[babl] extensions/CIE: do cbrt computation in single precision float

[Øyvind Kolås <ok src gnome org>]

commit b0e7abf2a6bc95b999d4377afead775f5080d135
Author: Øyvind Kolås <pippin gimp org>
Date:   Fri Jan 13 03:45:35 2017 +0100

    extensions/CIE: do cbrt computation in single precision float
    
    This is sufficient for our purposes, and gives a ~40% boost for
    the affecte CIE Lab float conversions.

 extensions/CIE.c |   16 +++-------------
 1 files changed, 3 insertions(+), 13 deletions(-)
---
diff --git a/extensions/CIE.c b/extensions/CIE.c
index fa9df11..e1e2fa7 100644
--- a/extensions/CIE.c
+++ b/extensions/CIE.c
@@ -472,7 +472,7 @@ B2 = 642849266; /* B2 = (127-127.0/3-24/3-0.03306235651)*2**23 */
 
 static inline float _cbrtf(float x)
 {
-       double_t r,T;
+       float r,T;
        union {float f; uint32_t i;} u = {x};
        uint32_t hx = u.i & 0x7fffffff;
 
@@ -491,23 +491,13 @@ static inline float _cbrtf(float x)
        u.i &= 0x80000000;
        u.i |= hx;
 
-       /*
-        * First step Newton iteration (solving t*t-x/t == 0) to 16 bits.  In
-        * double precision so that its terms can be arranged for efficiency
-        * without causing overflow or underflow.
-        */
        T = u.f;
        r = T*T*T;
-       T = T*((double_t)x+x+r)/(x+r+r);
+       T = T*((float)x+x+r)/(x+r+r);
 
-       /*
-        * Second step Newton iteration to 47 bits.  In double precision for
-        * efficiency and accuracy.
-        */
        r = T*T*T;
-       T = T*((double_t)x+x+r)/(x+r+r);
+       T = T*((float)x+x+r)/(x+r+r);
 
-       /* rounding to 24 bits is perfect in round-to-nearest mode */
        return T;
 }