Unroll secp256k1_fe_(get|set)_b32 for 10x26.

field_get_b32: min 0.890us / avg 0.905us / max 0.956us
field_set_b32: min 1.12us / avg 1.15us / max 1.19us

becomes

field_get_b32: min 0us / avg 0.000000119us / max 0.000000238us
field_set_b32: min 0.0532us / avg 0.0584us / max 0.0782us

Author: gmaxwell

src/field_10x26_impl.h

@@ -321,17 +321,17 @@ static int secp256k1_fe_cmp_var(const secp256k1_fe *a, const secp256k1_fe *b) {
 }
 
 static int secp256k1_fe_set_b32(secp256k1_fe *r, const unsigned char *a) {
-    int i;
-    r->n[0] = r->n[1] = r->n[2] = r->n[3] = r->n[4] = 0;
-    r->n[5] = r->n[6] = r->n[7] = r->n[8] = r->n[9] = 0;
-    for (i=0; i<32; i++) {
-        int j;
-        for (j=0; j<4; j++) {
-            int limb = (8*i+2*j)/26;
-            int shift = (8*i+2*j)%26;
-            r->n[limb] |= (uint32_t)((a[31-i] >> (2*j)) & 0x3) << shift;
-        }
-    }
+    r->n[0] = (uint32_t)a[31] | ((uint32_t)a[30] << 8) | ((uint32_t)a[29] << 16) | ((uint32_t)(a[28] & 0x3) << 24);
+    r->n[1] = (uint32_t)((a[28] >> 2) & 0x3f) | ((uint32_t)a[27] << 6) | ((uint32_t)a[26] << 14) | ((uint32_t)(a[25] & 0xf) << 22);
+    r->n[2] = (uint32_t)((a[25] >> 4) & 0xf) | ((uint32_t)a[24] << 4) | ((uint32_t)a[23] << 12) | ((uint32_t)(a[22] & 0x3f) << 20);
+    r->n[3] = (uint32_t)((a[22] >> 6) & 0x3) | ((uint32_t)a[21] << 2) | ((uint32_t)a[20] << 10) | ((uint32_t)a[19] << 18);
+    r->n[4] = (uint32_t)a[18] | ((uint32_t)a[17] << 8) | ((uint32_t)a[16] << 16) | ((uint32_t)(a[15] & 0x3) << 24);
+    r->n[5] = (uint32_t)((a[15] >> 2) & 0x3f) | ((uint32_t)a[14] << 6) | ((uint32_t)a[13] << 14) | ((uint32_t)(a[12] & 0xf) << 22);
+    r->n[6] = (uint32_t)((a[12] >> 4) & 0xf) | ((uint32_t)a[11] << 4) | ((uint32_t)a[10] << 12) | ((uint32_t)(a[9] & 0x3f) << 20);
+    r->n[7] = (uint32_t)((a[9] >> 6) & 0x3) | ((uint32_t)a[8] << 2) | ((uint32_t)a[7] << 10) | ((uint32_t)a[6] << 18);
+    r->n[8] = (uint32_t)a[5] | ((uint32_t)a[4] << 8) | ((uint32_t)a[3] << 16) | ((uint32_t)(a[2] & 0x3) << 24);
+    r->n[9] = (uint32_t)((a[2] >> 2) & 0x3f) | ((uint32_t)a[1] << 6) | ((uint32_t)a[0] << 14);
+
     if (r->n[9] == 0x3FFFFFUL && (r->n[8] & r->n[7] & r->n[6] & r->n[5] & r->n[4] & r->n[3] & r->n[2]) == 0x3FFFFFFUL && (r->n[1] + 0x40UL + ((r->n[0] + 0x3D1UL) >> 26)) > 0x3FFFFFFUL) {
         return 0;
     }
@@ -345,21 +345,42 @@ static int secp256k1_fe_set_b32(secp256k1_fe *r, const unsigned char *a) {
 
 /** Convert a field element to a 32-byte big endian value. Requires the input to be normalized */
 static void secp256k1_fe_get_b32(unsigned char *r, const secp256k1_fe *a) {
-    int i;
 #ifdef VERIFY
     VERIFY_CHECK(a->normalized);
     secp256k1_fe_verify(a);
 #endif
-    for (i=0; i<32; i++) {
-        int j;
-        int c = 0;
-        for (j=0; j<4; j++) {
-            int limb = (8*i+2*j)/26;
-            int shift = (8*i+2*j)%26;
-            c |= ((a->n[limb] >> shift) & 0x3) << (2 * j);
-        }
-        r[31-i] = c;
-    }
+    r[0] = (a->n[9] >> 14) & 0xff;
+    r[1] = (a->n[9] >> 6) & 0xff;
+    r[2] = ((a->n[9] & 0x3F) << 2) | ((a->n[8] >> 24) & 0x3);
+    r[3] = (a->n[8] >> 16) & 0xff;
+    r[4] = (a->n[8] >> 8) & 0xff;
+    r[5] = a->n[8] & 0xff;
+    r[6] = (a->n[7] >> 18) & 0xff;
+    r[7] = (a->n[7] >> 10) & 0xff;
+    r[8] = (a->n[7] >> 2) & 0xff;
+    r[9] = ((a->n[7] & 0x3) << 6) | ((a->n[6] >> 20) & 0x3f);
+    r[10] = (a->n[6] >> 12) & 0xff;
+    r[11] = (a->n[6] >> 4) & 0xff;
+    r[12] = ((a->n[6] & 0xf) << 4) | ((a->n[5] >> 22) & 0xf);
+    r[13] = (a->n[5] >> 14) & 0xff;
+    r[14] = (a->n[5] >> 6) & 0xff;
+    r[15] = ((a->n[5] & 0x3f) << 2) | ((a->n[4] >> 24) & 0x3);
+    r[16] = (a->n[4] >> 16) & 0xff;
+    r[17] = (a->n[4] >> 8) & 0xff;
+    r[18] = a->n[4] & 0xff;
+    r[19] = (a->n[3] >> 18) & 0xff;
+    r[20] = (a->n[3] >> 10) & 0xff;
+    r[21] = (a->n[3] >> 2) & 0xff;
+    r[22] = ((a->n[3] & 0x3) << 6) | ((a->n[2] >> 20) & 0x3f);
+    r[23] = (a->n[2] >> 12) & 0xff;
+    r[24] = (a->n[2] >> 4) & 0xff;
+    r[25] = ((a->n[2] & 0xf) << 4) | ((a->n[1] >> 22) & 0xf);
+    r[26] = (a->n[1] >> 14) & 0xff;
+    r[27] = (a->n[1] >> 6) & 0xff;
+    r[28] = ((a->n[1] & 0x3f) << 2) | ((a->n[0] >> 24) & 0x3);
+    r[29] = (a->n[0] >> 16) & 0xff;
+    r[30] = (a->n[0] >> 8) & 0xff;
+    r[31] = a->n[0] & 0xff;
 }
 
 SECP256K1_INLINE static void secp256k1_fe_negate(secp256k1_fe *r, const secp256k1_fe *a, int m) {