tls: P256: propagate constants, create dedicated "subtract p256_mod" function

8 instances of this subtraction probably warrant a few bytes more of code.

function                                             old     new   delta
sp_256_sub_8_p256_mod                                  -      71     +71
sp_256_mont_sub_8                                      -      29     +29
sp_256_mont_dbl_8                                      -      26     +26
sp_256_mont_reduce_8                                 262     257      -5
sp_256_ecc_mulmod_8                                 1171    1161     -10
sp_256_proj_point_dbl_8                              374     359     -15
static.sp_256_mont_sub_8                              29       -     -29
static.sp_256_mont_dbl_8                              31       -     -31
------------------------------------------------------------------------------
(add/remove: 3/2 grow/shrink: 0/3 up/down: 126/-90)            Total: 36 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 108 insertions, 32 deletions

diff --git a/networking/tls_sp_c32.c b/networking/tls_sp_c32.c
index 0773a2d47..1ab6106a7 100644
--- a/networking/tls_sp_c32.c
+++ b/networking/tls_sp_c32.c
@@ -291,6 +291,74 @@ static int sp_256_sub_8(sp_digit* r, const sp_digit* a, const sp_digit* b)
 #endif
 }
 
+/* Sub p256_mod from a into r. (r = a - p256_mod). */
+static void sp_256_sub_8_p256_mod(sp_digit* r, const sp_digit* a)
+{
+#if ALLOW_ASM && defined(__GNUC__) && defined(__i386__)
+        sp_digit reg;
+//p256_mod[7..0] = ffffffff 00000001 00000000 00000000 00000000 ffffffff ffffffff ffffffff
+        asm volatile (
+"\n             movl    (%0), %2"
+"\n             subl    $0xffffffff, %2"
+"\n             movl    %2, (%1)"
+"\n"
+"\n             movl    1*4(%0), %2"
+"\n             sbbl    $0xffffffff, %2"
+"\n             movl    %2, 1*4(%1)"
+"\n"
+"\n             movl    2*4(%0), %2"
+"\n             sbbl    $0xffffffff, %2"
+"\n             movl    %2, 2*4(%1)"
+"\n"
+"\n             movl    3*4(%0), %2"
+"\n             sbbl    $0, %2"
+"\n             movl    %2, 3*4(%1)"
+"\n"
+"\n             movl    4*4(%0), %2"
+"\n             sbbl    $0, %2"
+"\n             movl    %2, 4*4(%1)"
+"\n"
+"\n             movl    5*4(%0), %2"
+"\n             sbbl    $0, %2"
+"\n             movl    %2, 5*4(%1)"
+"\n"
+"\n             movl    6*4(%0), %2"
+"\n             sbbl    $1, %2"
+"\n             movl    %2, 6*4(%1)"
+"\n"
+"\n             movl    7*4(%0), %2"
+"\n             sbbl    $0xffffffff, %2"
+"\n             movl    %2, 7*4(%1)"
+"\n"
+                : "=r" (a), "=r" (r), "=r" (reg)
+                : "0" (a), "1" (r)
+                : "memory"
+        );
+#else
+        const sp_digit* b = p256_mod;
+        int i;
+        sp_digit borrow;
+
+        borrow = 0;
+        for (i = 0; i < 8; i++) {
+                sp_digit w, v;
+                w = b[i] + borrow;
+                v = a[i];
+                if (w != 0) {
+                        v = a[i] - w;
+                        borrow = (v > a[i]);
+                        /* hope compiler detects above as "carry flag set" */
+                }
+                /* else: b + borrow == 0, two cases:
+                 * b:ffffffff, borrow:1
+                 * b:00000000, borrow:0
+                 * in either case, r[i] = a[i] and borrow remains unchanged
+                 */
+                r[i] = v;
+        }
+#endif
+}
+
 /* Multiply a and b into r. (r = a * b) */
 static void sp_256_mul_8(sp_digit* r, const sp_digit* a, const sp_digit* b)
 {
@@ -425,21 +493,25 @@ static void sp_256_div2_8(sp_digit* r, const sp_digit* a, const sp_digit* m)
 }
 
 /* Add two Montgomery form numbers (r = a + b % m) */
-static void sp_256_mont_add_8(sp_digit* r, const sp_digit* a, const sp_digit* b,
-                const sp_digit* m)
+static void sp_256_mont_add_8(sp_digit* r, const sp_digit* a, const sp_digit* b
+                /*, const sp_digit* m*/)
 {
+//      const sp_digit* m = p256_mod;
+
         int carry = sp_256_add_8(r, a, b);
         sp_256_norm_8(r);
         if (carry) {
-                sp_256_sub_8(r, r, m);
+                sp_256_sub_8_p256_mod(r, r /*, m*/);
                 sp_256_norm_8(r);
         }
 }
 
