src/compiler-rt/lib/builtins/fp_trunc_impl.inc

   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     }
 102     else if (aAbs >= overflow) {
 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 }