--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libtwamr/inv_sqrt.c	Fri Apr 05 06:08:15 2024 +0000
@@ -0,0 +1,98 @@
+/*
+********************************************************************************
+*
+*      GSM AMR-NB speech codec   R98   Version 7.6.0   December 12, 2001
+*                                R99   Version 3.3.0                
+*                                REL-4 Version 4.1.0                
+*
+********************************************************************************
+*
+*      File             : inv_sqrt.c
+*      Purpose          : Computes 1/sqrt(L_x),  where  L_x is positive.
+*                       : If L_x is negative or zero, 
+*                       : the result is 1 (3fff ffff).
+*      Description      :
+*            The function 1/sqrt(L_x) is approximated by a table and linear
+*            interpolation. The inverse square root is computed using the
+*            following steps:
+*                1- Normalization of L_x.
+*                2- If (30-exponent) is even then shift right once.
+*                3- exponent = (30-exponent)/2  +1
+*                4- i = bit25-b31 of L_x;  16<=i<=63  because of normalization.
+*                5- a = bit10-b24
+*                6- i -=16
+*                7- L_y = table[i]<<16 - (table[i] - table[i+1]) * a * 2
+*                8- L_y >>= exponent
+*
+********************************************************************************
+*/
+/*
+********************************************************************************
+*                         MODULE INCLUDE FILE AND VERSION ID
+********************************************************************************
+*/
+#include "namespace.h"
+#include "inv_sqrt.h"
+const char inv_sqrt_id[] = "@(#)$Id $" inv_sqrt_h;
+
+/*
+********************************************************************************
+*                         INCLUDE FILES
+********************************************************************************
+*/
+#include "typedef.h"
+#include "basic_op.h"
+#include "no_count.h"
+
+/*
+********************************************************************************
+*                         LOCAL VARIABLES AND TABLES
+********************************************************************************
+*/
+#include "inv_sqrt.tab" /* Table for inv_sqrt() */
+
+/*
+********************************************************************************
+*                         PUBLIC PROGRAM CODE
+********************************************************************************
+*/
+
+Word32 Inv_sqrt (       /* (o) : output value   */
+    Word32 L_x          /* (i) : input value    */
+)
+{
+    Word16 exp, i, a, tmp;
+    Word32 L_y;
+
+    test (); 
+    if (L_x <= (Word32) 0)
+        return ((Word32) 0x3fffffffL);
+
+    exp = norm_l (L_x);
+    L_x = L_shl (L_x, exp);     /* L_x is normalize */
+
+    exp = sub (30, exp);
+    test (); logic16 (); 
+    if ((exp & 1) == 0)         /* If exponent even -> shift right */
+    {
+        L_x = L_shr (L_x, 1);
+    }
+    exp = shr (exp, 1);
+    exp = add (exp, 1);
+
+    L_x = L_shr (L_x, 9);
+    i = extract_h (L_x);        /* Extract b25-b31 */
+    L_x = L_shr (L_x, 1);
+    a = extract_l (L_x);        /* Extract b10-b24 */
+    a = a & (Word16) 0x7fff;    logic16 (); 
+
+    i = sub (i, 16);
+
+    L_y = L_deposit_h (table[i]);       /* table[i] << 16          */
+    tmp = sub (table[i], table[i + 1]); /* table[i] - table[i+1])  */
+    L_y = L_msu (L_y, tmp, a);  /* L_y -=  tmp*a*2         */
+
+    L_y = L_shr (L_y, exp);     /* denormalization */
+
+    return (L_y);
+}