--- a/ShortFloat.st	Tue May 03 11:08:52 2011 +0200
+++ b/ShortFloat.st	Wed May 04 15:16:38 2011 +0200
@@ -649,6 +649,63 @@
     "
 !
 
+fastInverseSqrt
+    "return a rough but fast approximation of (1 / self sqrt).
+     The error is some 1%, which is ok for some 3D computatins.
+     Do not use this for now: it is non-portable and probably not speeding things up
+     much, unless inlined into the sender code.
+     The code is here as a reminder and might be later used as a hint for the inliner
+     (to speed up 3D computations, for example)"
+
+%{  /* NOCONTEXT */
+#if sizeof(float) == 4
+    float x, rslt;
+    OBJ newFloat;
+
+    x = __shortFloatVal(self);
+    {
+        float xhalf = 0.5f * x;
+        int i = *(int*)&x; // store floating-point bits in integer
+
+        i = 0x5f3759d5 - (i >> 1); // initial guess for Newton's method
+        x = *(float*)&i; // convert new bits into float
+        x = x*(1.5f - xhalf*x*x); // One round of Newton's method
+        __qMKSFLOAT(newFloat, x);
+        RETURN ( newFloat );
+    }
+#endif
+%}.
+    ^ 1 / self sqrt
+
+    "
+     10.0 asShortFloat fastInverseSqrt   
+     (1 / 10.0 asShortFloat sqrt)
+    "
+
+    "
+     |a b t0 t1 t2|
+
+     a := 345 asShortFloat.
+     t0 := Time millisecondsToRun:[
+        1000000 timesRepeat:[
+        ]
+     ].
+     t1 := Time millisecondsToRun:[
+        1000000 timesRepeat:[
+            a fastInverseSqrt
+        ]
+     ].
+     t2 := Time millisecondsToRun:[
+        1000000 timesRepeat:[
+            (1 / a sqrt)
+        ]
+     ].
+     Transcript show:'empty: '; showCR:t0.
+     Transcript show:'fast: '; showCR:t1.
+     Transcript show:'regular: '; showCR:t2.
+    "
+!
+
 negated
     "return myself negated"
 
@@ -1724,9 +1781,9 @@
 !ShortFloat class methodsFor:'documentation'!
 
 version
-    ^ '$Header: /cvs/stx/stx/libbasic/ShortFloat.st,v 1.104 2010-05-05 13:56:55 stefan Exp $'
+    ^ '$Header: /cvs/stx/stx/libbasic/ShortFloat.st,v 1.105 2011-05-04 13:16:38 cg Exp $'
 !
 
 version_CVS
-    ^ '$Header: /cvs/stx/stx/libbasic/ShortFloat.st,v 1.104 2010-05-05 13:56:55 stefan Exp $'
+    ^ '$Header: /cvs/stx/stx/libbasic/ShortFloat.st,v 1.105 2011-05-04 13:16:38 cg Exp $'
 ! !