IRIX Base Documentation 2001 May

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MMMMAAAATTTTHHHH((((3333MMMM)))) MMMMAAAATTTTHHHH((((3333MMMM)))) NNNNAAAAMMMMEEEE math - introduction to mathematical library functions DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN These functions constitute the C math library _l_i_b_m. There are four versions of the math library _l_i_b_m._a, _l_i_b_m_x._a, _l_i_b_m_4_3._a and _l_i_b_f_a_s_t_m._a The first, _l_i_b_m._a, contains routines newly implemented (1994) using algorithms which take advantage of the Mips architecture and includes many routines for the _f_l_o_a_t data type. For the -64 and -n32 versions of _l_i_b_m._a, a second version of the math library, _l_i_b_m_x._a, contains functions which give identical results to those in libm.a, but which use System V error handling. See matherr(3M) for a description of error handling for _l_i_b_m_x._a functions. The third version of the math library, _l_i_b_m_4_3._a, contains routines all based on the original codes in the 4.3BSD release. The difference between the error bounds for libm.a and libm43.a is typically around 1 unit in the last place, whereas the performance difference may be a factor of two or more. The link editor searches this library under the "-lm", "-lmx", or "-lm43" option. Declarations for these functions may be obtained from the include file <_m_a_t_h._h>. The fourth library, _l_i_b_f_a_s_t_m._a, contains faster, lower-precision versions of various routines from libm.a. LLLLIIIISSSSTTTT OOOOFFFF FFFFUUUUNNNNCCCCTTTTIIIIOOOONNNNSSSS Error bounds listed below apply only to the -64 and -n32 versions of _l_i_b_m._a and _l_i_b_m_x._a The error bound sometimes applies only to the primary range. _E_r_r_o_r _B_o_u_n_d (_U_L_P_s) _N_a_m_e _A_p_p_e_a_r_s _o_n _P_a_g_e _D_e_s_c_r_i_p_t_i_o_n _l_i_b_m._a _l_i_b_m_4_3._a acos sin(3M) inverse trigonometric function 2 3 acosf sin(3M) inverse trigonometric function 1 acosh asinh(3M) inverse hyperbolic function 3 3 asin sin(3M) inverse trigonometric function 2 3 asinf sin(3M) inverse trigonometric function 1 asinh asinh(3M) inverse hyperbolic function 3 3 atan sin(3M) inverse trigonometric function 1.5 1 atanf sin(3M) inverse trigonometric function 1 atanh asinh(3M) inverse hyperbolic function 3 3 atan2 sin(3M) inverse trigonometric function 2 2 atan2f sin(3M) inverse trigonometric function 1 cabs hypot(3M) complex absolute value 1 1 cabsf hypot(3M) complex absolute value 1 cbrt sqrt(3M) cube root 1 1 PPPPaaaaggggeeee 1111 MMMMAAAATTTTHHHH((((3333MMMM)))) MMMMAAAATTTTHHHH((((3333MMMM)))) ceil floor(3M) integer no less than 0 0 ceilf floor(3M) integer no less than 0 0 copysign ieee(3M) copy sign bit 0 0 cos sin(3M) trigonometric function 2 1 cosf sin(3M) trigonometric function 1 cosh sinh(3M) hyperbolic function 2 3 coshf sinh(3M) hyperbolic function 1 drem ieee(3M) remainder 0 0 erf erf(3M) error function ? ? erfc erf(3M) complementary error function ? ? exp exp(3M) exponential 1 1 expf exp(3M) exponential 1 expm1 exp(3M) exp(x)-1 1 1 expm1f exp(3M) exp(x)-1 1 fabs floor(3M) absolute value 0 0 fabsf floor(3M) absolute value 0 0 finite ieee(3M) floating point arithmetic (N/A) floor floor(3M) integer no greater than 0 0 floorf floor(3M) integer no greater than 0 0 fmod floor(3M) remainder function 0 fmodf floor(3M) remainder function 0 hypot hypot(3M) Euclidean distance 1 1 hypotf hypot(3M) Euclidean distance 1 1 j0 j0(3M) bessel function ? ? j1 j0(3M) bessel function ? ? jn j0(3M) bessel function ? ? lgamma lgamma(3M) log gamma function ? ? log exp(3M) natural logarithm 1 1 logf exp(3M) natural logarithm 1 logb ieee(3M) exponent extraction 0 0 log10 exp(3M) logarithm to base 10 2 3 log10f exp(3M) logarithm to base 10 1.5 log1p exp(3M) log(1+x) 1 1 log1pf exp(3M) log(1+x) 1 1 pow exp(3M) exponential x**y 2 60-500 powf exp(3M) exponential x**y 1 rint floor(3M) round to nearest integer 0 0 sin sin(3M) trigonometric function 2 1 sinf sin(3M) trigonometric function 1 sinh sinh(3M) hyperbolic function 2 3 sinhf sinh(3M) hyperbolic function 1 sqrt sqrt(3M) square root 1 1 sqrtf sqrt(3M) square root 1 tan sin(3M) trigonometric function 2 3 tanf sin(3M) trigonometric