enforce explicit experimental configuration through annotation

Introduces a new support header that exposes the macro
EXPERIMENTAL_FEATURE that (a) allows us to clearly mark blocks
of code as experimental (or entire files) and (b) enforces that
none of the annotated code gets compiled if --enable-experimental
was not configured through a static assertion.

Existing experimental features are annotated:

- AVX2 using the intel-ipsec-mb dependency for SHA1/256/512
- ARMv8 intrinsics for SHA1/256
- ARMv82 intrinsics for SHA512
- SSE2 for scrypt

src/Makefile.am

@@ -143,6 +143,7 @@ BITCOIN_CORE_H = \
   support/allocators/zeroafterfree.h \
   support/cleanse.h \
   support/events.h \
+  support/experimental.h \
   support/lockedpool.h \
   sync.h \
   threadsafety.h \

src/crypto/scrypt-sse2.cpp

@@ -28,12 +28,16 @@
  */
 
 #include "crypto/scrypt.h"
+#include "support/experimental.h"
 #include <stdlib.h>
 #include <stdint.h>
 #include <string.h>
 
 #include <emmintrin.h>
 
+// this entire functionality is experimental
+EXPERIMENTAL_FEATURE
+
 static inline void xor_salsa8_sse2(__m128i B[4], const __m128i Bx[4])
 {
 	__m128i X0, X1, X2, X3;

src/crypto/sha1.cpp

@@ -6,6 +6,7 @@
 #include "crypto/sha1.h"
 
 #include "crypto/common.h"
+#include "support/experimental.h"
 
 #include <string.h>
 
@@ -71,6 +72,9 @@ const uint32_t k4 = 0xCA62C1D6ul;
 void Transform(uint32_t* s, const unsigned char* chunk)
 {
 #if defined(USE_ARMV8) || defined(USE_ARMV82)
+    // this entire block is experimental
+    EXPERIMENTAL_FEATURE
+
     uint32x4_t ABCD, ABCD_SAVED;
     uint32x4_t TMP0, TMP1;
     uint32x4_t MSG0, MSG1, MSG2, MSG3;
@@ -241,6 +245,7 @@ void Transform(uint32_t* s, const unsigned char* chunk)
 
 #elif USE_AVX2
     // Perform SHA1 one block (Intel AVX2)
+    EXPERIMENTAL_FEATURE
     sha1_one_block_avx2(chunk, s);
 #else
     // Perform SHA one block (legacy)

src/crypto/sha256.cpp

@@ -6,6 +6,7 @@
 #include "crypto/sha256.h"
 
 #include "crypto/common.h"
+#include "support/experimental.h"
 
 #include <string.h>
 
@@ -91,6 +92,9 @@ void inline Initialize(uint32_t* s)
 void Transform(uint32_t* s, const unsigned char* chunk)
 {
 #if defined(USE_ARMV8) || defined(USE_ARMV82)
+    // entire block is experimental
+    EXPERIMENTAL_FEATURE
+
     uint32x4_t STATE0, STATE1, ABEF_SAVE, CDGH_SAVE;
     uint32x4_t MSG0, MSG1, MSG2, MSG3;
     uint32x4_t TMP0, TMP1, TMP2;
@@ -247,6 +251,7 @@ void Transform(uint32_t* s, const unsigned char* chunk)
 
 #elif USE_AVX2
     // Perform SHA256 one block (Intel AVX2)
+    EXPERIMENTAL_FEATURE
     sha256_one_block_avx2(chunk, s);
 #else
     // Perform SHA256 one block (legacy)

src/crypto/sha512.cpp

@@ -6,6 +6,7 @@
 #include "crypto/sha512.h"
 
 #include "crypto/common.h"
+#include "support/experimental.h"
 
 #include <string.h>
 
@@ -100,6 +101,8 @@ void inline Round(uint64_t a, uint64_t b, uint64_t c, uint64_t& d, uint64_t e, u
 
 #ifdef USE_ARMV82
 
+EXPERIMENTAL_FEATURE
+
 /* ----------------------------------------------------------------------
  * Hardware-accelerated implementation of SHA-512 using Arm NEON.
  */
@@ -311,6 +314,7 @@ void Transform(uint64_t* s, const unsigned char* chunk)
 {
 #ifdef USE_AVX2
     // Perform SHA512 one block (Intel AVX2)
+    EXPERIMENTAL_FEATURE
     sha512_one_block_avx2(chunk, s);
 #elif USE_ARMV82
     sha512_neon_core core;

src/support/experimental.h

@@ -0,0 +1,22 @@
+// Copyright (c) 2022 The Dogecoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef DOGECOIN_SUPPORT_EXPERIMENTAL_H
+#define DOGECOIN_SUPPORT_EXPERIMENTAL_H
+
+// include config for experimental flag
+#if defined(HAVE_CONFIG_H)
+#include "config/bitcoin-config.h"
+#endif //HAVE_CONFIG_H
+
+#if defined(ALLOW_DOGECOIN_EXPERIMENTAL)
+#define EXPERIMENTAL_FEATURES_ALLOWED 1
+#else
+#define EXPERIMENTAL_FEATURES_ALLOWED 0
+#endif // ALLOW_DOGECOIN_EXPERIMENTAL
+
+#define EXPERIMENTAL_FEATURE static_assert(EXPERIMENTAL_FEATURES_ALLOWED == 1, \
+     "Experimental features need to be explicitly enabled during configuration.");
+
+#endif //BITCOIN_SUPPORT_EXPERIMENTAL_H