glsl: move half<->float convertion to util

Needed in NIR too, so move out of mesa/main/imports.c

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Rob Clark <robclark@freedesktop.org>

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;
+}