function 1 tanh sinh(3M) hyperbolic function 2 3 tanhf sinh(3M) hyperbolic function 1 trunc floor(3M) truncate to whole number 0 0 truncf floor(3M) truncate to whole number 0 0 y0 j0(3M) bessel function ? ? y1 j0(3M) bessel function ? ? yn j0(3M) bessel function ? ? PPPPaaaaggggeeee 2222 MMMMAAAATTTTHHHH((((3333MMMM)))) MMMMAAAATTTTHHHH((((3333MMMM)))) VVVVEEEECCCCTTTTOOOORRRR IIIINNNNTTTTRRRRIIIINNNNSSSSIIIICCCCSSSS Beginning with IRIX 6.2, libm now supports the following vector intrinsics: /* single precision vector routines */ vvvvaaaaccccoooossssffff(((( ffffllllooooaaaatttt ****xxxx,,,, ffffllllooooaaaatttt ****yyyy,,,, lllloooonnnngggg ccccoooouuuunnnntttt,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeexxxx,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeeyyyy )))) vvvvaaaassssiiiinnnnffff(((( ffffllllooooaaaatttt ****xxxx,,,, ffffllllooooaaaatttt ****yyyy,,,, lllloooonnnngggg ccccoooouuuunnnntttt,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeexxxx,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeeyyyy )))) vvvvaaaattttaaaannnnffff(((( ffffllllooooaaaatttt ****xxxx,,,, ffffllllooooaaaatttt ****yyyy,,,, lllloooonnnngggg ccccoooouuuunnnntttt,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeexxxx,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeeyyyy )))) vvvvccccoooossssffff(((( ffffllllooooaaaatttt ****xxxx,,,, ffffllllooooaaaatttt ****yyyy,,,, lllloooonnnngggg ccccoooouuuunnnntttt,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeexxxx,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeeyyyy )))) vvvveeeexxxxppppffff(((( ffffllllooooaaaatttt ****xxxx,,,, ffffllllooooaaaatttt ****yyyy,,,, lllloooonnnngggg ccccoooouuuunnnntttt,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeexxxx,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeeyyyy )))) vvvvllllooooggggffff(((( ffffllllooooaaaatttt ****xxxx,,,, ffffllllooooaaaatttt ****yyyy,,,, lllloooonnnngggg ccccoooouuuunnnntttt,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeexxxx,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeeyyyy )))) vvvvlllloooogggg11110000ffff(((( ffffllllooooaaaatttt ****xxxx,,,, ffffllllooooaaaatttt ****yyyy,,,, lllloooonnnngggg ccccoooouuuunnnntttt,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeexxxx,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeeyyyy )))) vvvvssssiiiinnnnffff(((( ffffllllooooaaaatttt ****xxxx,,,, ffffllllooooaaaatttt ****yyyy,,,, lllloooonnnngggg ccccoooouuuunnnntttt,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeexxxx,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeeyyyy )))) vvvvssssqqqqrrrrttttffff(((( ffffllllooooaaaatttt ****xxxx,,,, ffffllllooooaaaatttt ****yyyy,,,, lllloooonnnngggg ccccoooouuuunnnntttt,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeexxxx,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeeyyyy )))) vvvvttttaaaannnnffff(((( ffffllllooooaaaatttt ****xxxx,,,, ffffllllooooaaaatttt ****yyyy,,,, lllloooonnnngggg ccccoooouuuunnnntttt,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeexxxx,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeeyyyy )))) /* double precision vector routines */ vvvvaaaaccccoooossss(((( ddddoooouuuubbbblllleeee ****xxxx,,,, ddddoooouuuubbbblllleeee ****yyyy,,,, lllloooonnnngggg ccccoooouuuunnnntttt,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeexxxx,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeeyyyy )))) vvvvaaaassssiiiinnnn(((( ddddoooouuuubbbblllleeee ****xxxx,,,, ddddoooouuuubbbblllleeee ****yyyy,,,, lllloooonnnngggg ccccoooouuuunnnntttt,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeexxxx,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeeyyyy )))) vvvvaaaattttaaaannnn(((( ddddoooouuuubbbblllleeee ****xxxx,,,, ddddoooouuuubbbblllleeee ****yyyy,,,, lllloooonnnngggg ccccoooouuuunnnntttt,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeexxxx,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeeyyyy )))) vvvvccccoooossss(((( ddddoooouuuubbbblllleeee ****xxxx,,,, ddddoooouuuubbbblllleeee ****yyyy,,,, lllloooonnnngggg ccccoooouuuunnnntttt,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeexxxx,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeeyyyy )))) vvvveeeexxxxpppp(((( ddddoooouuuubbbblllleeee ****xxxx,,,, ddddoooouuuubbbblllleeee ****yyyy,,,, lllloooonnnngggg ccccoooouuuunnnntttt,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeexxxx,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeeyyyy )))) vvvvlllloooogggg(((( ddddoooouuuubbbblllleeee ****xxxx,,,, ddddoooouuuubbbblllleeee ****yyyy,,,, lllloooonnnngggg ccccoooouuuunnnntttt,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeexxxx,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeeyyyy )))) vvvvlllloooogggg11110000(((( ddddoooouuuubbbblllleeee ****xxxx,,,, ddddoooouuuubbbblllleeee ****yyyy,,,, lllloooonnnngggg ccccoooouuuunnnntttt,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeexxxx,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeeyyyy )))) vvvvssssiiiinnnn(((( ddddoooouuuubbbblllleeee ****xxxx,,,, ddddoooouuuubbbblllleeee ****yyyy,,,, lllloooonnnngggg ccccoooouuuunnnntttt,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeexxxx,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeeyyyy )))) vvvvssssqqqqrrrrtttt(((( ddddoooouuuubbbblllleeee ****xxxx,,,, ddddoooouuuubbbblllleeee ****yyyy,,,, lllloooonnnngggg ccccoooouuuunnnntttt,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeexxxx,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeeyyyy )))) vvvvttttaaaannnn(((( ddddoooouuuubbbblllleeee ****xxxx,,,, ddddoooouuuubbbblllleeee ****yyyy,,,, lllloooonnnngggg ccccoooouuuunnnntttt,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeexxxx,,,, lllloooonnnngggg ssssttttrrrriiiiddddeeeeyyyy )))) Input and output arrays for the above routines should either be identical or non-overlapping. On Mips4 processors, these routines are software pipelined to take advantage of the multiple execution units. On that machine, throughput is up to several times greater than one gets by calling the scalar intrinsics repeatedly. On processors other than the Mips4, these routines are still available; although not software pipelined on those processors, they still eliminate considerable call overhead when they can be used. Note that the vector routines do not support denormals on the Mips4 processors. The single precision vector routines can also be called by the names vfacos, vfasin, etc. Semantics of these routines: i=0, 1, ..., count-1: y[i*stridey] = f(x[i*stridex]) Example: PPPPaaaaggggeeee 3333 MMMMAAAATTTTHHHH((((3333MMMM)))) MMMMAAAATTTTHHHH((((3333MMMM)))) double x[10000], y[10000]; for (i=0; i<1000; i++ ) y[2*i] = sin(x[3*i]); Transform (by hand) into vsin(x, y, 1000, 3, 2); Vector and scalar routines may differ slightly, however none of the results differ from the mathematically correct result by more than 2 ulps (units in the last place). Note that the vector square root routines are less accurate than the hardware versions; vsqrt and vsqrtf use the reciprocal square root instruction and lose up to about 2 bits of accuracy. vsqrt and vfsqrt give correct answers for zero and infinite arguments. LLLLOOOONNNNGGGG DDDDOOOOUUUUBBBBLLLLEEEE AAAARRRRIIIITTTTHHHHMMMMEEEETTTTIIIICCCC Long double arithmetic is supported by the MIPSpro compiler. The representation used is not IEEE compliant; long doubles are represented on this system as the sum or difference of two doubles, normalized so that the smaller double is <= .5 ulp of the larger. This is equivalent to a 107 bit mantissa with an 11 bit biased exponent (bias = 1023), and 1 sign bit. In terms of decimal precision, this is approximately 34 decimal digits. Long double constants are coded as double precision constants followed by the letter 'l' (upper or lower case). The largest (finite) long double constant is 1.797693134862315807937289714053023e308L . The smallest long double precision constant is 4.940656458412465441765687928682213e-324L . Long doubles less than 1.805194375864829576069262081173746e-276L may require a double denormal in their representation and therefore contain less than 107 bits precision. Long double NaNs and (signed) infinities are supported by the MIPSpro compiler. Long double infinity is represented as the sum of a double infinity and a double zero; similarly for NaNs. In