 /* Subtract two Montgomery form numbers (r = a - b % m) */
-static void sp_256_mont_sub_8(sp_digit* r, const sp_digit* a, const sp_digit* b,
-                const sp_digit* m)
+static void sp_256_mont_sub_8(sp_digit* r, const sp_digit* a, const sp_digit* b
+                /*, const sp_digit* m*/)
 {
+        const sp_digit* m = p256_mod;
+
         int borrow;
         borrow = sp_256_sub_8(r, a, b);
         sp_256_norm_8(r);
@@ -450,28 +522,32 @@ static void sp_256_mont_sub_8(sp_digit* r, const sp_digit* a, const sp_digit* b,
 }
 
 /* Double a Montgomery form number (r = a + a % m) */
-static void sp_256_mont_dbl_8(sp_digit* r, const sp_digit* a, const sp_digit* m)
+static void sp_256_mont_dbl_8(sp_digit* r, const sp_digit* a /*, const sp_digit* m*/)
 {
+//      const sp_digit* m = p256_mod;
+
         int carry = sp_256_add_8(r, a, a);
         sp_256_norm_8(r);
         if (carry)
-                sp_256_sub_8(r, r, m);
+                sp_256_sub_8_p256_mod(r, r /*, m*/);
         sp_256_norm_8(r);
 }
 
 /* Triple a Montgomery form number (r = a + a + a % m) */
-static void sp_256_mont_tpl_8(sp_digit* r, const sp_digit* a, const sp_digit* m)
+static void sp_256_mont_tpl_8(sp_digit* r, const sp_digit* a /*, const sp_digit* m*/)
 {
+//      const sp_digit* m = p256_mod;
+
         int carry = sp_256_add_8(r, a, a);
         sp_256_norm_8(r);
         if (carry) {
-                sp_256_sub_8(r, r, m);
+                sp_256_sub_8_p256_mod(r, r /*, m*/);
                 sp_256_norm_8(r);
         }
         carry = sp_256_add_8(r, r, a);
         sp_256_norm_8(r);
         if (carry) {
-                sp_256_sub_8(r, r, m);
+                sp_256_sub_8_p256_mod(r, r /*, m*/);
                 sp_256_norm_8(r);
         }
 }
@@ -612,7 +688,7 @@ static int sp_256_mul_add_8(sp_digit* r /*, const sp_digit* a, sp_digit b*/)
  */
 static void sp_256_mont_reduce_8(sp_digit* a/*, const sp_digit* m, sp_digit mp*/)
 {
-        const sp_digit* m = p256_mod;
+//      const sp_digit* m = p256_mod;
         sp_digit mp = p256_mp_mod;
 
         int i;
@@ -635,13 +711,13 @@ static void sp_256_mont_reduce_8(sp_digit* a/*, const sp_digit* m, sp_digit mp*/
                 }
                 sp_256_mont_shift_8(a, a);
                 if (word16th != 0)
-                        sp_256_sub_8(a, a, m);
+                        sp_256_sub_8_p256_mod(a, a /*, m*/);
                 sp_256_norm_8(a);
         }
         else { /* Same code for explicit mp == 1 (which is always the case for P256) */
                 sp_digit word16th = 0;
                 for (i = 0; i < 8; i++) {
-//                      mu = a[i];
+                        /*mu = a[i];*/
                         if (sp_256_mul_add_8(a+i /*, m, mu*/)) {
                                 int j = i + 8;
  inc_next_word:
@@ -655,7 +731,7 @@ static void sp_256_mont_reduce_8(sp_digit* a/*, const sp_digit* m, sp_digit mp*/
                 }
                 sp_256_mont_shift_8(a, a);
                 if (word16th != 0)
-                        sp_256_sub_8(a, a, m);
+                        sp_256_sub_8_p256_mod(a, a /*, m*/);
                 sp_256_norm_8(a);
         }
 }
@@ -909,7 +985,7 @@ static void sp_256_map_8(sp_point* r, sp_point* p)
         sp_256_mont_reduce_8(r->x /*, 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(r->x, r->x, p256_mod);
+                sp_256_sub_8_p256_mod(r->x, r->x /*, p256_mod*/);
         sp_256_norm_8(r->x);
 
         /* y /= z^3 */
@@ -918,7 +994,7 @@ static void sp_256_map_8(sp_point* r, sp_point* p)
         sp_256_mont_reduce_8(r->y /*, 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(r->y, r->y, p256_mod);
+                sp_256_sub_8_p256_mod(r->y, r->y /*, p256_mod*/);
         sp_256_norm_8(r->y);
 
