tls: P256: x86-64 optimized sp_256_sub_8_p256_mod

function                                             old     new   delta
sp_256_sub_8_p256_mod                                  -      53     +53
sp_256_mont_reduce_8                                 223     217      -6
sp_256_mont_dbl_8                                     38      32      -6
sp_256_ecc_mulmod_8                                 1535    1529      -6
sp_256_proj_point_dbl_8                              469     454     -15
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 0/4 up/down: 53/-33)             Total: 20 bytes

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

1 files changed, 31 insertions, 1 deletions

diff --git a/networking/tls_sp_c32.c b/networking/tls_sp_c32.c
index 14a7c7066..1391cb405 100644
--- a/networking/tls_sp_c32.c
+++ b/networking/tls_sp_c32.c
@@ -346,8 +346,8 @@ static int sp_256_sub_8(sp_digit* r, const sp_digit* a, const sp_digit* b)
 #endif
 }
 
-#if ALLOW_ASM && defined(__GNUC__) && defined(__i386__)
 /* Sub p256_mod from a into r. (r = a - p256_mod). */
+#if ALLOW_ASM && defined(__GNUC__) && defined(__i386__)
 static void sp_256_sub_8_p256_mod(sp_digit* r, const sp_digit* a)
 {
         sp_digit reg;
@@ -390,6 +390,36 @@ static void sp_256_sub_8_p256_mod(sp_digit* r, const sp_digit* a)
                 : "memory"
         );
 }
+#elif ALLOW_ASM && defined(__GNUC__) && defined(__x86_64__)
+static void sp_256_sub_8_p256_mod(sp_digit* r, const sp_digit* a)
+{
+        uint64_t reg;
+        uint64_t ooff;
+//p256_mod[3..0] = ffffffff00000001 0000000000000000 00000000ffffffff ffffffffffffffff
+        asm volatile (
+"\n             movq    (%0), %3"
+"\n             addq    $1, %3"         // adding 1 is the same as subtracting ffffffffffffffff
+"\n             movq    %3, (%1)"       //
+"\n             cmc"                    // only carry bit needs inverting
+"\n"
+"\n             movq    1*8(%0), %3"
+"\n             sbbq    %2, %3"         // %2 holds 00000000ffffffff
+"\n             movq    %3, 1*8(%1)"
+"\n"
+"\n             movq    2*8(%0), %3"
+"\n             sbbq    $0, %3"
+"\n             movq    %3, 2*8(%1)"
+"\n"
+"\n             movq    3*8(%0), %3"
+"\n             sbbq    $0, %3"         // adding 00000000ffffffff (in %2)
+"\n             addq    %2, %3"         // is the same as subtracting ffffffff00000001
+"\n             movq    %3, 3*8(%1)"
+"\n"
+                : "=r" (a), "=r" (r), "=r" (ooff), "=r" (reg)
+                : "0" (a), "1" (r), "2" (0x00000000ffffffff)
+                : "memory"
+        );
+}
 #else
 # define sp_256_sub_8_p256_mod(r, a) sp_256_sub_8((r), (a), p256_mod)
 #endif