1 //===-- lib/extendsfdf2.c - single -> double conversion -----------*- C -*-===// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file is dual licensed under the MIT and the University of Illinois Open 6 // Source Licenses. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 // 10 // This file implements a fairly generic conversion from a narrower to a wider 11 // IEEE-754 floating-point type. The constants and types defined following the 12 // includes below parameterize the conversion. 13 // 14 // This routine can be trivially adapted to support conversions from 15 // half-precision or to quad-precision. It does not support types that don't 16 // use the usual IEEE-754 interchange formats; specifically, some work would be 17 // needed to adapt it to (for example) the Intel 80-bit format or PowerPC 18 // double-double format. 19 // 20 // Note please, however, that this implementation is only intended to support 21 // *widening* operations; if you need to convert to a *narrower* floating-point 22 // type (e.g. double -> float), then this routine will not do what you want it 23 // to. 24 // 25 // It also requires that integer types at least as large as both formats 26 // are available on the target platform; this may pose a problem when trying 27 // to add support for quad on some 32-bit systems, for example. You also may 28 // run into trouble finding an appropriate CLZ function for wide source types; 29 // you will likely need to roll your own on some platforms. 30 // 31 // Finally, the following assumptions are made: 32 // 33 // 1. floating-point types and integer types have the same endianness on the 34 // target platform 35 // 36 // 2. quiet NaNs, if supported, are indicated by the leading bit of the 37 // significand field being set 38 // 39 //===----------------------------------------------------------------------===// 40 41 #include "int_lib.h" 42 43 typedef float src_t; 44 typedef uint32_t src_rep_t; 45 #define SRC_REP_C UINT32_C 46 static const int srcSigBits = 23; 47 #define src_rep_t_clz __builtin_clz 48 49 typedef double dst_t; 50 typedef uint64_t dst_rep_t; 51 #define DST_REP_C UINT64_C 52 static const int dstSigBits = 52; 53 54 // End of specialization parameters. Two helper routines for conversion to and 55 // from the representation of floating-point data as integer values follow. 56 57 static inline src_rep_t srcToRep(src_t x) { 58 const union { src_t f; src_rep_t i; } rep = {.f = x}; 59 return rep.i; 60 } 61 62 static inline dst_t dstFromRep(dst_rep_t x) { 63 const union { dst_t f; dst_rep_t i; } rep = {.i = x}; 64 return rep.f; 65 } 66 67 // End helper routines. Conversion implementation follows. 68 69 ARM_EABI_FNALIAS(f2d, extendsfdf2) 70 71 COMPILER_RT_ABI dst_t 72 __extendsfdf2(src_t a) { 73 74 // Various constants whose values follow from the type parameters. 75 // Any reasonable optimizer will fold and propagate all of these. 76 const int srcBits = sizeof(src_t)*CHAR_BIT; 77 const int srcExpBits = srcBits - srcSigBits - 1; 78 const int srcInfExp = (1 << srcExpBits) - 1; 79 const int srcExpBias = srcInfExp >> 1; 80 81 const src_rep_t srcMinNormal = SRC_REP_C(1) << srcSigBits; 82 const src_rep_t srcInfinity = (src_rep_t)srcInfExp << srcSigBits; 83 const src_rep_t srcSignMask = SRC_REP_C(1) << (srcSigBits + srcExpBits); 84 const src_rep_t srcAbsMask = srcSignMask - 1; 85 const src_rep_t srcQNaN = SRC_REP_C(1) << (srcSigBits - 1); 86 const src_rep_t srcNaNCode = srcQNaN - 1; 87 88 const int dstBits = sizeof(dst_t)*CHAR_BIT; 89 const int dstExpBits = dstBits - dstSigBits - 1; 90 const int dstInfExp = (1 << dstExpBits) - 1; 91 const int dstExpBias = dstInfExp >> 1; 92 93 const dst_rep_t dstMinNormal = DST_REP_C(1) << dstSigBits; 94 95 // Break a into a sign and representation of the absolute value 96 const src_rep_t aRep = srcToRep(a); 97 const src_rep_t aAbs = aRep & srcAbsMask; 98 const src_rep_t sign = aRep & srcSignMask; 99 dst_rep_t absResult; 100 101 if (aAbs - srcMinNormal < srcInfinity - srcMinNormal) { 102 // a is a normal number. 103 // Extend to the destination type by shifting the significand and 104 // exponent into the proper position and rebiasing the exponent. 105 absResult = (dst_rep_t)aAbs << (dstSigBits - srcSigBits); 106 absResult += (dst_rep_t)(dstExpBias - srcExpBias) << dstSigBits; 107 } 108 109 else if (aAbs >= srcInfinity) { 110 // a is NaN or infinity. 111 // Conjure the result by beginning with infinity, then setting the qNaN 112 // bit (if needed) and right-aligning the rest of the trailing NaN 113 // payload field. 114 absResult = (dst_rep_t)dstInfExp << dstSigBits; 115 absResult |= (dst_rep_t)(aAbs & srcQNaN) << (dstSigBits - srcSigBits); 116 absResult |= aAbs & srcNaNCode; 117 } 118 119 else if (aAbs) { 120 // a is denormal. 121 // renormalize the significand and clear the leading bit, then insert 122 // the correct adjusted exponent in the destination type. 123 const int scale = src_rep_t_clz(aAbs) - src_rep_t_clz(srcMinNormal); 124 absResult = (dst_rep_t)aAbs << (dstSigBits - srcSigBits + scale); 125 absResult ^= dstMinNormal; 126 const int resultExponent = dstExpBias - srcExpBias - scale + 1; 127 absResult |= (dst_rep_t)resultExponent << dstSigBits; 128 } 129 130 else { 131 // a is zero. 132 absResult = 0; 133 } 134 135 // Apply the signbit to (dst_t)abs(a). 136 const dst_rep_t result = absResult | (dst_rep_t)sign << (dstBits - srcBits); 137 return dstFromRep(result); 138 } 139