serialization: detect byteswap builtins without autoconf tests

Rather than a complicated set of tests to decide which bswap functions to
use, always prefer the compiler built-ins when available.

These builtins and fallbacks can all be removed once we're using c++23, which
adds std::byteswap.

src/compat/byteswap.h

@@ -5,44 +5,66 @@
 #ifndef BITCOIN_COMPAT_BYTESWAP_H
 #define BITCOIN_COMPAT_BYTESWAP_H
 
-#if defined(HAVE_CONFIG_H)
-#include <config/bitcoin-config.h>
-#endif
-
 #include <cstdint>
-
-#if defined(HAVE_BYTESWAP_H)
-#include <byteswap.h>
+#ifdef _MSC_VER
+#include <cstdlib>
 #endif
 
-#if defined(MAC_OSX)
 
-#include <libkern/OSByteOrder.h>
-#define bswap_16(x) OSSwapInt16(x)
-#define bswap_32(x) OSSwapInt32(x)
-#define bswap_64(x) OSSwapInt64(x)
+// All internal_bswap_* functions can be replaced with std::byteswap once we
+// require c++23. Both libstdc++ and libc++ implement std::byteswap via these
+// builtins.
 
+#ifndef DISABLE_BUILTIN_BSWAPS
+#  if defined __has_builtin
+#    if __has_builtin(__builtin_bswap16)
+#      define bitcoin_builtin_bswap16(x) __builtin_bswap16(x)
+#    endif
+#    if __has_builtin(__builtin_bswap32)
+#      define bitcoin_builtin_bswap32(x) __builtin_bswap32(x)
+#    endif
+#    if __has_builtin(__builtin_bswap64)
+#      define bitcoin_builtin_bswap64(x) __builtin_bswap64(x)
+#    endif
+#  elif defined(_MSC_VER)
+#      define bitcoin_builtin_bswap16(x) _byteswap_ushort(x)
+#      define bitcoin_builtin_bswap32(x) _byteswap_ulong(x)
+#      define bitcoin_builtin_bswap64(x) _byteswap_uint64(x)
+#  endif
+#endif
+
+// MSVC's _byteswap_* functions are not constexpr
+
+#ifndef _MSC_VER
+#define BSWAP_CONSTEXPR constexpr
 #else
-// Non-MacOS / non-Darwin
+#define BSWAP_CONSTEXPR
+#endif
 
-#if HAVE_DECL_BSWAP_16 == 0
-inline uint16_t bswap_16(uint16_t x)
+inline BSWAP_CONSTEXPR uint16_t internal_bswap_16(uint16_t x)
 {
+#ifdef bitcoin_builtin_bswap16
+    return bitcoin_builtin_bswap16(x);
+#else
     return (x >> 8) | (x << 8);
+#endif
 }
-#endif // HAVE_DECL_BSWAP16 == 0
 
-#if HAVE_DECL_BSWAP_32 == 0
-inline uint32_t bswap_32(uint32_t x)
+inline BSWAP_CONSTEXPR uint32_t internal_bswap_32(uint32_t x)
 {
+#ifdef bitcoin_builtin_bswap32
+    return bitcoin_builtin_bswap32(x);
+#else
     return (((x & 0xff000000U) >> 24) | ((x & 0x00ff0000U) >>  8) |
             ((x & 0x0000ff00U) <<  8) | ((x & 0x000000ffU) << 24));
+#endif
 }
-#endif // HAVE_DECL_BSWAP32 == 0
 
-#if HAVE_DECL_BSWAP_64 == 0
-inline uint64_t bswap_64(uint64_t x)
+inline BSWAP_CONSTEXPR uint64_t internal_bswap_64(uint64_t x)
 {
+#ifdef bitcoin_builtin_bswap64
+    return bitcoin_builtin_bswap64(x);
+#else
      return (((x & 0xff00000000000000ull) >> 56)
           | ((x & 0x00ff000000000000ull) >> 40)
           | ((x & 0x0000ff0000000000ull) >> 24)
@@ -51,9 +73,7 @@ inline uint64_t bswap_64(uint64_t x)
           | ((x & 0x0000000000ff0000ull) << 24)
           | ((x & 0x000000000000ff00ull) << 40)
           | ((x & 0x00000000000000ffull) << 56));
+#endif
 }
-#endif // HAVE_DECL_BSWAP64 == 0
-
-#endif // defined(MAC_OSX)
 
