Example 1 - avoid optimizing out stack variable clearing by using an __attribute__

Author: isle2983

src/ecmult_gen_impl.h

@@ -150,7 +150,7 @@ static void secp256k1_ecmult_gen(const secp256k1_ecmult_gen_context *ctx, secp25
         secp256k1_ge_from_storage(&add, &adds);
         secp256k1_gej_add_ge(r, r, &add);
     }
-    bits = 0;
+    secp256k1_int_clear(&bits);
     secp256k1_ge_clear(&add);
     secp256k1_scalar_clear(&gnb);
 }

src/group_impl.h

@@ -7,6 +7,7 @@
 #ifndef _SECP256K1_GROUP_IMPL_H_
 #define _SECP256K1_GROUP_IMPL_H_
 
+#include <string.h>
 #include "num.h"
 #include "field.h"
 #include "group.h"
@@ -213,6 +214,14 @@ static void secp256k1_ge_clear(secp256k1_ge *r) {
     secp256k1_fe_clear(&r->y);
 }
 
+static void __attribute__((optimize("O0"))) secp256k1_secure_clear(void *s, size_t n) {
+    (void) memset(s, 0, n);
+}
+
+static void secp256k1_int_clear(int *r) {
+    secp256k1_secure_clear(r, sizeof(*r));
+}
+
 static int secp256k1_ge_set_xquad(secp256k1_ge *r, const secp256k1_fe *x) {
     secp256k1_fe x2, x3, c;
     r->x = *x;