Merge bitcoin-core/secp256k1#1794: ecmult: Use size_t for array indices

47eb70959a ecmult: Use size_t for array indices in _odd_multiplies_table (Tim Ruffing)
bb1d199de5 ecmult: Use size_t for array indices into tables (Tim Ruffing)

Pull request description:

  I don't think the current code is incorrect, but using `size_t` improves readability because the type makes it clear that we're dealing with array indices.

  Also, making the result of the `ECMULT_TABLE_SIZE` macro (hopefully) a `size_t` fixes a compiler warning on MSVC, see #1791.

ACKs for top commit:
  hebasto:
    re-ACK 47eb70959a.
  jonasnick:
    ACK 47eb70959a
  theStack:
    ACK 47eb70959a

Tree-SHA512: e484fd610d50e972021c0184a683993364290eb58e09b65f9521b4507ec8d0639b402c67002005630b389bc863a7aa05b75f7224524dbcbafbfa5f9a4812b4a5

src/ecmult.h

@@ -38,7 +38,7 @@
 #endif
 
 /** The number of entries a table with precomputed multiples needs to have. */
-#define ECMULT_TABLE_SIZE(w) (1L << ((w)-2))
+#define ECMULT_TABLE_SIZE(w) ((size_t)1 << ((w)-2))
 
 /** Double multiply: R = na*A + ng*G */
 static void secp256k1_ecmult(secp256k1_gej *r, const secp256k1_gej *a, const secp256k1_scalar *na, const secp256k1_scalar *ng);

src/ecmult_compute_table_impl.h

@@ -16,7 +16,7 @@
 static void secp256k1_ecmult_compute_table(secp256k1_ge_storage* table, int window_g, const secp256k1_gej* gen) {
     secp256k1_gej gj;
     secp256k1_ge ge, dgen;
-    int j;
+    size_t j;
 
     gj = *gen;
     secp256k1_ge_set_gej_var(&ge, &gj);

src/ecmult_impl.h

@@ -70,10 +70,10 @@
  *  Lastly the zr[0] value, which isn't used above, is set so that:
  *  - a.z = z(pre_a[0]) / zr[0]
  */
-static void secp256k1_ecmult_odd_multiples_table(int n, secp256k1_ge *pre_a, secp256k1_fe *zr, secp256k1_fe *z, const secp256k1_gej *a) {
+static void secp256k1_ecmult_odd_multiples_table(size_t n, secp256k1_ge *pre_a, secp256k1_fe *zr, secp256k1_fe *z, const secp256k1_gej *a) {
     secp256k1_gej d, ai;
     secp256k1_ge d_ge;
-    int i;
+    size_t i;
 
     VERIFY_CHECK(!secp256k1_gej_is_infinity(a));
 
@@ -311,8 +311,9 @@ static void secp256k1_ecmult_strauss_wnaf(const struct secp256k1_strauss_state *
     }
 
     for (np = 0; np < no; ++np) {
-        for (i = 0; i < ECMULT_TABLE_SIZE(WINDOW_A); i++) {
-            secp256k1_fe_mul(&state->aux[np * ECMULT_TABLE_SIZE(WINDOW_A) + i], &state->pre_a[np * ECMULT_TABLE_SIZE(WINDOW_A) + i].x, &secp256k1_const_beta);
+        size_t j;
+        for (j = 0; j < ECMULT_TABLE_SIZE(WINDOW_A); j++) {
+            secp256k1_fe_mul(&state->aux[np * ECMULT_TABLE_SIZE(WINDOW_A) + j], &state->pre_a[np * ECMULT_TABLE_SIZE(WINDOW_A) + j].x, &secp256k1_const_beta);
         }
     }
 
@@ -517,7 +518,6 @@ static int secp256k1_ecmult_pippenger_wnaf(secp256k1_gej *buckets, int bucket_wi
     size_t np;
     size_t no = 0;
     int i;
-    int j;
 
     for (np = 0; np < num; ++np) {
         if (secp256k1_scalar_is_zero(&sc[np]) || secp256k1_ge_is_infinity(&pt[np])) {
@@ -535,16 +535,17 @@ static int secp256k1_ecmult_pippenger_wnaf(secp256k1_gej *buckets, int bucket_wi
 
     for (i = n_wnaf - 1; i >= 0; i--) {
         secp256k1_gej running_sum;
+        int j;
+        size_t buc;
 
-        for(j = 0; j < ECMULT_TABLE_SIZE(bucket_window+2); j++) {
-            secp256k1_gej_set_infinity(&buckets[j]);
+        for (buc = 0; buc < ECMULT_TABLE_SIZE(bucket_window+2); buc++) {
+            secp256k1_gej_set_infinity(&buckets[buc]);
         }
 
         for (np = 0; np < no; ++np) {
             int n = state->wnaf_na[np*n_wnaf + i];
             struct secp256k1_pippenger_point_state point_state = state->ps[np];
             secp256k1_ge tmp;
-            int idx;
 
             if (i == 0) {
                 /* correct for wnaf skew */
@@ -555,16 +556,16 @@ static int secp256k1_ecmult_pippenger_wnaf(secp256k1_gej *buckets, int bucket_wi
                 }
             }
             if (n > 0) {
-                idx = (n - 1)/2;
-                secp256k1_gej_add_ge_var(&buckets[idx], &buckets[idx], &pt[point_state.input_pos], NULL);
+                buc = (n - 1)/2;
+                secp256k1_gej_add_ge_var(&buckets[buc], &buckets[buc], &pt[point_state.input_pos], NULL);
             } else if (n < 0) {
-                idx = -(n + 1)/2;
+                buc = -(n + 1)/2;
                 secp256k1_ge_neg(&tmp, &pt[point_state.input_pos]);
-                secp256k1_gej_add_ge_var(&buckets[idx], &buckets[idx], &tmp, NULL);
+                secp256k1_gej_add_ge_var(&buckets[buc], &buckets[buc], &tmp, NULL);
             }
         }
 
-        for(j = 0; j < bucket_window; j++) {
+        for (j = 0; j < bucket_window; j++) {
             secp256k1_gej_double_var(r, r, NULL);
         }
 
@@ -577,8 +578,8 @@ static int secp256k1_ecmult_pippenger_wnaf(secp256k1_gej *buckets, int bucket_wi
          *
          * The doubling is done implicitly by deferring the final window doubling (of 'r').
          */
-        for(j = ECMULT_TABLE_SIZE(bucket_window+2) - 1; j > 0; j--) {
-            secp256k1_gej_add_var(&running_sum, &running_sum, &buckets[j], NULL);
+        for (buc = ECMULT_TABLE_SIZE(bucket_window+2) - 1; buc > 0; buc--) {
+            secp256k1_gej_add_var(&running_sum, &running_sum, &buckets[buc], NULL);
             secp256k1_gej_add_var(r, r, &running_sum, NULL);
         }
 

src/precompute_ecmult.c

@@ -20,7 +20,7 @@
 #include "ecmult_compute_table_impl.h"
 
 static void print_table(FILE *fp, const char *name, int window_g, const secp256k1_ge_storage* table) {
-    int j;
+    size_t j;
     int i;
 
     fprintf(fp, "const secp256k1_ge_storage %s[ECMULT_TABLE_SIZE(WINDOW_G)] = {\n", name);