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Diffstat (limited to 'src/util/half_float.c')
| -rw-r--r-- | src/util/half_float.c | 177 |
1 files changed, 177 insertions, 0 deletions
diff --git a/src/util/half_float.c b/src/util/half_float.c new file mode 100644 index 00000000000..4df64c2ccf9 --- /dev/null +++ b/src/util/half_float.c @@ -0,0 +1,177 @@ +/* + * Mesa 3-D graphics library + * + * Copyright (C) 1999-2007 Brian Paul All Rights Reserved. + * + * Permission is hereby granted, free of charge, to any person obtaining a + * copy of this software and associated documentation files (the "Software"), + * to deal in the Software without restriction, including without limitation + * the rights to use, copy, modify, merge, publish, distribute, sublicense, + * and/or sell copies of the Software, and to permit persons to whom the + * Software is furnished to do so, subject to the following conditions: + * + * The above copyright notice and this permission notice shall be included + * in all copies or substantial portions of the Software. + * + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS + * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, + * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL + * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR + * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, + * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR + * OTHER DEALINGS IN THE SOFTWARE. + */ + +#include <math.h> +#include <assert.h> +#include "half_float.h" +#include "rounding.h" + +typedef union { float f; int32_t i; uint32_t u; } fi_type; + +/** + * Convert a 4-byte float to a 2-byte half float. + * + * Not all float32 values can be represented exactly as a float16 value. We + * round such intermediate float32 values to the nearest float16. When the + * float32 lies exactly between to float16 values, we round to the one with + * an even mantissa. + * + * This rounding behavior has several benefits: + * - It has no sign bias. + * + * - It reproduces the behavior of real hardware: opcode F32TO16 in Intel's + * GPU ISA. + * + * - By reproducing the behavior of the GPU (at least on Intel hardware), + * compile-time evaluation of constant packHalf2x16 GLSL expressions will + * result in the same value as if the expression were executed on the GPU. + */ +uint16_t +_mesa_float_to_half(float val) +{ + const fi_type fi = {val}; + const int flt_m = fi.i & 0x7fffff; + const int flt_e = (fi.i >> 23) & 0xff; + const int flt_s = (fi.i >> 31) & 0x1; + int s, e, m = 0; + uint16_t result; + + /* sign bit */ + s = flt_s; + + /* handle special cases */ + if ((flt_e == 0) && (flt_m == 0)) { + /* zero */ + /* m = 0; - already set */ + e = 0; + } + else if ((flt_e == 0) && (flt_m != 0)) { + /* denorm -- denorm float maps to 0 half */ + /* m = 0; - already set */ + e = 0; + } + else if ((flt_e == 0xff) && (flt_m == 0)) { + /* infinity */ + /* m = 0; - already set */ + e = 31; + } + else if ((flt_e == 0xff) && (flt_m != 0)) { + /* NaN */ + m = 1; + e = 31; + } + else { + /* regular number */ + const int new_exp = flt_e - 127; + if (new_exp < -14) { + /* The float32 lies in the range (0.0, min_normal16) and is rounded + * to a nearby float16 value. The result will be either zero, subnormal, + * or normal. + */ + e = 0; + m = _mesa_lroundevenf((1 << 24) * fabsf(fi.f)); + } + else if (new_exp > 15) { + /* map this value to infinity */ + /* m = 0; - already set */ + e = 31; + } + else { + /* The float32 lies in the range + * [min_normal16, max_normal16 + max_step16) + * and is rounded to a nearby float16 value. The result will be + * either normal or infinite. + */ + e = new_exp + 15; + m = _mesa_lroundevenf(flt_m / (float) (1 << 13)); + } + } + + assert(0 <= m && m <= 1024); + if (m == 1024) { + /* The float32 was rounded upwards into the range of the next exponent, + * so bump the exponent. This correctly handles the case where f32 + * should be rounded up to float16 infinity. + */ + ++e; + m = 0; + } + + result = (s << 15) | (e << 10) | m; + return result; +} + + +/** + * Convert a 2-byte half float to a 4-byte float. + * Based on code from: + * http://www.opengl.org/discussion_boards/ubb/Forum3/HTML/008786.html + */ +float +_mesa_half_to_float(uint16_t val) +{ + /* XXX could also use a 64K-entry lookup table */ + const int m = val & 0x3ff; + const int e = (val >> 10) & 0x1f; + const int s = (val >> 15) & 0x1; + int flt_m, flt_e, flt_s; + fi_type fi; + float result; + + /* sign bit */ + flt_s = s; + + /* handle special cases */ + if ((e == 0) && (m == 0)) { + /* zero */ + flt_m = 0; + flt_e = 0; + } + else if ((e == 0) && (m != 0)) { + /* denorm -- denorm half will fit in non-denorm single */ + const float half_denorm = 1.0f / 16384.0f; /* 2^-14 */ + float mantissa = ((float) (m)) / 1024.0f; + float sign = s ? -1.0f : 1.0f; + return sign * mantissa * half_denorm; + } + else if ((e == 31) && (m == 0)) { + /* infinity */ + flt_e = 0xff; + flt_m = 0; + } + else if ((e == 31) && (m != 0)) { + /* NaN */ + flt_e = 0xff; + flt_m = 1; + } + else { + /* regular */ + flt_e = e + 112; + flt_m = m << 13; + } + + fi.i = (flt_s << 31) | (flt_e << 23) | flt_m; + result = fi.f; + return result; +} |