Merge bitcoin/bitcoin#29263: serialization: c++20 endian/byteswap/clz modernization

86b7f28 serialization: use internal endian conversion functions (Cory Fields)
432b18c serialization: detect byteswap builtins without autoconf tests (Cory Fields)
297367b crypto: replace CountBits with std::bit_width (Cory Fields)
52f9bba crypto: replace non-standard CLZ builtins with c++20's bit_width (Cory Fields)

Pull request description:

  This replaces #28674, #29036, and #29057. Now ready for testing and review.

  Replaces platform-specific endian and byteswap functions. This is especially useful for kernel, as it means that our deep serialization code no longer requires bitcoin-config.h.

  I apologize for the size of the last commit, but it's hard to avoid making those changes at once.

  All platforms now use our internal functions rather than libc or platform-specific ones, with the exception of MSVC.

  Sadly, benchmarking showed that not all compilers are capable of detecting and optimizing byteswap functions, so compiler builtins are instead used where possible. However, they're now detected via macros rather than autoconf checks.

  This[ matches how libc++ implements std::byteswap for c++23](https://github.com/llvm/llvm-project/blob/main/libcxx/include/__bit/byteswap.h#L26).

  I suggest we move/rename `compat/endian.h`, but I left that out of this PR to avoid bikeshedding.

  #29057 pointed out some irregularities in benchmarks. After messing with various compilers and configs for a few weeks with these changes, I'm of the opinion that we can't win on every platform every time, so we should take the code that makes sense going forward. That said, if any real-world slowdowns are caused here, we should obviously investigate.

ACKs for top commit:
  maflcko:
    ACK 86b7f28 📘
  fanquake:
    ACK 86b7f28 - we can finish pruning out the __builtin_clz* checks/usage once the minisketch code has been updated. This is more good cleanup pre-CMake & for the kernal.

Tree-SHA512: 715a32ec190c70505ffbce70bfe81fc7b6aa33e376b60292e801f60cf17025aabfcab4e8c53ebb2e28ffc5cf4c20b74fe3dd8548371ad772085c13aec8b7970e

Author: fanquake

configure.ac

@@ -958,7 +958,7 @@ if test "$TARGET_OS" = "darwin"; then
   AX_CHECK_LINK_FLAG([-Wl,-fixup_chains], [HARDENED_LDFLAGS="$HARDENED_LDFLAGS -Wl,-fixup_chains"], [], [$LDFLAG_WERROR])
 fi
 
-AC_CHECK_HEADERS([endian.h sys/endian.h byteswap.h sys/select.h sys/prctl.h sys/sysctl.h vm/vm_param.h sys/vmmeter.h sys/resources.h])
+AC_CHECK_HEADERS([sys/select.h sys/prctl.h sys/sysctl.h vm/vm_param.h sys/vmmeter.h sys/resources.h])
 
 AC_CHECK_DECLS([getifaddrs, freeifaddrs],[CHECK_SOCKET],,
     [#include <sys/types.h>
@@ -973,18 +973,6 @@ AC_CHECK_DECLS([pipe2])
 
 AC_CHECK_FUNCS([timingsafe_bcmp])
 
-AC_CHECK_DECLS([le16toh, le32toh, le64toh, htole16, htole32, htole64, be16toh, be32toh, be64toh, htobe16, htobe32, htobe64],,,
-    [#if HAVE_ENDIAN_H
-                 #include <endian.h>
-                 #elif HAVE_SYS_ENDIAN_H
-                 #include <sys/endian.h>
-                 #endif])
-
-AC_CHECK_DECLS([bswap_16, bswap_32, bswap_64],,,
-    [#if HAVE_BYTESWAP_H
-                 #include <byteswap.h>
-                 #endif])
-
 AC_MSG_CHECKING([for __builtin_clzl])
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[ ]], [[
  (void) __builtin_clzl(0);

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>
+#include <cstdint>
+#ifdef _MSC_VER
+#include <cstdlib>
 #endif
 
-#include <cstdint>
 
-#if defined(HAVE_BYTESWAP_H)
-#include <byteswap.h>
-#endif
+// 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.
 
-#if defined(MAC_OSX)
+#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
 
-#include <libkern/OSByteOrder.h>
-#define bswap_16(x) OSSwapInt16(x)
-#define bswap_32(x) OSSwapInt32(x)
-#define bswap_64(x) OSSwapInt64(x)
+// 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