util: Reimplement half <-> float conversions.

Removed u_half.py used to generate the table for previous method.

Previous implementation of float to half conversion was faulty for
denormalised and NaNs and would require extra logic to fix,
thus making the speedup of using tables irrelevant.

Reviewed-by: Jose Fonseca <[email protected]>

1 files changed, 67 insertions, 34 deletions

diff --git a/src/gallium/auxiliary/util/u_half.h b/src/gallium/auxiliary/util/u_half.h
index ad030e90c67..f7009f54844 100644
--- a/src/gallium/auxiliary/util/u_half.h
+++ b/src/gallium/auxiliary/util/u_half.h
@@ -35,51 +35,84 @@
 extern "C" {
 #endif
 
-extern const uint32_t util_half_to_float_mantissa_table[2048];
-extern const uint32_t util_half_to_float_exponent_table[64];
-extern const uint32_t util_half_to_float_offset_table[64];
-extern const uint16_t util_float_to_half_base_table[512];
-extern const uint8_t util_float_to_half_shift_table[512];
-
 /*
- * Note that if the half float is a signaling NaN, the x87 FPU will turn
- * it into a quiet NaN immediately upon loading into a float.
- *
- * Additionally, denormals may be flushed to zero.
+ * References for float <-> half conversions
  *
- * To avoid this, use the floatui functions instead of the float ones
- * when just doing conversion rather than computation on the resulting
- * floats.
+ *  http://fgiesen.wordpress.com/2012/03/28/half-to-float-done-quic/
+ *  https://gist.github.com/2156668
+ *  https://gist.github.com/2144712
  */
 
-static INLINE uint32_t
-util_half_to_floatui(uint16_t h)
+static INLINE uint16_t
+util_float_to_half(float f)
 {
-   unsigned exp = h >> 10;
-   return util_half_to_float_mantissa_table[util_half_to_float_offset_table[exp] + (h & 0x3ff)] + util_half_to_float_exponent_table[exp];
+   uint32_t sign_mask  = 0x80000000;
+   uint32_t round_mask = ~0xfff;
+   uint32_t f32inf = 0xff << 23;
+   uint32_t f16inf = 0x1f << 23;
+   uint32_t sign;
+   union fi magic;
+   union fi f32;
+   uint16_t f16;
+
+   magic.ui = 0xf << 23;
+
+   f32.f = f;
+
+   /* Sign */
+   sign = f32.ui & sign_mask;
+   f32.ui ^= sign;
+
+   if (f32.ui == f32inf) {
+      /* Inf */
+      f16 = 0x7c00;
+   } else if (f32.ui > f32inf) {
+      /* NaN */
+      f16 = 0x7e00;
+   } else {
+      /* Number */
+      f32.ui &= round_mask;
+      f32.f  *= magic.f;
+      f32.ui -= round_mask;
+
+      /* Clamp to infinity if overflowed */
+      if (f32.ui > f16inf)
+         f32.ui = f16inf;
+
+      f16 = f32.ui >> 13;
+   }
+
+   /* Sign */
+   f16 |= sign >> 16;
+
+   return f16;
 }
 
 static INLINE float
-util_half_to_float(uint16_t h)
+util_half_to_float(uint16_t f16)
 {
-   union fi r;
-   r.ui = util_half_to_floatui(h);
-   return r.f;
-}
+   union fi infnan;
+   union fi magic;
+   union fi f32;
 
-static INLINE uint16_t
-util_floatui_to_half(uint32_t v)
-{
-   unsigned signexp = v >> 23;
-   return util_float_to_half_base_table[signexp] + ((v & 0x007fffff) >> util_float_to_half_shift_table[signexp]);
-}
+   infnan.ui = 0x8f << 23;
+   infnan.f = 65536.0f;
+   magic.ui  = 0xef << 23;
 
-static INLINE uint16_t
-util_float_to_half(float f)
-{
-   union fi i;
-   i.f = f;
-   return util_floatui_to_half(i.ui);
+   /* Exponent / Mantissa */
+   f32.ui = (f16 & 0x7fff) << 13;
+
+   /* Adjust */
+   f32.f *= magic.f;
+
+   /* Inf / NaN */
+   if (f32.f >= infnan.f)
+      f32.ui |= 0xff << 23;
+
+   /* Sign */
+   f32.ui |= (f16 & 0x8000) << 16;
+
+   return f32.f;
 }
 
 #ifdef __cplusplus