Fortran, long doubles are denoted by the term REAL *16. In general, long double arithmetic operations (+, -, *, /) are not precisely rounded, but are accurate to approximately 3 ulps. Note that long double arithmetic operations are done in software by MIPSpro compilers; results of these operations may vary slightly from release to release due to improvements in the algorithms which implement them. PPPPaaaaggggeeee 4444 MMMMAAAATTTTHHHH((((3333MMMM)))) MMMMAAAATTTTHHHH((((3333MMMM)))) Long double operations on this system are only supported in round to nearest rounding mode (the default). The system must be in round to nearest rounding mode when issuing long double arithmetic operations or calling any of the long double functions, or incorrect answers will result. DDDDIIIIFFFFFFFFEEEERRRREEEENNNNCCCCEEEESSSS BBBBEEEETTTTWWWWEEEEEEEENNNN ----oooo33332222,,,, ----nnnn33332222,,,, ----66664444 For the IRIX 6.2 release, faster and more accurate algorithms were implemented, and vector functions were added to the math library. In order to maintain numerical compatibility with older releases, these changes were made only in the -n32 and -64 versions of the library and not in the -o32 version. ( Where there are differences in accuracy, this document describes the behavior of the -n32 and -64 versions of the library. ) To take advantage of the new functions and algorithms, you need to compile and link using either the -n32 or the -64 option. Note however, that the -o32 version of libmx contains all routines present in the -n32 and -64 versions of libmx except the quad precision and vector routines, and gives results identical to the -n32 and -64 versions. NNNNOOOOTTTTEEEESSSS Users concerned with portability to other computer systems should note that the long double and float versions of these functions are optional according to the ANSI C Programming Language Specification ISO/IEC 9899 : 1990 (E). Long double functions have been renamed to be compliant with the ANSI-C standard, however to be backward compatible, they may still be called with the double precision function name prefixed with a q. (Exceptions: functions _f_a_b_s_l and _f_m_o_d_l may be called with names _q_a_b_s and _q_m_o_d, resp.) In 4.3BSD, distributed from the University of California in late 1985, most of the foregoing functions come in two versions, one for the double-precision "D" format in the DEC VAX-11 family of computers, another for double-precision arithmetic conforming to the IEEE Standard 754 for Binary Floating-point Arithmetic. The two versions behave very similarly, as should be expected from programs more accurate and robust than was the norm when UNIX was born. For instance, the programs are accurate to within the numbers of _u_l_ps tabulated above; an _u_l_p is one _Unit in the _Last _Place. And the programs have been cured of anomalies that afflicted the older math library _l_i_b_m in which incidents like the following had been reported: sqrt(-1.0) = 0.0 and log(-1.0) = -1.7e38. cos(1.0e-11) > cos(0.0) > 1.0. pow(x,1.0) != x when x = 2.0, 3.0, 4.0, ..., 9.0. pow(-1.0,1.0e10) trapped on Integer Overflow. sqrt(1.0e30) and sqrt(1.0e-30) were very slow. This machine conforms to the IEEE Standard 754 for Binary Floating-point Arithmetic, to which only the notes for IEEE floating-point apply and are PPPPaaaaggggeeee 5555 MMMMAAAATTTTHHHH((((3333MMMM)))) MMMMAAAATTTTHHHH((((3333MMMM)))) included here. (See however, the notes regarding long double precision below.) IIIIEEEEEEEEEEEE SSSSTTTTAAAANNNNDDDDAAAARRRRDDDD 777755554444 FFFFllllooooaaaattttiiiinnnngggg----ppppooooiiiinnnntttt AAAArrrriiiitttthhhhmmmmeeeettttiiiicccc:::: This standard is on its way to becoming more widely adopted than any other design for computer arithmetic. Properties of IEEE 754 Double-precision: Wordsize: 64 bits, 8 bytes. Radix: Binary. Precision: 53 sig. bits, roughly 16 sig. decimals. If x and x' are consecutive positive Double-precision numbers (they differ by 1 _u_l_p), then 1.1e-16 < 0.5**53 < (x'-x)/x _< 0.5**52 < 2.3e-16. Range: Overflow threshold = 2.0**1024 = 1.8e308 Underflow threshold = 0.5**1022 = 2.2e-308 Overflow goes by default to a signed Infinity. Underflow