forked from OSchip/llvm-project
81 lines
2.5 KiB
C
81 lines
2.5 KiB
C
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//===-- floatuntidf.c - Implement __floatuntidf ---------------------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements __floatuntidf for the compiler_rt library.
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//
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//===----------------------------------------------------------------------===//
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#if __x86_64
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#include "int_lib.h"
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#include <float.h>
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// Returns: convert a to a double, rounding toward even.
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// Assumption: double is a IEEE 64 bit floating point type
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// tu_int is a 128 bit integral type
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// seee eeee eeee mmmm mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm mmmm mmmm mmmm
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si_int __clzti2(ti_int a);
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double
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__floatuntidf(tu_int a)
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{
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if (a == 0)
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return 0.0;
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const unsigned N = sizeof(tu_int) * CHAR_BIT;
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int sd = N - __clzti2(a); // number of significant digits
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int e = sd - 1; // exponent
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if (sd > DBL_MANT_DIG)
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{
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// start: 0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
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// finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
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// 12345678901234567890123456
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// 1 = msb 1 bit
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// P = bit DBL_MANT_DIG-1 bits to the right of 1
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// Q = bit DBL_MANT_DIG bits to the right of 1
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// R = "or" of all bits to the right of Q
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switch (sd)
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{
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case DBL_MANT_DIG + 1:
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a <<= 1;
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break;
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case DBL_MANT_DIG + 2:
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break;
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default:
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a = (a >> (sd - (DBL_MANT_DIG+2))) |
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((a & ((tu_int)(-1) >> ((N + DBL_MANT_DIG+2) - sd))) != 0);
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};
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// finish:
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a |= (a & 4) != 0; // Or P into R
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++a; // round - this step may add a significant bit
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a >>= 2; // dump Q and R
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// a is now rounded to DBL_MANT_DIG or DBL_MANT_DIG+1 bits
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if (a & ((tu_int)1 << DBL_MANT_DIG))
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{
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a >>= 1;
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++e;
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}
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// a is now rounded to DBL_MANT_DIG bits
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}
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else
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{
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a <<= (DBL_MANT_DIG - sd);
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// a is now rounded to DBL_MANT_DIG bits
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}
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double_bits fb;
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fb.u.high = ((e + 1023) << 20) | // exponent
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((su_int)(a >> 32) & 0x000FFFFF); // mantissa-high
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fb.u.low = (su_int)a; // mantissa-low
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return fb.f;
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}
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#endif
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