Revert r40623, "Refactor bigint _sqrTo in preparation of intrinsification."

runtime/lib/bigint.dart

Index: runtime/lib/bigint.dart
diff --git a/runtime/lib/bigint.dart b/runtime/lib/bigint.dart
index 52228effea62fc1072991a4b6af2a4eaa018e67a..92cdde039994b7735cb6ff01d301c4c5f0756f06 100644
--- a/runtime/lib/bigint.dart
+++ b/runtime/lib/bigint.dart
@@ -815,46 +815,34 @@ class _Bigint extends _IntegerImplementation implements int {
     args[MA_CARRY_OUT] = c;
   }
 
-  // Square and accumulate.
+  // Multiply and accumulate with carry in.
   // Input:
-  //   x_digits[i..used-1]: digits of operand being squared.
-  //   a_digits[2*i..i+used-1]: accumulator digits.
+  //   x: multiplier digit, 0 <= x < 2*DIGIT_BASE (i.e. 33-bit multiplier).
+  //   m_digits[i..i+n-1]: multiplicand digits.
+  //   a_digits[j..j+n-1]: accumulator digits.
+  //   c: carry in.
   // Operation:
-  //   a_digits[2*i..i+used-1] += x_digits[i]*x_digits[i] +
-  //                              2*x_digits[i]*x_digits[i+1..used-1].
-  static void _sqrAdd(Uint32List x_digits, int i,
-                      Uint32List a_digits, int used) {
-    int x = x_digits[i];
-    if (x == 0) return;
-    int j = 2*i;
-    int c = 0;
+  //   a_digits[j..j+n-1] += x*m_digits[i..i+n-1] + c.
+  // Output:
+  //   carry out.
+  // TODO(regis): Use an argument buffer as in _mulAdd.
+  static int _mulAddc(int x, Uint32List m_digits, int i,
+                      Uint32List a_digits, int j, int c, int n) {
+    if (x == 0 && c == 0) {
+      // No-op if both x and c are 0.
+      return 0;
+    }
     int xl = x & DIGIT2_MASK;
     int xh = x >> DIGIT2_BITS;
-    int m = 2*xh*xl;
-    int l = xl*xl + ((m & DIGIT2_MASK) << DIGIT2_BITS) + a_digits[j];
-    c = (l >> DIGIT_BITS) + (m >> DIGIT2_BITS) + xh*xh;
-    a_digits[j] = l & DIGIT_MASK;
-    x <<= 1;
-    xl = x & DIGIT2_MASK;
-    xh = x >> DIGIT2_BITS;
-    int n = used - i - 1;
-    int k = i + 1;
-    j++;
     while (--n >= 0) {
-      int l = x_digits[k] & DIGIT2_MASK;
-      int h = x_digits[k++] >> DIGIT2_BITS;
+      int l = m_digits[i] & DIGIT2_MASK;
+      int h = m_digits[i++] >> DIGIT2_BITS;
       int m = xh*l + h*xl;
       l = xl*l + ((m & DIGIT2_MASK) << DIGIT2_BITS) + a_digits[j] + c;
       c = (l >> DIGIT_BITS) + (m >> DIGIT2_BITS) + xh*h;
       a_digits[j++] = l & DIGIT_MASK;
     }
-    c += a_digits[i + used];
-    if (c >= DIGIT_BASE) {
-      a_digits[i + used] = c - DIGIT_BASE;
-      a_digits[i + used + 1] = 1;
-    } else {
-      a_digits[i + used] = c;
-    }
+    return c;
   }
 
   // r = this * a.
@@ -892,7 +880,18 @@ class _Bigint extends _IntegerImplementation implements int {
       r_digits[i] = 0;
     }
     for (i = 0; i < used - 1; ++i) {
-      _sqrAdd(digits, i, r_digits, used);
+      _MulAddArgs[MA_MULTIPLIER] = digits[i];
+      _mulAdd(_MulAddArgs, digits, i, r_digits, 2*i, 1);
+      var c = _MulAddArgs[MA_CARRY_OUT];
+      var d = r_digits[i + used];
+      d += _mulAddc(digits[i] << 1, digits, i + 1,
+                    r_digits, 2*i + 1, c, used - i - 1);
+      if (d >= DIGIT_BASE) {
+        r_digits[i + used] = d - DIGIT_BASE;
+        r_digits[i + used + 1] = 1;
+      } else {
+        r_digits[i + used] = d;
+      }
     }
     if (r_used > 0) {
       _MulAddArgs[MA_MULTIPLIER] = digits[i];