Stephen Hines | 6a211c5 | 2014-07-21 00:49:56 -0700 | [diff] [blame] | 1 | //= lib/fp_trunc_impl.inc - high precision -> low precision conversion *-*-===// |
| 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 wider to a narrower |
| 11 | // IEEE-754 floating-point type in the default (round to nearest, ties to even) |
| 12 | // rounding mode. The constants and types defined following the includes below |
| 13 | // parameterize the conversion. |
| 14 | // |
| 15 | // This routine can be trivially adapted to support conversions to |
| 16 | // half-precision or from quad-precision. It does not support types that don't |
| 17 | // use the usual IEEE-754 interchange formats; specifically, some work would be |
| 18 | // needed to adapt it to (for example) the Intel 80-bit format or PowerPC |
| 19 | // double-double format. |
| 20 | // |
| 21 | // Note please, however, that this implementation is only intended to support |
| 22 | // *narrowing* operations; if you need to convert to a *wider* floating-point |
| 23 | // type (e.g. float -> double), then this routine will not do what you want it |
| 24 | // to. |
| 25 | // |
| 26 | // It also requires that integer types at least as large as both formats |
| 27 | // are available on the target platform; this may pose a problem when trying |
| 28 | // to add support for quad on some 32-bit systems, for example. |
| 29 | // |
| 30 | // Finally, the following assumptions are made: |
| 31 | // |
| 32 | // 1. floating-point types and integer types have the same endianness on the |
| 33 | // target platform |
| 34 | // |
| 35 | // 2. quiet NaNs, if supported, are indicated by the leading bit of the |
| 36 | // significand field being set |
| 37 | // |
| 38 | //===----------------------------------------------------------------------===// |
| 39 | |
| 40 | #include "fp_trunc.h" |
| 41 | |
| 42 | static inline dst_t __truncXfYf2__(src_t a) { |
| 43 | // Various constants whose values follow from the type parameters. |
| 44 | // Any reasonable optimizer will fold and propagate all of these. |
| 45 | const int srcBits = sizeof(src_t)*CHAR_BIT; |
| 46 | const int srcExpBits = srcBits - srcSigBits - 1; |
| 47 | const int srcInfExp = (1 << srcExpBits) - 1; |
| 48 | const int srcExpBias = srcInfExp >> 1; |
| 49 | |
| 50 | const src_rep_t srcMinNormal = SRC_REP_C(1) << srcSigBits; |
| 51 | const src_rep_t srcSignificandMask = srcMinNormal - 1; |
| 52 | const src_rep_t srcInfinity = (src_rep_t)srcInfExp << srcSigBits; |
| 53 | const src_rep_t srcSignMask = SRC_REP_C(1) << (srcSigBits + srcExpBits); |
| 54 | const src_rep_t srcAbsMask = srcSignMask - 1; |
| 55 | const src_rep_t roundMask = (SRC_REP_C(1) << (srcSigBits - dstSigBits)) - 1; |
| 56 | const src_rep_t halfway = SRC_REP_C(1) << (srcSigBits - dstSigBits - 1); |
| 57 | const src_rep_t srcQNaN = SRC_REP_C(1) << (srcSigBits - 1); |
| 58 | const src_rep_t srcNaNCode = srcQNaN - 1; |
| 59 | |
| 60 | const int dstBits = sizeof(dst_t)*CHAR_BIT; |
| 61 | const int dstExpBits = dstBits - dstSigBits - 1; |
| 62 | const int dstInfExp = (1 << dstExpBits) - 1; |
| 63 | const int dstExpBias = dstInfExp >> 1; |
| 64 | |
| 65 | const int underflowExponent = srcExpBias + 1 - dstExpBias; |
