1 Long double format 2 ================== 3 4 Each long double is made up of two IEEE doubles. The value of the 5 long double is the sum of the values of the two parts (except for 6 -0.0). The most significant part is required to be the value of the 7 long double rounded to the nearest double, as specified by IEEE. For 8 Inf values, the least significant part is required to be one of +0.0 9 or -0.0. No other requirements are made; so, for example, 1.0 may be 10 represented as (1.0, +0.0) or (1.0, -0.0), and the low part of a NaN 11 is don't-care. 12 13 Classification 14 -------------- 15 16 A long double can represent any value of the form 17 s * 2^e * sum(k=0...105: f_k * 2^(-k)) 18 where 's' is +1 or -1, 'e' is between 1022 and -968 inclusive, f_0 is 19 1, and f_k for k>0 is 0 or 1. These are the 'normal' long doubles. 20 21 A long double can also represent any value of the form 22 s * 2^-968 * sum(k=0...105: f_k * 2^(-k)) 23 where 's' is +1 or -1, f_0 is 0, and f_k for k>0 is 0 or 1. These are 24 the 'subnormal' long doubles. 25 26 There are four long doubles that represent zero, two that represent 27 +0.0 and two that represent -0.0. The sign of the high part is the 28 sign of the long double, and the sign of the low part is ignored. 29 30 Likewise, there are four long doubles that represent infinities, two 31 for +Inf and two for -Inf. 32 33 Each NaN, quiet or signalling, that can be represented as a 'double' 34 can be represented as a 'long double'. In fact, there are 2^64 35 equivalent representations for each one. 36 37 There are certain other valid long doubles where both parts are 38 nonzero but the low part represents a value which has a bit set below 39 2^(e-105). These, together with the subnormal long doubles, make up 40 the denormal long doubles. 41 42 Many possible long double bit patterns are not valid long doubles. 43 These do not represent any value. 44 45 Limits 46 ------ 47 48 The maximum representable long double is 2^1024-2^918. The smallest 49 *normal* positive long double is 2^-968. The smallest denormalised 50 positive long double is 2^-1074 (this is the same as for 'double'). 51 52 Conversions 53 ----------- 54 55 A double can be converted to a long double by adding a zero low part. 56 57 A long double can be converted to a double by removing the low part. 58 59 Comparisons 60 ----------- 61 62 Two long doubles can be compared by comparing the high parts, and if 63 those compare equal, comparing the low parts. 64 65 Arithmetic 66 ---------- 67 68 The unary negate operation operates by negating the low and high parts. 69 70 An absolute or absolute-negate operation must be done by comparing 71 against zero and negating if necessary. 72 73 Addition and subtraction are performed using library routines. They 74 are not at present performed perfectly accurately, the result produced 75 will be within 1ulp of the range generated by adding or subtracting 76 1ulp from the input values, where a 'ulp' is 2^(e-106) given the 77 exponent 'e'. In the presence of cancellation, this may be 78 arbitrarily inaccurate. Subtraction is done by negation and addition. 79 80 Multiplication is also performed using a library routine. Its result 81 will be within 2ulp of the correct result. 82 83 Division is also performed using a library routine. Its result will 84 be within 3ulp of the correct result. 85 86 88 Copyright (C) 2004-2024 Free Software Foundation, Inc. 89 90 Copying and distribution of this file, with or without modification, 91 are permitted in any medium without royalty provided the copyright 92 notice and this notice are preserved. 93
Indexes created Thu Aug 13 00:25:44 UTC 2026