emo/busybox
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

tls: optimize sp_256_mont_reduce_8 in P256

Browse Source 
The code size decrease is small, but we eliminate ALL multiplies!

function                                             old     new   delta
sp_256_mont_reduce_8                                 268     262      -6

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
This commit is contained in:
Denys Vlasenko 2021-10-06 00:19:30 +02:00
parent bbd723ebec
commit 2430fcfd8d
1 changed files with 125 additions and 21 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

146
networking/tls_sp_c32.c
View File

@ -488,19 +488,118 @@ static void sp_256_mont_shift_8(sp_digit* r, const sp_digit* a)
}
/* Mul a by scalar b and add into r. (r += a * b) */
static int sp_256_mul_add_8(sp_digit* r, const sp_digit* a, sp_digit b)
static int sp_256_mul_add_8(sp_digit* r /*, const sp_digit* a, sp_digit b*/)
{
// 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;
int i;
for (i = 0; i < 8; i++) {
uint32_t t_hi;
uint64_t m = ((uint64_t)b * a[i]) + r[i];
// for (i = 0; i < 8; i++) {
// uint32_t t_hi;
// uint64_t m = ((uint64_t)b * a[i]) + r[i];
// t += m;
// t_hi = (t < m);
// r[i] = (sp_digit)t;
// t = (t >> 32) | ((uint64_t)t_hi << 32);
// }
// r[8] += (sp_digit)t;
// Unroll, then optimize the above loop:
//uint32_t t_hi;
uint64_t m;
//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_hi = (t < m);
// t_hi = 0;
//r[0] = (sp_digit)t;
r[0] = 0;
//t = (t >> 32) | ((uint64_t)t_hi << 32);
// t = b;
//m = ((uint64_t)b * a[1]) + r[1];
// Since a[1] is ffffffff, the above optimizes to:
// m = b * ffffffff + r[1] = (b * 100000000 - b) + r[1] = (b << 32) - b + r[1];
//t += m;
// t = b + (b << 32) - b + r[1] = (b << 32) + r[1];
//t_hi = (t < m);
// t_hi = 0;
//r[1] = (sp_digit)t;
// r[1] = r[1];
//t = (t >> 32) | ((uint64_t)t_hi << 32);
// t = b;
//m = ((uint64_t)b * a[2]) + r[2];
// Since a[2] is ffffffff, the above optimizes to:
// m = b * ffffffff + r[2] = (b * 100000000 - b) + r[2] = (b << 32) - b + r[2];
//t += m;
// t = b + (b << 32) - b + r[2] = (b << 32) + r[2]
//t_hi = (t < m);
// t_hi = 0;
//r[2] = (sp_digit)t;
// r[2] = r[2];
//t = (t >> 32) | ((uint64_t)t_hi << 32);
// t = b;
//m = ((uint64_t)b * a[3]) + r[3];
// Since a[3] is 00000000, the above optimizes to:
// m = b * 0 + r[3] = r[3];
//t += m;
// t += 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);
//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_hi = (t < m);
// t_hi = 0;
r[4] = (sp_digit)t;
//t = (t >> 32) | ((uint64_t)t_hi << 32);
t = (t >> 32);
//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_hi = (t < m);
// t_hi = 0;
r[5] = (sp_digit)t;
//t = (t >> 32) | ((uint64_t)t_hi << 32);
t = (t >> 32);
//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;
t_hi = (t < m);
r[i] = (sp_digit)t;
t = (t >> 32) | ((uint64_t)t_hi << 32);
}
//t_hi = (t < m);
// t_hi = 0; //32bit_value + 33bit_value can't overflow 64 bits
r[6] = (sp_digit)t;
//t = (t >> 32) | ((uint64_t)t_hi << 32);
t = (t >> 32);
//m = ((uint64_t)b * a[7]) + r[7];
// Since a[7] is ffffffff, the above optimizes to:
// m = b * ffffffff + r[7] = (b * 100000000 - b) + r[7]
m = ((uint64_t)b << 32) - b + r[7];
t += m;
//t_hi = (t < m);
// t_hi in fact is always 0 here
r[7] = (sp_digit)t;
//t = (t >> 32) | ((uint64_t)t_hi << 32);
t = (t >> 32);
r[8] += (sp_digit)t;
return (r[8] < (sp_digit)t); /* 1 if addition overflowed */
}
@ -517,28 +616,33 @@ static void sp_256_mont_reduce_8(sp_digit* a/*, const sp_digit* m, sp_digit mp*/
sp_digit mp = p256_mp_mod;
int i;
sp_digit mu;
// sp_digit mu;
if (mp != 1) {
int too_wide;
for (i = 0; i < 7; i++) {
mu = (sp_digit)(a[i] * mp);
if (sp_256_mul_add_8(a+i, m, mu))
(a+i)[9]++;
sp_digit word16th = 0;
for (i = 0; i < 8; i++) {
// mu = (sp_digit)(a[i] * mp);
if (sp_256_mul_add_8(a+i /*, m, mu*/)) {
int j = i + 8;
inc_next_word0:
if (++j > 15) { /* a[16] array has no more words? */
word16th++;
continue;
}
if (++a[j] == 0) /* did this overflow too? */
goto inc_next_word0;
}
}
mu = (sp_digit)(a[7] * mp);
too_wide = sp_256_mul_add_8(a+7, m, mu);
sp_256_mont_shift_8(a, a);
if (too_wide)
if (word16th != 0)
sp_256_sub_8(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];
//m = ffffffff 00000001 00000000 00000000 00000000 ffffffff ffffffff ffffffff
if (sp_256_mul_add_8(a+i, m, mu)) {
// mu = a[i];
if (sp_256_mul_add_8(a+i /*, m, mu*/)) {
int j = i + 8;
inc_next_word:
if (++j > 15) { /* a[16] array has no more words? */