On 21 January 2013 00:49, Chad Versace <chad.vers...@linux.intel.com> wrote:
> Not all float32 values can be exactly represented as a float16. > _mesa_float_to_half() rounded such intermediate float32 values to zero by > truncating unrepresentable bits in the mantissa. This behavior is bia > Sentence fragment above. Did you lose your place while editing the commit message? With the commit mesage fixed, this patch is: Reviewed-by: Paul Berry <stereotype...@gmail.com> > > This patch improves _mesa_float_to_half() by rounding intermediate float32 > values to the nearest float16; when the float32 is exactly between two > float16 values we round to the one with an even mantissa. This behavior is > preferred over the old behavior because: > - It has reduced bias relative to the old behavior. > > - 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. > > CC: Paul Berry <stereotype...@gmail.com> > Signed-off-by: Chad Versace <chad.vers...@linux.intel.com> > --- > src/mesa/main/imports.c | 66 > +++++++++++++++++++++++++++---------------------- > 1 file changed, 37 insertions(+), 29 deletions(-) > > diff --git a/src/mesa/main/imports.c b/src/mesa/main/imports.c > index 26c91dc..e6f7542 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; > } > -- > 1.8.1.1 > >
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