Merge bitcoin#678: Preventing compiler optimizations in benchmarks wi…

…thout a memory fence

362bb25 Modified bench_scalar_split so it won't get optimized out (Elichai Turkel)
73a30c6 Added accumulators and checks on benchmarks so they won't get optimized out (Elichai Turkel)

Pull request description:

  As asked bitcoin-core/secp256k1#667 (comment) this is the parts of bitcoin#667 that don't require an assembly memory fence.

  I splitted them to 2 commits, one with obvious easy ones. and another that changes the logic a bit to achieve this (See bitcoin-core/secp256k1#667 (comment) )

ACKs for top commit:
  jonasnick:
    ACK 362bb25
  real-or-random:
    ACK 362bb25 I read the diff and I ran the benchmarks

Tree-SHA512: d5e47f5d64c3b035155276f057671ceb7f5852f24c7102fee4d0141aabebf882039f3eae0d152bae89d0603bc09fa6ad9f7bc6b8c0f74a668ee252c727517804

src/bench_internal.c

@@ -57,12 +57,13 @@ void bench_setup(void* arg) {
 }
 
 void bench_scalar_add(void* arg) {
-    int i;
+    int i, j = 0;
     bench_inv *data = (bench_inv*)arg;
 
     for (i = 0; i < 2000000; i++) {
-        secp256k1_scalar_add(&data->scalar_x, &data->scalar_x, &data->scalar_y);
+        j += secp256k1_scalar_add(&data->scalar_x, &data->scalar_x, &data->scalar_y);
     }
+    CHECK(j <= 2000000);
 }
 
 void bench_scalar_negate(void* arg) {
@@ -94,35 +95,37 @@ void bench_scalar_mul(void* arg) {
 
 #ifdef USE_ENDOMORPHISM
 void bench_scalar_split(void* arg) {
-    int i;
+    int i, j = 0;
     bench_inv *data = (bench_inv*)arg;
 
     for (i = 0; i < 20000; i++) {
-        secp256k1_scalar l, r;
-        secp256k1_scalar_split_lambda(&l, &r, &data->scalar_x);
-        secp256k1_scalar_add(&data->scalar_x, &data->scalar_x, &data->scalar_y);
+        secp256k1_scalar_split_lambda(&data->scalar_x, &data->scalar_y, &data->scalar_x);
+        j += secp256k1_scalar_add(&data->scalar_x, &data->scalar_x, &data->scalar_y);
     }
+    CHECK(j <= 20000);
 }
 #endif
 
 void bench_scalar_inverse(void* arg) {
-    int i;
+    int i, j = 0;
     bench_inv *data = (bench_inv*)arg;
 
     for (i = 0; i < 2000; i++) {
         secp256k1_scalar_inverse(&data->scalar_x, &data->scalar_x);
-        secp256k1_scalar_add(&data->scalar_x, &data->scalar_x, &data->scalar_y);
+        j += secp256k1_scalar_add(&data->scalar_x, &data->scalar_x, &data->scalar_y);
     }
+    CHECK(j <= 2000);
 }
 
 void bench_scalar_inverse_var(void* arg) {
-    int i;
+    int i, j = 0;
     bench_inv *data = (bench_inv*)arg;
 
     for (i = 0; i < 2000; i++) {
         secp256k1_scalar_inverse_var(&data->scalar_x, &data->scalar_x);
-        secp256k1_scalar_add(&data->scalar_x, &data->scalar_x, &data->scalar_y);
+        j += secp256k1_scalar_add(&data->scalar_x, &data->scalar_x, &data->scalar_y);
     }
+    CHECK(j <= 2000);
 }
 
 void bench_field_normalize(void* arg) {
@@ -182,15 +185,16 @@ void bench_field_inverse_var(void* arg) {
 }
 
 void bench_field_sqrt(void* arg) {
-    int i;
+    int i, j = 0;
     bench_inv *data = (bench_inv*)arg;
     secp256k1_fe t;
 
     for (i = 0; i < 20000; i++) {
         t = data->fe_x;
-        secp256k1_fe_sqrt(&data->fe_x, &t);
+        j += secp256k1_fe_sqrt(&data->fe_x, &t);
         secp256k1_fe_add(&data->fe_x, &data->fe_y);
     }
+    CHECK(j <= 20000);
 }
 
 void bench_group_double_var(void* arg) {
@@ -230,32 +234,37 @@ void bench_group_add_affine_var(void* arg) {
 }
 
 void bench_group_jacobi_var(void* arg) {
-    int i;
+    int i, j = 0;
     bench_inv *data = (bench_inv*)arg;
 
     for (i = 0; i < 20000; i++) {
-        secp256k1_gej_has_quad_y_var(&data->gej_x);
+        j += secp256k1_gej_has_quad_y_var(&data->gej_x);
     }
+    CHECK(j == 20000);
 }
 
 void bench_ecmult_wnaf(void* arg) {
-    int i;
+    int i, bits = 0, overflow = 0;
     bench_inv *data = (bench_inv*)arg;
 
     for (i = 0; i < 20000; i++) {
-        secp256k1_ecmult_wnaf(data->wnaf, 256, &data->scalar_x, WINDOW_A);
-        secp256k1_scalar_add(&data->scalar_x, &data->scalar_x, &data->scalar_y);
+        bits += secp256k1_ecmult_wnaf(data->wnaf, 256, &data->scalar_x, WINDOW_A);
+        overflow += secp256k1_scalar_add(&data->scalar_x, &data->scalar_x, &data->scalar_y);
     }
+    CHECK(overflow >= 0);
+    CHECK(bits <= 256*20000);
 }
 
 void bench_wnaf_const(void* arg) {
-    int i;
+    int i, bits = 0, overflow = 0;
     bench_inv *data = (bench_inv*)arg;
 
     for (i = 0; i < 20000; i++) {
-        secp256k1_wnaf_const(data->wnaf, &data->scalar_x, WINDOW_A, 256);
-        secp256k1_scalar_add(&data->scalar_x, &data->scalar_x, &data->scalar_y);
+        bits += secp256k1_wnaf_const(data->wnaf, &data->scalar_x, WINDOW_A, 256);
+        overflow += secp256k1_scalar_add(&data->scalar_x, &data->scalar_x, &data->scalar_y);
     }
+    CHECK(overflow >= 0);
+    CHECK(bits <= 256*20000);
 }
 
 
@@ -312,7 +321,7 @@ void bench_context_sign(void* arg) {
 
 #ifndef USE_NUM_NONE
 void bench_num_jacobi(void* arg) {
-    int i;
+    int i, j = 0;
     bench_inv *data = (bench_inv*)arg;
     secp256k1_num nx, norder;
 
@@ -321,8 +330,9 @@ void bench_num_jacobi(void* arg) {
     secp256k1_scalar_get_num(&norder, &data->scalar_y);
 
     for (i = 0; i < 200000; i++) {
-        secp256k1_num_jacobi(&nx, &norder);
+        j += secp256k1_num_jacobi(&nx, &norder);
     }
+    CHECK(j <= 200000);
 }
 #endif