tls: P256: shrink sp_256_mul_add_8 a bit more
function old new delta
sp_256_mont_reduce_8 257 245 -12
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/networking/tls_sp_c32.c b/networking/tls_sp_c32.c
index 1ab6106..6fca2aa 100644
--- a/networking/tls_sp_c32.c
+++ b/networking/tls_sp_c32.c
@@ -569,8 +569,10 @@
// const sp_digit* a = p256_mod;
//a[7..0] = ffffffff 00000001 00000000 00000000 00000000 ffffffff ffffffff ffffffff
sp_digit b = r[0];
- uint64_t t = 0;
+ uint64_t t;
+
+// t = 0;
// for (i = 0; i < 8; i++) {
// uint32_t t_hi;
// uint64_t m = ((uint64_t)b * a[i]) + r[i];
@@ -584,12 +586,13 @@
// Unroll, then optimize the above loop:
//uint32_t t_hi;
uint64_t m;
+ uint32_t t32;
//m = ((uint64_t)b * a[0]) + r[0];
// Since b is r[0] and a[0] is ffffffff, the above optimizes to:
// m = r[0] * ffffffff + r[0] = (r[0] * 100000000 - r[0]) + r[0] = r[0] << 32;
//t += m;
- // t = (uint64_t)r[0] << 32;
+ // t = r[0] << 32 = b << 32;
//t_hi = (t < m);
// t_hi = 0;
//r[0] = (sp_digit)t;
@@ -625,42 +628,49 @@
// Since a[3] is 00000000, the above optimizes to:
// m = b * 0 + r[3] = r[3];
//t += m;
- // t += r[3];
+ // t = b + r[3];
//t_hi = (t < m);
// t_hi = 0;
//r[3] = (sp_digit)t;
r[3] = r[3] + b;
//t = (t >> 32) | ((uint64_t)t_hi << 32);
- t = (r[3] < b);
+ t32 = (r[3] < b); // 0 or 1
//m = ((uint64_t)b * a[4]) + r[4];
// Since a[4] is 00000000, the above optimizes to:
// m = b * 0 + r[4] = r[4];
//t += m;
- t += r[4];
+ // t = t32 + r[4];
//t_hi = (t < m);
// t_hi = 0;
- r[4] = (sp_digit)t;
+ //r[4] = (sp_digit)t;
//t = (t >> 32) | ((uint64_t)t_hi << 32);
- t = (t >> 32);
+ if (t32 != 0) {
+ r[4]++;
+ t32 = (r[4] == 0); // 0 or 1
//m = ((uint64_t)b * a[5]) + r[5];
// Since a[5] is 00000000, the above optimizes to:
// m = b * 0 + r[5] = r[5];
//t += m;
- t += r[5];
+ // t = t32 + r[5]; (t32 is 0 or 1)
//t_hi = (t < m);
// t_hi = 0;
- r[5] = (sp_digit)t;
+ //r[5] = (sp_digit)t;
//t = (t >> 32) | ((uint64_t)t_hi << 32);
- t = (t >> 32);
+ if (t32 != 0) {
+ r[5]++;
+ t32 = (r[5] == 0); // 0 or 1
+ }
+ }
//m = ((uint64_t)b * a[6]) + r[6];
// Since a[6] is 00000001, the above optimizes to:
- m = (uint64_t)b + r[6]; // 33 bits at most
- t += m;
+ // m = (uint64_t)b + r[6]; // 33 bits at most
+ //t += m;
+ t = t32 + (uint64_t)b + r[6];
//t_hi = (t < m);
- // t_hi = 0; //32bit_value + 33bit_value can't overflow 64 bits
+ // t_hi = 0;
r[6] = (sp_digit)t;
//t = (t >> 32) | ((uint64_t)t_hi << 32);
t = (t >> 32);
@@ -671,7 +681,7 @@
m = ((uint64_t)b << 32) - b + r[7];
t += m;
//t_hi = (t < m);
- // t_hi in fact is always 0 here
+ // t_hi in fact is always 0 here (256bit * 32bit can't have more than 32 bits of overflow)
r[7] = (sp_digit)t;
//t = (t >> 32) | ((uint64_t)t_hi << 32);
t = (t >> 32);