Merge bitcoin-core/secp256k1#1054: tests: Fix test whose result is implementation-defined

3d7cbaf tests: Fix test whose result is implementation-defined (Tim Ruffing)

Pull request description:

  A compiler may add struct padding and fe_cmov is not guaranteed to
  preserve it.

  On the way, we restore the name of the function. It was mistakenly
  renamed in 6173839 using
  "search and replace".

ACKs for top commit:
  robot-dreams:
    ACK 3d7cbaf
  sipa:
    utACK 3d7cbaf

Tree-SHA512: f8bb643d4915e9ce9c4fe45b48a2878f6cf1f29e654be1c150cdf65c6959cf65f8491928cf098da5a01f1d488ba475914905ca96b232abed499eb6ed65e53fb8

Author: real-or-random

src/tests.c

@@ -2451,13 +2451,16 @@ void run_field_convert(void) {
     CHECK(secp256k1_memcmp_var(&fes2, &fes, sizeof(fes)) == 0);
 }
 
-int fe_secp256k1_memcmp_var(const secp256k1_fe *a, const secp256k1_fe *b) {
-    secp256k1_fe t = *b;
+/* Returns true if two field elements have the same representation. */
+int fe_identical(const secp256k1_fe *a, const secp256k1_fe *b) {
+    int ret = 1;
 #ifdef VERIFY
-    t.magnitude = a->magnitude;
-    t.normalized = a->normalized;
+    ret &= (a->magnitude == b->magnitude);
+    ret &= (a->normalized == b->normalized);
 #endif
-    return secp256k1_memcmp_var(a, &t, sizeof(secp256k1_fe));
+    /* Compare the struct member that holds the limbs. */
+    ret &= (secp256k1_memcmp_var(a->n, b->n, sizeof(a->n)) == 0);
+    return ret;
 }
 
 void run_field_misc(void) {
@@ -2483,13 +2486,13 @@ void run_field_misc(void) {
         CHECK(x.normalized && x.magnitude == 1);
 #endif
         secp256k1_fe_cmov(&x, &x, 1);
-        CHECK(fe_secp256k1_memcmp_var(&x, &z) != 0);
-        CHECK(fe_secp256k1_memcmp_var(&x, &q) == 0);
+        CHECK(!fe_identical(&x, &z));
+        CHECK(fe_identical(&x, &q));
         secp256k1_fe_cmov(&q, &z, 1);
 #ifdef VERIFY
         CHECK(!q.normalized && q.magnitude == z.magnitude);
 #endif
-        CHECK(fe_secp256k1_memcmp_var(&q, &z) == 0);
+        CHECK(fe_identical(&q, &z));
         secp256k1_fe_normalize_var(&x);
         secp256k1_fe_normalize_var(&z);
         CHECK(!secp256k1_fe_equal_var(&x, &z));