+ * Compute the modulus (product of the two factors), to compare
+ * it with the source value. We use br_i31_mulacc(), since it's
+ * already used later on.
+ */
+ t2 = mq + 2 * fwlen;
+ br_i31_zero(t2, mq[0]);
+ br_i31_mulacc(t2, mq, t1);
+
+ /*
+ * We encode the modulus into bytes, to perform the comparison
+ * with bytes. We know that the product length, in bytes, is
+ * exactly xlen.
+ * The comparison actually computes the carry when subtracting
+ * the modulus from the source value; that carry must be 1 for
+ * a value in the correct range. We keep it in r, which is our
+ * accumulator for the error code.
+ */
+ t3 = mq + 4 * fwlen;
+ br_i31_encode(t3, xlen, t2);
+ u = xlen;
+ r = 0;
+ while (u > 0) {
+ uint32_t wn, wx;
+
+ u --;
+ wn = ((unsigned char *)t3)[u];
+ wx = x[u];
+ r = ((wx - (wn + r)) >> 8) & 1;
+ }
+
+ /*
+ * Move the decoded p to another temporary buffer.