is _G_r_a_d_u_a_l, rounding to the nearest integer multiple of 0.5**1074 = 4.9e-324. Zero is represented ambiguously as +0 or -0. Its sign transforms correctly through multiplication or division, and is preserved by addition of zeros with like signs; but x-x yields +0 for every finite x. The only operations that reveal zero's sign are division by zero and copysign(x,+_0). In particular, comparison (x > y, x _> y, etc.) cannot be affected by the sign of zero; but if finite x = y then Infinity = 1/(x-y) != -1/(y-x) = -Infinity. Infinity is signed. it persists when added to itself or to any finite number. Its sign transforms correctly through multiplication and division, and (finite)/+_Infinity = +_0 (nonzero)/0 = +_Infinity. But Infinity-Infinity, Infinity*0 and Infinity/Infinity are, like 0/0 and sqrt(-3), invalid operations that produce _N_a_N. Reserved operands: there are 2**53-2 of them, all called _N_a_N (_Not _a _Number). Some, called Signaling _N_a_Ns, trap any floating-point operation performed upon them; they could be used to mark missing or uninitialized values, or nonexistent elements of arrays. The rest are Quiet _N_a_Ns; they are the default results of Invalid Operations, and propagate through subsequent arithmetic operations. If x != x then x is _N_a_N; every other predicate (x > y, x = y, x < y, ...) is FALSE if _N_a_N is involved. NOTE: Trichotomy is violated by _N_a_N. Besides being FALSE, predicates that entail ordered comparison, rather than mere (in)equality, signal Invalid Operation when _N_a_N is involved. Rounding: Every algebraic operation (+, -, *, /, sqrt) is rounded by default to within half an _u_l_p, and when the rounding error is exactly half an _u_l_p then the rounded value's least significant bit is zero. This kind of rounding is usually the best kind, sometimes provably so; for instance, for every x = 1.0, 2.0, PPPPaaaaggggeeee 6666 MMMMAAAATTTTHHHH((((3333MMMM)))) MMMMAAAATTTTHHHH((((3333MMMM)))) 3.0, 4.0, ..., 2.0**52, we find (x/3.0)*3.0 == x and (x/10.0)*10.0 == x and ... despite that both the quotients and the products have been rounded. Only rounding like IEEE 754 can do that. But no single kind of rounding can be proved best for every circumstance, so IEEE 754 provides rounding towards zero or towards +Infinity or towards -Infinity at the programmer's option. Exceptions: IEEE 754 recognizes five kinds of floating-point exceptions, listed below in declining order of probable importance. Exception Default Result ------- ------- Invalid Operation _N_a_N, or FALSE Overflow +_Infinity Divide by Zero +_Infinity Underflow Gradual Underflow Inexact Rounded value NOTE: An Exception is not an Error unless handled badly. What makes a class of exceptions exceptional is that no single default response can be satisfactory in every instance. On the other hand, if a default response will serve most instances satisfactorily, the unsatisfactory instances cannot justify aborting computation every time the exception occurs. For each kind of floating-point exception, IEEE 754 provides a Flag that is raised each time its exception is signaled, and stays raised until the program resets it. Programs may also test, save and restore a flag. Thus, IEEE 754 provides three ways by which programs may cope with exceptions for which the default result might be unsatisfactory: 1) Test for a condition that might cause an exception later, and branch to avoid the exception. 2) Test a flag to see whether an exception has occurred since the program last reset its flag. 3) Test a result to see whether it is a value that only an exception could have produced. CAUTION: The only reliable ways to discover whether Underflow has occurred are to test whether products or quotients lie closer to zero than the underflow threshold, or to test the Underflow flag. (Sums and differences cannot underflow in IEEE 754; if x != y then x-y is correct to full precision and certainly nonzero regardless of how tiny it may be.) Products and quotients that underflow gradually can lose accuracy gradually without vanishing, so comparing them with zero (as one might on a VAX) will not reveal the loss. Fortunately, if a