 #endif // BITCOIN_COMPAT_BYTESWAP_H

src/compat/endian.h

@@ -76,7 +76,7 @@ inline uint16_t htobe16(uint16_t host_16bits)
 #if HAVE_DECL_HTOLE16 == 0
 inline uint16_t htole16(uint16_t host_16bits)
 {
-    return bswap_16(host_16bits);
+    return internal_bswap_16(host_16bits);
 }
 #endif // HAVE_DECL_HTOLE16
 
@@ -90,7 +90,7 @@ inline uint16_t be16toh(uint16_t big_endian_16bits)
 #if HAVE_DECL_LE16TOH == 0
 inline uint16_t le16toh(uint16_t little_endian_16bits)
 {
-    return bswap_16(little_endian_16bits);
+    return internal_bswap_16(little_endian_16bits);
 }
 #endif // HAVE_DECL_LE16TOH
 
@@ -104,7 +104,7 @@ inline uint32_t htobe32(uint32_t host_32bits)
 #if HAVE_DECL_HTOLE32 == 0
 inline uint32_t htole32(uint32_t host_32bits)
 {
-    return bswap_32(host_32bits);
+    return internal_bswap_32(host_32bits);
 }
 #endif // HAVE_DECL_HTOLE32
 
@@ -118,7 +118,7 @@ inline uint32_t be32toh(uint32_t big_endian_32bits)
 #if HAVE_DECL_LE32TOH == 0
 inline uint32_t le32toh(uint32_t little_endian_32bits)
 {
-    return bswap_32(little_endian_32bits);
+    return internal_bswap_32(little_endian_32bits);
 }
 #endif // HAVE_DECL_LE32TOH
 
@@ -132,7 +132,7 @@ inline uint64_t htobe64(uint64_t host_64bits)
 #if HAVE_DECL_HTOLE64 == 0
 inline uint64_t htole64(uint64_t host_64bits)
 {
-    return bswap_64(host_64bits);
+    return internal_bswap_64(host_64bits);
 }
 #endif // HAVE_DECL_HTOLE64
 
@@ -146,7 +146,7 @@ inline uint64_t be64toh(uint64_t big_endian_64bits)
 #if HAVE_DECL_LE64TOH == 0
 inline uint64_t le64toh(uint64_t little_endian_64bits)
 {
-    return bswap_64(little_endian_64bits);
+    return internal_bswap_64(little_endian_64bits);
 }
 #endif // HAVE_DECL_LE64TOH
 
@@ -155,7 +155,7 @@ inline uint64_t le64toh(uint64_t little_endian_64bits)
 #if HAVE_DECL_HTOBE16 == 0
 inline uint16_t htobe16(uint16_t host_16bits)
 {
-    return bswap_16(host_16bits);
+    return internal_bswap_16(host_16bits);
 }
 #endif // HAVE_DECL_HTOBE16
 
@@ -169,7 +169,7 @@ inline uint16_t htole16(uint16_t host_16bits)
 #if HAVE_DECL_BE16TOH == 0
 inline uint16_t be16toh(uint16_t big_endian_16bits)
 {
-    return bswap_16(big_endian_16bits);
+    return internal_bswap_16(big_endian_16bits);
 }
 #endif // HAVE_DECL_BE16TOH
 
@@ -183,7 +183,7 @@ inline uint16_t le16toh(uint16_t little_endian_16bits)
 #if HAVE_DECL_HTOBE32 == 0
 inline uint32_t htobe32(uint32_t host_32bits)
 {
-    return bswap_32(host_32bits);
+    return internal_bswap_32(host_32bits);
 }
 #endif // HAVE_DECL_HTOBE32
 
@@ -197,7 +197,7 @@ inline uint32_t htole32(uint32_t host_32bits)
 #if HAVE_DECL_BE32TOH == 0
 inline uint32_t be32toh(uint32_t big_endian_32bits)
 {
-    return bswap_32(big_endian_32bits);
+    return internal_bswap_32(big_endian_32bits);
 }
 #endif // HAVE_DECL_BE32TOH
 
@@ -211,7 +211,7 @@ inline uint32_t le32toh(uint32_t little_endian_32bits)
 #if HAVE_DECL_HTOBE64 == 0
 inline uint64_t htobe64(uint64_t host_64bits)
 {
-    return bswap_64(host_64bits);
+    return internal_bswap_64(host_64bits);
 }
 #endif // HAVE_DECL_HTOBE64
 
@@ -225,7 +225,7 @@ inline uint64_t htole64(uint64_t host_64bits)
 #if HAVE_DECL_BE64TOH == 0
 inline uint64_t be64toh(uint64_t big_endian_64bits)
 {
-    return bswap_64(big_endian_64bits);
+    return internal_bswap_64(big_endian_64bits);
 }
 #endif // HAVE_DECL_BE64TOH