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-rw-r--r--src/mesa/main/imports.c66
1 files changed, 37 insertions, 29 deletions
diff --git a/src/mesa/main/imports.c b/src/mesa/main/imports.c
index 26c91dce535..e6f754254af 100644
--- a/src/mesa/main/imports.c
+++ b/src/mesa/main/imports.c
@@ -336,8 +336,21 @@ _mesa_round_to_even(float val)
/**
* Convert a 4-byte float to a 2-byte half float.
- * Based on code from:
- * http://www.opengl.org/discussion_boards/ubb/Forum3/HTML/008786.html
+ *
+ * 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.
*/
GLhalfARB
_mesa_float_to_half(float val)
@@ -376,32 +389,13 @@ _mesa_float_to_half(float val)
else {
/* regular number */
const int new_exp = flt_e - 127;
- if (new_exp < -24) {
- /* this maps to 0 */
- /* m = 0; - already set */
- e = 0;
- }
- else if (new_exp < -14) {
- /* this maps to a denorm */
- unsigned int exp_val = (unsigned int) (-14 - new_exp); /* 2^-exp_val*/
+ 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;
- switch (exp_val) {
- case 0:
- _mesa_warning(NULL,
- "float_to_half: logical error in denorm creation!\n");
- /* m = 0; - already set */
- break;
- case 1: m = 512 + (flt_m >> 14); break;
- case 2: m = 256 + (flt_m >> 15); break;
- case 3: m = 128 + (flt_m >> 16); break;
- case 4: m = 64 + (flt_m >> 17); break;
- case 5: m = 32 + (flt_m >> 18); break;
- case 6: m = 16 + (flt_m >> 19); break;
- case 7: m = 8 + (flt_m >> 20); break;
- case 8: m = 4 + (flt_m >> 21); break;
- case 9: m = 2 + (flt_m >> 22); break;
- case 10: m = 1; break;
- }
+ m = _mesa_round_to_even((1 << 24) * fabsf(fi.f));
}
else if (new_exp > 15) {
/* map this value to infinity */
@@ -409,12 +403,26 @@ _mesa_float_to_half(float val)
e = 31;
}
else {
- /* regular */
+ /* 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 = flt_m >> 13;
+ m = _mesa_round_to_even(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;
}