Optimize bitlen <= 53 as pure 32-bit

Author: sverker

erts/emulator/beam/big.c

@@ -2429,20 +2429,26 @@ big_to_double(Eterm x, double* resp)
     msd_bits = erts_fit_in_bits_uint(msd);
     bitlen = (Uint64)(xl-1) * D_EXP + msd_bits;
 
+#if D_EXP == 64
+    ERTS_CT_ASSERT(SMALL_BITS > 53);
+    ASSERT(bitlen > 53);
+#elif D_EXP == 32
     if (bitlen <= 53) {
         /*
          * The value fits the double mantissa exactly, so accumulating
          * digit by digit cannot round.
          */
-        double dbase = ((double)(D_MASK)+1);
+        ASSERT(xl == 1 || xl == 2);
 
-        d = 0.0;
-        for (i = xl; i-- > 0; ) {
-            d = d * dbase + v[i];
+        d = (double) v[0];
+        if (xl == 2) {
+            const double dbase = ((double)(D_MASK)+1);
+            d += ((double) v[1]) * dbase;
         }
         *resp = xsgn ? -d : d;
         return 0;
     }
+#endif
 
     /*
      * More than 1024 bits is at least 2^1024, above the largest finite double