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tls: P256: enable 64-bit version of montgomery reduction

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After more testing, (1) I'm more sure it is indeed correct, and
(2) it is a significant speedup - we do a lot of those multiplications.

function                                             old     new   delta
sp_512to256_mont_reduce_8                            191     223     +32

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
This commit is contained in:
Denys Vlasenko 2021-11-28 21:40:23 +01:00
parent 90b0d33044
commit 8514b4166d
1 changed files with 8 additions and 7 deletions
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15
networking/tls_sp_c32.c
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@ -711,12 +711,13 @@ static void sp_512to256_mont_shift_8(sp_digit* r, sp_digit* a)
memcpy(r, a + 8, sizeof(*r) * 8); memcpy(r, a + 8, sizeof(*r) * 8);
} }
// Disabled for now. Seems to work, but ugly and 40 bytes larger on x86-64. #if UNALIGNED_LE_64BIT
#if 0 //UNALIGNED_LE_64BIT
/* 64-bit little-endian optimized version. /* 64-bit little-endian optimized version.
* See generic 32-bit version below for explanation. * See generic 32-bit version below for explanation.
* The benefit of this version is: even though r[3] calculation is atrocious, * The benefit of this version is: even though r[3] calculation is atrocious,
* we call sp_256_mul_add_4() four times, not 8. * we call sp_256_mul_add_4() four times, not 8.
* Measured run time improvement of curve_P256_compute_pubkey_and_premaster()
* call on x86-64: from ~1500us to ~900us. Code size +32 bytes.
*/ */
static int sp_256_mul_add_4(uint64_t *r /*, const uint64_t* a, uint64_t b*/) static int sp_256_mul_add_4(uint64_t *r /*, const uint64_t* a, uint64_t b*/)
{ {
@ -794,18 +795,18 @@ static int sp_256_mul_add_4(uint64_t *r /*, const uint64_t* a, uint64_t b*/)
t64u = (t64 < b); t64u = (t64 < b);
t64 += r[3]; t64 += r[3];
t64u += (t64 < r[3]); t64u += (t64 < r[3]);
{ { // add ((((uint128_t)b << 32) - b) << 32):
uint64_t lo,hi; uint64_t lo, hi;
//lo = (((b << 32) - b) << 32 //lo = (((b << 32) - b) << 32
//hi = (((uint128_t)b << 32) - b) >> 32 //hi = (((uint128_t)b << 32) - b) >> 32
//but without uint128_t: //but without uint128_t:
hi = (b << 32) - b; /* form lower 32 bits of "hi" part 1 */ hi = (b << 32) - b; /* make lower 32 bits of "hi", part 1 */
b = (b >> 32) - (/*borrowed above?*/(b << 32) < b); /* upper 32 bits of "hi" are in b */ b = (b >> 32) - (/*borrowed above?*/(b << 32) < b); /* upper 32 bits of "hi" are in b */
lo = hi << 32; /* (use "hi" value to calculate "lo",... */ lo = hi << 32; /* (use "hi" value to calculate "lo",... */
t64 += lo; /* ...consume... */ t64 += lo; /* ...consume... */
t64u += (t64 < lo); /* ..."lo") */ t64u += (t64 < lo); /* ..."lo") */
hi >>= 32; /* form lower 32 bits of "hi" part 2 */ hi >>= 32; /* make lower 32 bits of "hi", part 2 */
hi |= (b << 32); /* combine lower and upper */ hi |= (b << 32); /* combine lower and upper 32 bits */
t64u += hi; /* consume "hi" */ t64u += hi; /* consume "hi" */
} }
//t_hi = (t < m); //t_hi = (t < m);