gradually underflowed value is destined to be added to something bigger than the underflow threshold, as is almost always the case, digits lost to gradual underflow will not be missed because they would have been rounded off anyway. So gradual PPPPaaaaggggeeee 7777 MMMMAAAATTTTHHHH((((3333MMMM)))) MMMMAAAATTTTHHHH((((3333MMMM)))) underflows are usually _p_r_o_v_a_b_l_y ignorable. The same cannot be said of underflows flushed to 0. At the option of an implementor conforming to IEEE 754, other ways to cope with exceptions may be provided: 4) ABORT. This mechanism classifies an exception in advance as an incident to be handled by means traditionally associated with error-handling statements like "ON ERROR GO TO ...". Different languages offer different forms of this statement, but most share the following characteristics: - No means is provided to substitute a value for the offending operation's result and resume computation from what may be the middle of an expression. An exceptional result is abandoned. - In a subprogram that lacks an error-handling statement, an exception causes the subprogram to abort within whatever program called it, and so on back up the chain of calling subprograms until an error-handling statement is encountered or the whole task is aborted and memory is dumped. 5) STOP. This mechanism, requiring an interactive debugging environment, is more for the programmer than the program. It classifies an exception in advance as a symptom of a programmer's error; the exception suspends execution as near as it can to the offending operation so that the programmer can look around to see how it happened. Quite often the first several exceptions turn out to be quite unexceptionable, so the programmer ought ideally to be able to resume execution after each one as if execution had not been stopped. 6) ... Other ways lie beyond the scope of this document. The crucial problem for exception handling is the problem of Scope, and the problem's solution is understood, but not enough manpower was available to implement it fully in time to be distributed in 4.3BSD's _l_i_b_m. Ideally, each elementary function should act as if it were indivisible, or atomic, in the sense that ... i) No exception should be signaled that is not deserved by the data supplied to that function. ii) Any exception signaled should be identified with that function rather than with one of its subroutines. iii) The internal behavior of an atomic function should not be disrupted when a calling program changes from one to another of the five or so ways of handling exceptions listed above, although the definition of the function may be correlated intentionally with exception handling. PPPPaaaaggggeeee 8888 MMMMAAAATTTTHHHH((((3333MMMM)))) MMMMAAAATTTTHHHH((((3333MMMM)))) Ideally, every programmer should be able _c_o_n_v_e_n_i_e_n_t_l_y to turn a debugged subprogram into one that appears atomic to its users. But simulating all three characteristics of an atomic function is still a tedious affair, entailing hosts of tests and saves/restores; work is under way to ameliorate the inconvenience. Meanwhile, the functions in _l_i_b_m are only approximately atomic. They signal no inappropriate exception except possibly ... Over/Underflow when a result, if properly computed, might have lain barely within range, and Inexact in _c_a_b_s, _c_b_r_t, _h_y_p_o_t, _l_o_g_1_0 and _p_o_w when it happens to be exact, thanks to fortuitous cancellation of errors. Otherwise, ... Invalid Operation is signaled only when any result but _N_a_N would probably be misleading. Overflow is signaled only when the exact result would be finite but beyond the overflow threshold. Divide-by-Zero is signaled only when a function takes exactly infinite values at finite operands. Underflow is signaled only when the exact result would be nonzero but tinier than the underflow threshold. Inexact is signaled only when greater range or precision would be needed to represent the exact result. Exceptions on this machine: The exception enables and the flags that are raised when an exception occurs (as well as the rounding mode) are in the floating-point control and status register. This register