refactor: Make 64-bit shift explicit

In addition, this change enables MSVC warning C4334 for the entire
codebase.
See: https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-3-c4334

Required to enable level 3 warnings.

Author: hebasto

CMakeLists.txt

@@ -207,6 +207,7 @@ include(TryAppendCFlags)
 if(MSVC)
   # Keep the following commands ordered lexicographically.
   try_append_c_flags(/W2) # Moderate warning level.
+  try_append_c_flags(/w24334) # Enable warning C4334 "result of 32-bit shift implicitly converted to 64 bits" as a level 2 one.
   try_append_c_flags(/wd4146) # Disable warning C4146 "unary minus operator applied to unsigned type, result still unsigned".
 else()
   # Keep the following commands ordered lexicographically.

configure.ac

@@ -121,7 +121,9 @@ AC_DEFUN([SECP_TRY_APPEND_DEFAULT_CFLAGS], [
     # libtool makes the same assumption internally.
     # Note that "/opt" and "-opt" are equivalent for MSVC; we use "-opt" because "/opt" looks like a path.
     if test x"$GCC" != x"yes" && test x"$build_windows" = x"yes"; then
-      SECP_TRY_APPEND_CFLAGS([-W2 -wd4146], $1) # Moderate warning level, disable warning C4146 "unary minus operator applied to unsigned type, result still unsigned"
+      SECP_TRY_APPEND_CFLAGS([-W2], $1) # Moderate warning level.
+      SECP_TRY_APPEND_CFLAGS([-w24334], $1) # Enable warning C4334 "result of 32-bit shift implicitly converted to 64 bits" as a level 2 one.
+      SECP_TRY_APPEND_CFLAGS([-wd4146], $1) # Disable warning C4146 "unary minus operator applied to unsigned type, result still unsigned".
       # We pass -ignore:4217 to the MSVC linker to suppress warning 4217 when
       # importing variables from a statically linked secp256k1.
       # (See the libtool manual, section "Windows DLLs" for background.)

src/ecmult_impl.h

@@ -683,7 +683,7 @@ static int secp256k1_ecmult_pippenger_batch(const secp256k1_callback* error_call
     }
     state_space->ps = (struct secp256k1_pippenger_point_state *) secp256k1_scratch_alloc(error_callback, scratch, entries * sizeof(*state_space->ps));
     state_space->wnaf_na = (int *) secp256k1_scratch_alloc(error_callback, scratch, entries*(WNAF_SIZE(bucket_window+1)) * sizeof(int));
-    buckets = (secp256k1_gej *) secp256k1_scratch_alloc(error_callback, scratch, (1<<bucket_window) * sizeof(*buckets));
+    buckets = (secp256k1_gej *) secp256k1_scratch_alloc(error_callback, scratch, (1ULL << bucket_window) * sizeof(*buckets));
     if (state_space->ps == NULL || state_space->wnaf_na == NULL || buckets == NULL) {
         secp256k1_scratch_apply_checkpoint(error_callback, scratch, scratch_checkpoint);
         return 0;

src/tests.c

@@ -2221,7 +2221,7 @@ static void scalar_test(void) {
         for (i = 0; i < 100; ++i) {
             int low;
             int shift = 1 + secp256k1_testrand_int(15);
-            int expected = r.d[0] % (1 << shift);
+            int expected = r.d[0] % (1ULL << shift);
             low = secp256k1_scalar_shr_int(&r, shift);
             CHECK(expected == low);
         }