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authorSiddhesh Poyarekar <siddhesh@redhat.com>2013-02-13 14:49:50 +0530
committerSiddhesh Poyarekar <siddhesh@redhat.com>2013-02-13 14:49:50 +0530
commit4e92d59e26eb33c3ae523783e918a6e839e83ffd (patch)
tree1d10f92b166a89352df8701c6775ceb3b1795e44 /sysdeps/ieee754
parent909279a5cfa938c989c9b01c8f48a0276291ec45 (diff)
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Better exp polynomial
The lesser the __mul calls, the better it is for performance.
Diffstat (limited to 'sysdeps/ieee754')
-rw-r--r--sysdeps/ieee754/dbl-64/mpexp.c60
1 files changed, 37 insertions, 23 deletions
diff --git a/sysdeps/ieee754/dbl-64/mpexp.c b/sysdeps/ieee754/dbl-64/mpexp.c
index 8d288ff9a1..35c4e80b83 100644
--- a/sysdeps/ieee754/dbl-64/mpexp.c
+++ b/sysdeps/ieee754/dbl-64/mpexp.c
@@ -49,6 +49,15 @@ __mpexp (mp_no *x, mp_no *y, int p)
       0, 0, 0, 0, 3, 3, 4, 4, 5, 4, 4, 5, 5, 5, 6, 6, 6, 6, 6, 6,
       6, 6, 6, 6, 7, 7, 7, 7, 8, 8, 8, 8, 8
     };
+
+  /* Factorials for the values of np above.  */
+  static const double nfa[33] =
+    {
+      1.0, 1.0, 1.0, 1.0, 6.0, 6.0, 24.0, 24.0, 120.0, 24.0, 24.0,
+      120.0, 120.0, 120.0, 720.0, 720.0, 720.0, 720.0, 720.0, 720.0,
+      720.0, 720.0, 720.0, 720.0, 5040.0, 5040.0, 5040.0, 5040.0,
+      40320.0, 40320.0, 40320.0, 40320.0, 40320.0
+    };
   static const int m1p[33] =
     {
       0, 0, 0, 0,
@@ -71,16 +80,7 @@ __mpexp (mp_no *x, mp_no *y, int p)
       {0, 0, 0, 0, 0, 0, 0, 0, 27, 0, 0, 39, 43, 47, 51, 55, 59, 63},
       {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 43, 47, 50, 54}
     };
-  mp_no mpk =
-    {
-      0,
-      {
-	0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0,
-	0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0,
-	0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0
-      }
-    };
-  mp_no mps, mpak, mpt1, mpt2;
+  mp_no mps, mpk, mpt1, mpt2;
 
   /* Choose m,n and compute a=2**(-m).  */
   n = np[p];
@@ -115,24 +115,38 @@ __mpexp (mp_no *x, mp_no *y, int p)
 	  break;
     }
 
-  /* Compute s=x*2**(-m). Put result in mps.  */
+  /* Compute s=x*2**(-m). Put result in mps.  This is the range-reduced input
+     that we will use to compute e^s.  For the final result, simply raise it
+     to 2^m.  */
   __pow_mp (-m, &mpt1, p);
   __mul (x, &mpt1, &mps, p);
 
-  /* Evaluate the polynomial. Put result in mpt2.  */
-  mpk.e = 1;
-  mpk.d[0] = ONE;
-  mpk.d[1] = n;
-  __dvd (&mps, &mpk, &mpt1, p);
-  __add (&mpone, &mpt1, &mpak, p);
-  for (k = n - 1; k > 1; k--)
+  /* Compute the Taylor series for e^s:
+
+         1 + x/1! + x^2/2! + x^3/3! ...
+
+     for N iterations.  We compute this as:
+
+         e^x = 1 + (x * n!/1! + x^2 * n!/2! + x^3 * n!/3!) / n!
+             = 1 + (x * (n!/1! + x * (n!/2! + x * (n!/3! + x ...)))) / n!
+
+     n! is pre-computed and saved while k! is computed on the fly.  */
+  __cpy (&mps, &mpt2, p);
+
+  double kf = 1.0;
+
+  /* Evaluate the rest.  The result will be in mpt2.  */
+  for (k = n - 1; k > 0; k--)
     {
-      __mul (&mps, &mpak, &mpt1, p);
-      mpk.d[1] = k;
-      __dvd (&mpt1, &mpk, &mpt2, p);
-      __add (&mpone, &mpt2, &mpak, p);
+      /* n! / k! = n * (n - 1) ... * (n - k + 1) */
+      kf *= k + 1;
+
+      __dbl_mp (kf, &mpk, p);
+      __add (&mpt2, &mpk, &mpt1, p);
+      __mul (&mps, &mpt1, &mpt2, p);
     }
-  __mul (&mps, &mpak, &mpt1, p);
+  __dbl_mp (nfa[p], &mpk, p);
+  __dvd (&mpt2, &mpk, &mpt1, p);
   __add (&mpone, &mpt1, &mpt2, p);
 
   /* Raise polynomial value to the power of 2**m. Put result in y.  */