Protect SSE4 code behind a compile-time flag

configure.ac

@@ -177,6 +177,16 @@ AC_ARG_ENABLE([glibc-back-compat],
   [use_glibc_compat=$enableval],
   [use_glibc_compat=no])
 
+AC_ARG_ENABLE([experimental-asm],
+  [AS_HELP_STRING([--enable-experimental-asm],
+  [Enable experimental assembly routines (default is no)])],
+  [experimental_asm=$enableval],
+  [experimental_asm=no])
+
+if test "x$experimental_asm" = xyes; then
+  AC_DEFINE(EXPERIMENTAL_ASM, 1, [Define this symbol to build in experimental assembly routines])
+fi
+
 AC_ARG_WITH([system-univalue],
   [AS_HELP_STRING([--with-system-univalue],
   [Build with system UniValue (default is no)])],
@@ -1162,6 +1172,7 @@ AM_CONDITIONAL([USE_LCOV],[test x$use_lcov = xyes])
 AM_CONDITIONAL([GLIBC_BACK_COMPAT],[test x$use_glibc_compat = xyes])
 AM_CONDITIONAL([HARDEN],[test x$use_hardening = xyes])
 AM_CONDITIONAL([ENABLE_HWCRC32],[test x$enable_hwcrc32 = xyes])
+AM_CONDITIONAL([EXPERIMENTAL_ASM],[test x$experimental_asm = xyes])
 
 AC_DEFINE(CLIENT_VERSION_MAJOR, _CLIENT_VERSION_MAJOR, [Major version])
 AC_DEFINE(CLIENT_VERSION_MINOR, _CLIENT_VERSION_MINOR, [Minor version])

src/Makefile.am

@@ -263,11 +263,14 @@ crypto_libbitcoin_crypto_a_SOURCES = \
   crypto/sha1.cpp \
   crypto/sha1.h \
   crypto/sha256.cpp \
-  crypto/sha256_sse4.cpp \
   crypto/sha256.h \
   crypto/sha512.cpp \
   crypto/sha512.h
 
+if EXPERIMENTAL_ASM
+crypto_libbitcoin_crypto_a_SOURCES += crypto/sha256_sse4.cpp
+endif
+
 # consensus: shared between all executables that validate any consensus rules.
 libbitcoin_consensus_a_CPPFLAGS = $(AM_CPPFLAGS) $(BITCOIN_INCLUDES)
 libbitcoin_consensus_a_CXXFLAGS = $(AM_CXXFLAGS) $(PIE_FLAGS)

src/crypto/sha256.cpp

@@ -10,12 +10,14 @@
 #include <atomic>
 
 #if defined(__x86_64__) || defined(__amd64__)
+#if defined(EXPERIMENTAL_ASM)
 #include <cpuid.h>
 namespace sha256_sse4
 {
 void Transform(uint32_t* s, const unsigned char* chunk, size_t blocks);
 }
 #endif
+#endif
 
 // Internal implementation code.
 namespace
@@ -176,7 +178,7 @@ TransformType Transform = sha256::Transform;
 
 std::string SHA256AutoDetect()
 {
-#if defined(__x86_64__) || defined(__amd64__)
+#if defined(EXPERIMENTAL_ASM) && (defined(__x86_64__) || defined(__amd64__))
     uint32_t eax, ebx, ecx, edx;
     if (__get_cpuid(1, &eax, &ebx, &ecx, &edx) && (ecx >> 19) & 1) {
         Transform = sha256_sse4::Transform;