ec_cvt.c: ARM comparison results were wrong, clarify the background.

[openssl.git] / crypto / ec / ec_cvt.c

diff --git a/crypto/ec/ec_cvt.c b/crypto/ec/ec_cvt.c
index a99d762d3b93ad67febb6ea9a3b531f035599860..dffd70521a37164a3740ea828353ff3fee2df94e 100644 (file)
--- a/crypto/ec/ec_cvt.c
+++ b/crypto/ec/ec_cvt.c
@@ -85,15 +85,21 @@ EC_GROUP *EC_GROUP_new_curve_GFp(const BIGNUM *p, const BIGNUM *a, const BIGNUM
         * This might appear controversial, but the fact is that generic
         * prime method was observed to deliver better performance even
         * for NIST primes on a range of platforms, e.g.: 60%-15%
-        * improvement on IA-64, 50%-20% on ARM, 30%-90% on P4, 20%-25%
+        * improvement on IA-64, ~25% on ARM, 30%-90% on P4, 20%-25%
         * in 32-bit build and 35%--12% in 64-bit build on Core2...
         * Coefficients are relative to optimized bn_nist.c for most
         * intensive ECDSA verify and ECDH operations for 192- and 521-
-        * bit keys respectively. What effectively happens is that loop
-        * with bn_mul_add_words is put against bn_mul_mont, and latter
-        * wins on short vectors. Correct solution should be implementing
-        * dedicated NxN multiplication subroutines for small N. But till
-        * it materializes, let's stick to generic prime method...
+        * bit keys respectively. Choice of these boundary values is
+        * arguable, because the dependency of improvement coefficient
+        * from key length is not a "monotone" curve. For example while
+        * 571-bit result is 23% on ARM, 384-bit one is -1%. But it's
+        * generally faster, sometimes "respectfully" faster, or
+        * "tolerably" slower... What effectively happens is that loop
+        * with bn_mul_add_words is put against bn_mul_mont, and the
+        * latter "wins" on short vectors. Correct solution should be
+        * implementing dedicated NxN multiplication subroutines for
+        * small N. But till it materializes, let's stick to generic
+        * prime method...
         *                                              <appro>
         */
        meth = EC_GFp_mont_method();