         memset(r->z, 0, sizeof(r->z));
@@ -954,17 +1030,17 @@ static void sp_256_proj_point_dbl_8(sp_point* r, sp_point* p)
         /* Z = Y * Z */
         sp_256_mont_mul_8(r->z, r->y, r->z /*, p256_mod, p256_mp_mod*/);
         /* Z = 2Z */
-        sp_256_mont_dbl_8(r->z, r->z, p256_mod);
+        sp_256_mont_dbl_8(r->z, r->z /*, p256_mod*/);
         /* T2 = X - T1 */
-        sp_256_mont_sub_8(t2, r->x, t1, p256_mod);
+        sp_256_mont_sub_8(t2, r->x, t1 /*, p256_mod*/);
         /* T1 = X + T1 */
-        sp_256_mont_add_8(t1, r->x, t1, p256_mod);
+        sp_256_mont_add_8(t1, r->x, t1 /*, p256_mod*/);
         /* T2 = T1 * T2 */
         sp_256_mont_mul_8(t2, t1, t2 /*, p256_mod, p256_mp_mod*/);
         /* T1 = 3T2 */
-        sp_256_mont_tpl_8(t1, t2, p256_mod);
+        sp_256_mont_tpl_8(t1, t2 /*, p256_mod*/);
         /* Y = 2Y */
-        sp_256_mont_dbl_8(r->y, r->y, p256_mod);
+        sp_256_mont_dbl_8(r->y, r->y /*, p256_mod*/);
         /* Y = Y * Y */
         sp_256_mont_sqr_8(r->y, r->y /*, p256_mod, p256_mp_mod*/);
         /* T2 = Y * Y */
@@ -976,15 +1052,15 @@ static void sp_256_proj_point_dbl_8(sp_point* r, sp_point* p)
         /* X = T1 * T1 */
         sp_256_mont_mul_8(r->x, t1, t1 /*, p256_mod, p256_mp_mod*/);
         /* X = X - Y */
-        sp_256_mont_sub_8(r->x, r->x, r->y, p256_mod);
+        sp_256_mont_sub_8(r->x, r->x, r->y /*, p256_mod*/);
         /* X = X - Y */
-        sp_256_mont_sub_8(r->x, r->x, r->y, p256_mod);
+        sp_256_mont_sub_8(r->x, r->x, r->y /*, p256_mod*/);
         /* Y = Y - X */
-        sp_256_mont_sub_8(r->y, r->y, r->x, p256_mod);
+        sp_256_mont_sub_8(r->y, r->y, r->x /*, p256_mod*/);
         /* Y = Y * T1 */
         sp_256_mont_mul_8(r->y, r->y, t1 /*, p256_mod, p256_mp_mod*/);
         /* Y = Y - T2 */
-        sp_256_mont_sub_8(r->y, r->y, t2, p256_mod);
+        sp_256_mont_sub_8(r->y, r->y, t2 /*, p256_mod*/);
         dump_512("y2 %s\n", r->y);
 }
 
@@ -1043,9 +1119,9 @@ static void sp_256_proj_point_add_8(sp_point* r, sp_point* p, sp_point* q)
                 /* S2 = Y2*Z1^3 */
                 sp_256_mont_mul_8(t4, t4, q->y /*, p256_mod, p256_mp_mod*/);
                 /* H = U2 - U1 */
-                sp_256_mont_sub_8(t2, t2, t1, p256_mod);
+                sp_256_mont_sub_8(t2, t2, t1 /*, p256_mod*/);
                 /* R = S2 - S1 */
-                sp_256_mont_sub_8(t4, t4, t3, p256_mod);
+                sp_256_mont_sub_8(t4, t4, t3 /*, p256_mod*/);
                 /* Z3 = H*Z1*Z2 */
                 sp_256_mont_mul_8(v->z, v->z, q->z /*, p256_mod, p256_mp_mod*/);
                 sp_256_mont_mul_8(v->z, v->z, t2 /*, p256_mod, p256_mp_mod*/);
@@ -1054,14 +1130,14 @@ static void sp_256_proj_point_add_8(sp_point* r, sp_point* p, sp_point* q)
                 sp_256_mont_sqr_8(t5, t2 /*, p256_mod, p256_mp_mod*/);
                 sp_256_mont_mul_8(v->y, t1, t5 /*, p256_mod, p256_mp_mod*/);
                 sp_256_mont_mul_8(t5, t5, t2 /*, p256_mod, p256_mp_mod*/);
-                sp_256_mont_sub_8(v->x, v->x, t5, p256_mod);
-                sp_256_mont_dbl_8(t1, v->y, p256_mod);
-                sp_256_mont_sub_8(v->x, v->x, t1, p256_mod);
+                sp_256_mont_sub_8(v->x, v->x, t5 /*, p256_mod*/);
+                sp_256_mont_dbl_8(t1, v->y /*, p256_mod*/);
+                sp_256_mont_sub_8(v->x, v->x, t1 /*, p256_mod*/);
                 /* Y3 = R*(U1*H^2 - X3) - S1*H^3 */
-                sp_256_mont_sub_8(v->y, v->y, v->x, p256_mod);
+                sp_256_mont_sub_8(v->y, v->y, v->x /*, p256_mod*/);
                 sp_256_mont_mul_8(v->y, v->y, t4 /*, p256_mod, p256_mp_mod*/);
                 sp_256_mont_mul_8(t5, t5, t3 /*, p256_mod, p256_mp_mod*/);
-                sp_256_mont_sub_8(v->y, v->y, t5, p256_mod);
+                sp_256_mont_sub_8(v->y, v->y, t5 /*, p256_mod*/);
         }
 }