Merge bitcoin#550: Optimize secp256k1_fe_normalize_weak calls.

9bd89c8 Optimize secp256k1_fe_normalize_weak calls. Move secp256k1_fe_normalize_weak calls out of ECMULT_TABLE_GET_GE and ECMULT_TABLE_GET_GE_STORAGE and into secp256k1_ge_globalz_set_table_gej instead. (Russell O'Connor)

Pull request description:

  Move secp256k1_fe_normalize_weak calls out of ECMULT_TABLE_GET_GE and ECMULT_TABLE_GET_GE_STORAGE and into secp256k1_ge_globalz_set_table_gej instead.

Tree-SHA512: 7bbb1aca8e37a268a26d7061bd1f390db129e697792f1d5ddd10ea34927616edc26ef118b500c3e5e14d1d463196033ef64e4d34b765380325c24835458b7a9b

Author: gmaxwell

src/ecmult_impl.h

@@ -277,7 +277,8 @@ static void secp256k1_ecmult_odd_multiples_table_storage_var(const int n, secp25
     if ((n) > 0) { \
         *(r) = (pre)[((n)-1)/2]; \
     } else { \
-        secp256k1_ge_neg((r), &(pre)[(-(n)-1)/2]); \
+        *(r) = (pre)[(-(n)-1)/2]; \
+        secp256k1_fe_negate(&((r)->y), &((r)->y), 1); \
     } \
 } while(0)
 
@@ -289,7 +290,7 @@ static void secp256k1_ecmult_odd_multiples_table_storage_var(const int n, secp25
         secp256k1_ge_from_storage((r), &(pre)[((n)-1)/2]); \
     } else { \
         secp256k1_ge_from_storage((r), &(pre)[(-(n)-1)/2]); \
-        secp256k1_ge_neg((r), (r)); \
+        secp256k1_fe_negate(&((r)->y), &((r)->y), 1); \
     } \
 } while(0)
 

src/group_impl.h

@@ -175,6 +175,8 @@ static void secp256k1_ge_globalz_set_table_gej(size_t len, secp256k1_ge *r, secp
         /* The z of the final point gives us the "global Z" for the table. */
         r[i].x = a[i].x;
         r[i].y = a[i].y;
+        /* Ensure all y values are in weak normal form for fast negation of points */
+        secp256k1_fe_normalize_weak(&r[i].y);
         *globalz = a[i].z;
         r[i].infinity = 0;
         zs = zr[i];