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tls: P256: shrink sp_256_mul_add_8 a bit more

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function                                             old     new   delta
sp_256_mont_reduce_8                                 257     245     -12

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
This commit is contained in:
Denys Vlasenko 2021-10-06 10:15:29 +02:00
parent c784284615
commit 00f2cceb6a
1 changed files with 24 additions and 14 deletions
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38
networking/tls_sp_c32.c
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@ -569,8 +569,10 @@ 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;
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 @@ static int sp_256_mul_add_8(sp_digit* r /*, const sp_digit* a, sp_digit b*/)
// 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 @@ static int sp_256_mul_add_8(sp_digit* r /*, const sp_digit* a, sp_digit b*/)
// 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 @@ static int sp_256_mul_add_8(sp_digit* r /*, const sp_digit* a, sp_digit b*/)
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);