pow(-9.9999999999999978e-01,4.5035996273704970e+15) = 0
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Correct results are close to -e and -1/e.
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2. k_tan.c error was > 1 ulp target for FDLIBM
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5.2: Worst error at least 1.45 ulp at
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tan(1.7765241907548024E+269) = 1.7733884462610958E+16
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5.3: Worst error 0.96 ulp
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NOT FIXED YET
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3. Compiler failure on non-standard code
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Statements like
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*(1+(int*)&t1) = 0;
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are not standard C and cause some optimizing compilers (e.g. GCC)
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to generate bad code under optimization. These cases
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are to be addressed in the next release.
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FDLIBM (Freely Distributable LIBM) is a C math library
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for machines that support IEEE 754 floating-point arithmetic.
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In this release, only double precision is supported.
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FDLIBM is intended to provide a reasonably portable (see
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assumptions below), reference quality (below one ulp for
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major functions like sin,cos,exp,log) math library
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(libm.a). For a copy of FDLIBM, please see
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http://www.netlib.org/fdlibm/
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or
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http://www.validlab.com/software/
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--------------
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1. ASSUMPTIONS
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--------------
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FDLIBM (double precision version) assumes:
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a. IEEE 754 style (if not precise compliance) arithmetic;
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b. 32 bit 2's complement integer arithmetic;
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c. Each double precision floating-point number must be in IEEE 754
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double format, and that each number can be retrieved as two 32-bit
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integers through the using of pointer bashing as in the example
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below:
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Example: let y = 2.0
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double fp number y: 2.0
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IEEE double format: 0x4000000000000000
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Referencing y as two integers:
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*(int*)&y,*(1+(int*)&y) = {0x40000000,0x0} (on sparc)
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{0x0,0x40000000} (on 386)
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Note: Four macros are defined in fdlibm.h to handle this kind of
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retrieving:
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__HI(x) the high part of a double x
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(sign,exponent,the first 21 significant bits)
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__LO(x) the least 32 significant bits of x
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__HIp(x) same as __HI except that the argument is a pointer
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to a double
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__LOp(x) same as __LO except that the argument is a pointer
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to a double
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To ensure obtaining correct ordering, one must define __LITTLE_ENDIAN
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during compilation for little endian machine (like 386,486). The
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default is big endian.
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If the behavior of pointer bashing is undefined, one may hack on the
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macro in fdlibm.h.
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d. IEEE exceptions may trigger "signals" as is common in Unix
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implementations.
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-------------------
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2. EXCEPTION CASES
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-------------------
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All exception cases in the FDLIBM functions will be mapped
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to one of the following four exceptions:
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+-huge*huge, +-tiny*tiny, +-1.0/0.0, +-0.0/0.0
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(overflow) (underflow) (divided-by-zero) (invalid)
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For example, ieee_log(0) is a singularity and is thus mapped to
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-1.0/0.0 = -infinity.
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That is, FDLIBM's log will compute -one/zero and return the
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computed value. On an IEEE machine, this will trigger the
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divided-by-zero exception and a negative infinity is returned by
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default.
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Similarly, ieee_exp(-huge) will be mapped to tiny*tiny to generate
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an underflow signal.
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--------------------------------
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3. STANDARD CONFORMANCE WRAPPER
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--------------------------------
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The default FDLIBM functions (compiled with -D_IEEE_LIBM flag)
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are in "IEEE spirit" (i.e., return the most reasonable result in
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floating-point arithmetic). If one wants FDLIBM to comply with
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standards like SVID, X/OPEN, or POSIX/ANSI, then one can
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create a multi-standard compliant FDLIBM. In this case, each
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function in FDLIBM is actually a standard compliant wrapper
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function.
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File organization:
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1. For FDLIBM's kernel (internal) function,
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File name Entry point
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---------------------------
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k_sin.c __kernel_sin
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k_tan.c __kernel_tan
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---------------------------
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2. For functions that have no standards conflict
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File name Entry point
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---------------------------
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s_sin.c sin
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s_erf.c erf
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---------------------------
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3. Ieee754 core functions
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File name Entry point
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---------------------------
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e_exp.c __ieee754_exp
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e_sinh.c __ieee754_sinh
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---------------------------
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4. Wrapper functions
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File name Entry point
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---------------------------
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w_exp.c exp
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w_sinh.c sinh
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---------------------------
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Wrapper functions will twist the result of the ieee754
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function to comply to the standard specified by the value
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of _LIB_VERSION
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if _LIB_VERSION = _IEEE_, return the ieee754 result;
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if _LIB_VERSION = _SVID_, return SVID result;
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if _LIB_VERSION = _XOPEN_, return XOPEN result;
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if _LIB_VERSION = _POSIX_, return POSIX/ANSI result.
