asdfuser pushed a commit to branch master.
commit 5f1614e1d504d8b9ffe4509528791ef1009a730b
Author: Daniel Willmann <[email protected]>
Date: Fri May 17 14:43:19 2013 +0100
eina_fp: Fix eina_f32p32_mul rounding issues
Previous multiplication just truncated the fraction part to 16 bits. If
we round properly we can actually increase accuracy a bit.
Signed-off-by: Daniel Willmann <[email protected]>
---
src/lib/eina/eina_inline_f32p32.x | 23 +++++++++++++++++------
1 file changed, 17 insertions(+), 6 deletions(-)
diff --git a/src/lib/eina/eina_inline_f32p32.x
b/src/lib/eina/eina_inline_f32p32.x
index 73480de..f0c5fd8 100644
--- a/src/lib/eina/eina_inline_f32p32.x
+++ b/src/lib/eina/eina_inline_f32p32.x
@@ -36,10 +36,16 @@ eina_f32p32_sub(Eina_F32p32 a, Eina_F32p32 b)
static inline Eina_F32p32
eina_f32p32_mul(Eina_F32p32 a, Eina_F32p32 b)
{
- /* Prevent overflow and do '(a * b) >> 32' */
- /* Basically do: Eina_F16p16 * Eina_F16p16 = Eina_F32p32 */
+ /* To prevent overflow during multiplication
+ * we need to reduce the precision of the fraction part
+ * Shift both values to only contain 16 bit of the fraction part
+ * (rounded).
+ * After multiplication we again have a value with a 32-bit
+ * fraction part. See also
+ * http://en.wikipedia.org/wiki/Fixed-point_arithmetic#Operations
+ */
+
Eina_F32p32 up;
- Eina_F32p32 down;
Eina_F32p32 result;
uint64_t as, bs;
Eina_F32p32 sign;
@@ -48,10 +54,13 @@ eina_f32p32_mul(Eina_F32p32 a, Eina_F32p32 b)
as = eina_fp32p32_llabs(a);
bs = eina_fp32p32_llabs(b);
- up = (as >> 16) * (bs >> 16);
- down = (as & 0xFFFF) * (bs & 0xFFFF);
+ /* Reduce fraction to 16-bit (rounded) */
+ as = (as >> 16) + ((as >> 15) & 0x1);
+ bs = (bs >> 16) + ((bs >> 15) & 0x1);
+
+ up = as * bs;
- result = up + (down >> 32);
+ result = up;
return sign < 0 ? - result : result;
}
@@ -70,6 +79,8 @@ eina_f32p32_div(Eina_F32p32 a, Eina_F32p32 b)
sign = a ^ b;
+ /* FIXME: This should probably map to +/-inf when converting to double
+ or we should abort... */
if (b == 0)
return sign < 0 ? (Eina_F32p32) 0x8000000000000000ull : (Eina_F32p32)
0x7FFFFFFFFFFFFFFFull;
--
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