| 66 | const int overflowExponent = srcExpBias + dstInfExp - dstExpBias; |
| 67 | const src_rep_t underflow = (src_rep_t)underflowExponent << srcSigBits; |
| 68 | const src_rep_t overflow = (src_rep_t)overflowExponent << srcSigBits; |
| 69 | |
| 70 | const dst_rep_t dstQNaN = DST_REP_C(1) << (dstSigBits - 1); |
| 71 | const dst_rep_t dstNaNCode = dstQNaN - 1; |
| 72 | |
| 73 | // Break a into a sign and representation of the absolute value |
| 74 | const src_rep_t aRep = srcToRep(a); |
| 75 | const src_rep_t aAbs = aRep & srcAbsMask; |
| 76 | const src_rep_t sign = aRep & srcSignMask; |
| 77 | dst_rep_t absResult; |
| 78 | |
| 79 | if (aAbs - underflow < aAbs - overflow) { |
| 80 | // The exponent of a is within the range of normal numbers in the |
| 81 | // destination format. We can convert by simply right-shifting with |
| 82 | // rounding and adjusting the exponent. |
| 83 | absResult = aAbs >> (srcSigBits - dstSigBits); |
| 84 | absResult -= (dst_rep_t)(srcExpBias - dstExpBias) << dstSigBits; |
| 85 | |
| 86 | const src_rep_t roundBits = aAbs & roundMask; |
| 87 | // Round to nearest |
| 88 | if (roundBits > halfway) |
| 89 | absResult++; |
| 90 | // Ties to even |
| 91 | else if (roundBits == halfway) |
| 92 | absResult += absResult & 1; |
| 93 | } |
| 94 | else if (aAbs > srcInfinity) { |
| 95 | // a is NaN. |
| 96 | // Conjure the result by beginning with infinity, setting the qNaN |
| 97 | // bit and inserting the (truncated) trailing NaN field. |
| 98 | absResult = (dst_rep_t)dstInfExp << dstSigBits; |
| 99 | absResult |= dstQNaN; |
| 100 | absResult |= ((aAbs & srcNaNCode) >> (srcSigBits - dstSigBits)) & dstNaNCode; |
| 101 | } |
Pirama Arumuga Nainar | d5a4b89 | 2015-06-23 15:40:12 -0700 | [diff] [blame] | 102 | else if (aAbs >= overflow) { |
Stephen Hines | 6a211c5 | 2014-07-21 00:49:56 -0700 | [diff] [blame] | 103 | // a overflows to infinity. |
| 104 | absResult = (dst_rep_t)dstInfExp << dstSigBits; |
| 105 | } |
| 106 | else { |
| 107 | // a underflows on conversion to the destination type or is an exact |
| 108 | // zero. The result may be a denormal or zero. Extract the exponent |
| 109 | // to get the shift amount for the denormalization. |
| 110 | const int aExp = aAbs >> srcSigBits; |
| 111 | const int shift = srcExpBias - dstExpBias - aExp + 1; |
| 112 | |
| 113 | const src_rep_t significand = (aRep & srcSignificandMask) | srcMinNormal; |
| 114 | |
| 115 | // Right shift by the denormalization amount with sticky. |
| 116 | if (shift > srcSigBits) { |
| 117 | absResult = 0; |
| 118 | } else { |
| 119 | const bool sticky = significand << (srcBits - shift); |
| 120 | src_rep_t denormalizedSignificand = significand >> shift | sticky; |
| 121 | absResult = denormalizedSignificand >> (srcSigBits - dstSigBits); |
| 122 | const src_rep_t roundBits = denormalizedSignificand & roundMask; |
| 123 | // Round to nearest |
| 124 | if (roundBits > halfway) |
| 125 | absResult++; |
| 126 | // Ties to even |
| 127 | else if (roundBits == halfway) |
| 128 | absResult += absResult & 1; |
| 129 | } |
| 130 | } |
| 131 | |
| 132 | // Apply the signbit to (dst_t)abs(a). |
| 133 | const dst_rep_t result = absResult | sign >> (srcBits - dstBits); |
| 134 | return dstFromRep(result); |
| 135 | } |