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

   1 //=-lib/fp_extend_impl.inc - low precision -> high 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 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 // It does not support types that don't use the usual IEEE-754 interchange
  15 // formats; specifically, some work would be needed to adapt it to
  16 // (for example) the Intel 80-bit format or PowerPC double-double format.
  17 //
  18 // Note please, however, that this implementation is only intended to support
  19 // *widening* operations; if you need to convert to a *narrower* floating-point
  20 // type (e.g. double -> float), then this routine will not do what you want it
  21 // to.
  22 //
  23 // It also requires that integer types at least as large as both formats
  24 // are available on the target platform; this may pose a problem when trying
  25 // to add support for quad on some 32-bit systems, for example.  You also may
  26 // run into trouble finding an appropriate CLZ function for wide source types;
  27 // you will likely need to roll your own on some platforms.
  28 //
  29 // Finally, the following assumptions are made:
  30 //
  31 // 1. floating-point types and integer types have the same endianness on the
  32 //    target platform
  33 //
  34 // 2. quiet NaNs, if supported, are indicated by the leading bit of the
  35 //    significand field being set
  36 //
  37 //===----------------------------------------------------------------------===//
  38
  39 #include "fp_extend.h"
  40
  41 static __inline dst_t __extendXfYf2__(src_t a) {
  42     // Various constants whose values follow from the type parameters.
  43     // Any reasonable optimizer will fold and propagate all of these.
  44     const int srcBits = sizeof(src_t)*CHAR_BIT;
  45     const int srcExpBits = srcBits - srcSigBits - 1;
  46     const int srcInfExp = (1 << srcExpBits) - 1;
  47     const int srcExpBias = srcInfExp >> 1;
  48
  49     const src_rep_t srcMinNormal = SRC_REP_C(1) << srcSigBits;
  50     const src_rep_t srcInfinity = (src_rep_t)srcInfExp << srcSigBits;
  51     const src_rep_t srcSignMask = SRC_REP_C(1) << (srcSigBits + srcExpBits);
  52     const src_rep_t srcAbsMask = srcSignMask - 1;
  53     const src_rep_t srcQNaN = SRC_REP_C(1) << (srcSigBits - 1);
  54     const src_rep_t srcNaNCode = srcQNaN - 1;
  55
  56     const int dstBits = sizeof(dst_t)*CHAR_BIT;
  57     const int dstExpBits = dstBits - dstSigBits - 1;
  58     const int dstInfExp = (1 << dstExpBits) - 1;
  59     const int dstExpBias = dstInfExp >> 1;
  60
  61     const dst_rep_t dstMinNormal = DST_REP_C(1) << dstSigBits;
  62
  63     // Break a into a sign and representation of the absolute value
  64     const src_rep_t aRep = srcToRep(a);
  65     const src_rep_t aAbs = aRep & srcAbsMask;
  66     const src_rep_t sign = aRep & srcSignMask;
  67     dst_rep_t absResult;
  68
  69     // If sizeof(src_rep_t) < sizeof(int), the subtraction result is promoted
  70     // to (signed) int.  To avoid that, explicitly cast to src_rep_t.
  71     if ((src_rep_t)(aAbs - srcMinNormal) < srcInfinity - srcMinNormal) {
  72         // a is a normal number.
  73         // Extend to the destination type by shifting the significand and
  74         // exponent into the proper position and rebiasing the exponent.
  75         absResult = (dst_rep_t)aAbs << (dstSigBits - srcSigBits);
  76         absResult += (dst_rep_t)(dstExpBias - srcExpBias) << dstSigBits;
  77     }
  78
  79     else if (aAbs >= srcInfinity) {
  80         // a is NaN or infinity.
  81         // Conjure the result by beginning with infinity, then setting the qNaN
  82         // bit (if needed) and right-aligning the rest of the trailing NaN
  83         // payload field.
  84         absResult = (dst_rep_t)dstInfExp << dstSigBits;
  85         absResult |= (dst_rep_t)(aAbs & srcQNaN) << (dstSigBits - srcSigBits);
  86         absResult |= (dst_rep_t)(aAbs & srcNaNCode) << (dstSigBits - srcSigBits);
  87     }
  88
  89     else if (aAbs) {
  90         // a is denormal.
  91         // renormalize the significand and clear the leading bit, then insert
  92         // the correct adjusted exponent in the destination type.
  93         const int scale = src_rep_t_clz(aAbs) - src_rep_t_clz(srcMinNormal);
  94         absResult = (dst_rep_t)aAbs << (dstSigBits - srcSigBits + scale);
  95         absResult ^= dstMinNormal;
  96         const int resultExponent = dstExpBias - srcExpBias - scale + 1;
  97         absResult |= (dst_rep_t)resultExponent << dstSigBits;
  98     }
  99
 100     else {
 101         // a is zero.
 102         absResult = 0;
 103     }
 104
 105     // Apply the signbit to (dst_t)abs(a).
 106     const dst_rep_t result = absResult | (dst_rep_t)sign << (dstBits - srcBits);
 107     return dstFromRep(result);
 108 }