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(These are macros, see fdlibm.h for their definition.)
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--------------------------------
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4. HOW TO CREATE FDLIBM's libm.a
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--------------------------------
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There are two types of libm.a. One is IEEE only, and the other is
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multi-standard compliant (supports IEEE,XOPEN,POSIX/ANSI,SVID).
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To create the IEEE only libm.a, use
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make "CFLAGS = -D_IEEE_LIBM"
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This will create an IEEE libm.a, which is smaller in size, and
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somewhat faster.
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To create a multi-standard compliant libm, use
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make "CFLAGS = -D_IEEE_MODE" --- multi-standard fdlibm: default
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to IEEE
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make "CFLAGS = -D_XOPEN_MODE" --- multi-standard fdlibm: default
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to X/OPEN
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make "CFLAGS = -D_POSIX_MODE" --- multi-standard fdlibm: default
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to POSIX/ANSI
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make "CFLAGS = -D_SVID3_MODE" --- multi-standard fdlibm: default
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to SVID
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Here is how one makes a SVID compliant libm.
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Make the library by
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make "CFLAGS = -D_SVID3_MODE".
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The libm.a of FDLIBM will be multi-standard compliant and
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_LIB_VERSION is initialized to the value _SVID_ .
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example1:
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---------
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main()
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{
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double ieee_y0();
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printf("y0(1e300) = %1.20e\n",y0(1e300));
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exit(0);
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}
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% cc example1.c libm.a
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% a.out
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y0: TLOSS error
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ieee_y0(1e300) = 0.00000000000000000000e+00
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It is possible to change the default standard in multi-standard
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fdlibm. Here is an example of how to do it:
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example2:
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---------
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#include "fdlibm.h" /* must include FDLIBM's fdlibm.h */
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main()
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{
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double ieee_y0();
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_LIB_VERSION = _IEEE_;
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printf("IEEE: ieee_y0(1e300) = %1.20e\n",y0(1e300));
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_LIB_VERSION = _XOPEN_;
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printf("XOPEN ieee_y0(1e300) = %1.20e\n",y0(1e300));
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_LIB_VERSION = _POSIX_;
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printf("POSIX ieee_y0(1e300) = %1.20e\n",y0(1e300));
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_LIB_VERSION = _SVID_;
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printf("SVID ieee_y0(1e300) = %1.20e\n",y0(1e300));
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exit(0);
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}
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% cc example2.c libm.a
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% a.out
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IEEE: ieee_y0(1e300) = -1.36813604503424810557e-151
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XOPEN ieee_y0(1e300) = 0.00000000000000000000e+00
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POSIX ieee_y0(1e300) = 0.00000000000000000000e+00
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y0: TLOSS error
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SVID ieee_y0(1e300) = 0.00000000000000000000e+00
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Note: Here _LIB_VERSION is a global variable. If global variables
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are forbidden, then one should modify fdlibm.h to change
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_LIB_VERSION to be a global constant. In this case, one
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may not change the value of _LIB_VERSION as in example2.
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---------------------------
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5. NOTES ON PORTING FDLIBM
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---------------------------
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Care must be taken when installing FDLIBM over existing
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libm.a.
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All co-existing function prototypes must agree, otherwise
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users will encounter mysterious failures.
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So far, the only known likely conflict is the declaration
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of the IEEE recommended function scalb:
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double ieee_scalb(double,double) (1) SVID3 defined
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double ieee_scalb(double,int) (2) IBM,DEC,...
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FDLIBM follows Sun definition and use (1) as default.
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If one's existing libm.a uses (2), then one may raise
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the flags _SCALB_INT during the compilation of FDLIBM
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to get the correct function prototype.
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(E.g., make "CFLAGS = -D_IEEE_LIBM -D_SCALB_INT".)
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NOTE that if -D_SCALB_INT is raised, it won't be SVID3
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conformant.
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--------------
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6. PROBLEMS ?
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--------------
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Please send comments and bug reports to the electronic mail address
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suggested by:
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fdlibm-comments AT sun.com
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