can be read or written by the routines described on the man page _f_p_c(3C). This register's layout is described in the file <_s_y_s/_f_p_u._h>. A useful set of ``user trap handlers'' is available. See the man page _s_i_g_f_p_e(3C). The raw interface to the hardware registers is only intended to be used by the code to implement IEEE user trap handlers. IEEE floating-point exceptions are enabled by setting the enable bit for that exception in the floating-point control and status register. If an exception then occurs the UNIX signal SIGFPE is sent to the process. It is up to the signal handler to determine the instruction that caused the exception and to take the action specified by the user. The instruction that caused the exception is in one of two places. If the floating-point board is used (the floating-point implementation revision register indicates this in its implementation field) then the instruction that caused the exception is in the floating-point exception instruction register. In all other implementations the instruction that caused the PPPPaaaaggggeeee 9999 MMMMAAAATTTTHHHH((((3333MMMM)))) MMMMAAAATTTTHHHH((((3333MMMM)))) exception is at the address of the program counter as modified by the branch delay bit in the cause register. Both the program counter and cause register are in the sigcontext structure passed to the signal handler (see _s_i_g_n_a_l(2)). If the program is to be continued past the instruction that caused the exception the program counter in the signal context must be advanced. If the instruction is in a branch delay slot then the branch must be emulated to determine if the branch is taken and then the resulting program counter can be calculated (see _e_m_u_l_a_t_e__b_r_a_n_c_h(3X) and _s_i_g_n_a_l(2)). Note however, that on systems using the R8000 processor, floating point exceptions are generally fatal when trapped unless the process is being run in precise exception mode. PPPPLLLLAAAATTTTFFFFOOOORRRRMMMM SSSSPPPPEEEECCCCIIIIFFFFIIIICCCC LLLLIIIIBBBBRRRRAAAARRRRIIIIEEEESSSS When compiling -n32 or -64, each processor has specially tuned, hardware specific, versions of _l_i_b_m and _l_i_b_f_a_s_t_m, that the run time linker will use, by default, whenever available. The R10000 tuned libraries are found in the directories: /usr/lib32/mips4/r10000/ /usr/lib64/mips4/r10000/ The R8000 tuned libraries are found in the directories: /usr/lib32/mips4/r8000/ /usr/lib64/mips4/r8000/ The R5000 tuned libraries are found in the directories: /usr/lib32/mips4/ /usr/lib64/mips4/ And the R4000 tuned libraries are found in the directories: /usr/lib32/mips3/ /usr/lib64/mips3/ At runtime, each program automatically uses the "best" library for the system on which it is executing. For example, if the executing program is a mip3 program designed to run on an r4000 processor, it will still use the mips4 R1000-tuned math library when running on an r10000 system. BBBBUUUUGGGGSSSS When signals are appropriate, they are emitted by certain operations within the codes, so a subroutine-trace may be needed to identify the function with its signal in case method 5) above is in use. And the codes all take the IEEE 754 defaults for granted; this means that a decision to trap all divisions by zero could disrupt a code that would otherwise get correct results despite division by zero. PPPPaaaaggggeeee 11110000 MMMMAAAATTTTHHHH((((3333MMMM)))) MMMMAAAATTTTHHHH((((3333MMMM)))) SEE ALSO signal(2), fpc(3C), emulate_branch(3X), sigfpe(3C), matherr(3M) R2010 Floating Point Coprocessor Architecture R2360 Floating Point Board Product Description An explanation of IEEE 754 and its proposed extension p854 was published in the IEEE magazine MICRO in August 1984 under the title "A Proposed Radix- and Word-length-independent Standard for Floating-point Arithmetic" by W. J. Cody et al. Articles in the IEEE magazine COMPUTER vol. 14 no. 3 (Mar. 1981), and in the ACM SIGNUM Newsletter Special Issue of Oct. 1979, may be helpful although they pertain to superseded drafts of the standard. AAAAUUUUTTTTHHHHOOOORRRR W. Kahan, with the help of Z-S. Alex Liu, Stuart I. McDonald, Dr. Kwok-Choi Ng, Peter Tang. PPPPaaaaggggeeee 11111111