[git commit] tls: P256: factor out "multiply then reduce" operation

[Denys Vlasenko]

commit: https://git.busybox.net/busybox/commit/?id=27df6aeef2d0d4b726a8b3b1ce1b1cafbbce3431
branch: https://git.busybox.net/busybox/commit/?id=refs/heads/master

function                                             old     new   delta
sp_256_mont_mul_and_reduce_8                           -      44     +44
sp_256_ecc_mulmod_8                                  517     442     -75
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 0/1 up/down: 44/-75)            Total: -31 bytes

Signed-off-by: Denys Vlasenko <vda.linux at googlemail.com>
---
 networking/tls_sp_c32.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/networking/tls_sp_c32.c b/networking/tls_sp_c32.c
index cb166e413..292dda24e 100644
--- a/networking/tls_sp_c32.c
+++ b/networking/tls_sp_c32.c
@@ -1091,6 +1091,17 @@ static void sp_256_mont_sqr_8(sp_digit* r, const sp_digit* a
 	sp_256_mont_mul_8(r, a, a /*, m, mp*/);
 }
 
+static NOINLINE void sp_256_mont_mul_and_reduce_8(sp_digit* r,
+		const sp_digit* a, const sp_digit* b
+		/*, const sp_digit* m, sp_digit mp*/)
+{
+	sp_digit rr[2 * 8];
+
+	sp_256_mont_mul_8(rr, a, b /*, p256_mod, p256_mp_mod*/);
+	memset(rr + 8, 0, sizeof(rr) / 2);
+	sp_512to256_mont_reduce_8(r, rr /*, p256_mod, p256_mp_mod*/);
+}
+
 /* Invert the number, in Montgomery form, modulo the modulus (prime) of the
  * P256 curve. (r = 1 / a mod m)
  *
@@ -1186,7 +1197,6 @@ static void sp_256_map_8(sp_point* r, sp_point* p)
 {
 	sp_digit t1[8];
 	sp_digit t2[8];
-	sp_digit rr[2 * 8];
 
 	sp_256_mont_inv_8(t1, p->z);
 
@@ -1194,18 +1204,14 @@ static void sp_256_map_8(sp_point* r, sp_point* p)
 	sp_256_mont_mul_8(t1, t2, t1 /*, p256_mod, p256_mp_mod*/);
 
 	/* x /= z^2 */
-	sp_256_mont_mul_8(rr, p->x, t2 /*, p256_mod, p256_mp_mod*/);
-	memset(rr + 8, 0, sizeof(rr) / 2);
-	sp_512to256_mont_reduce_8(r->x, rr /*, p256_mod, p256_mp_mod*/);
+	sp_256_mont_mul_and_reduce_8(r->x, p->x, t2 /*, p256_mod, p256_mp_mod*/);
 	/* Reduce x to less than modulus */
 	if (sp_256_cmp_8(r->x, p256_mod) >= 0)
 		sp_256_sub_8_p256_mod(r->x);
 	sp_256_norm_8(r->x);
 
 	/* y /= z^3 */
-	sp_256_mont_mul_8(rr, p->y, t1 /*, p256_mod, p256_mp_mod*/);
-	memset(rr + 8, 0, sizeof(rr) / 2);
-	sp_512to256_mont_reduce_8(r->y, rr /*, p256_mod, p256_mp_mod*/);
+	sp_256_mont_mul_and_reduce_8(r->y, p->y, t1 /*, p256_mod, p256_mp_mod*/);
 	/* Reduce y to less than modulus */
 	if (sp_256_cmp_8(r->y, p256_mod) >= 0)
 		sp_256_sub_8_p256_mod(r->y);