for byte swapping, now use a seperate header in which the compatibility code lives

src/util/compat/bswap.h

@@ -0,0 +1,44 @@
+#pragma once
+
+#include <stdint.h>
+
+#if defined(__GNUC__) || defined(__clang__)
+#define bswap16 __builtin_bswap16
+#define bswap32 __builtin_bswap32
+#define bswap64 __builtin_bswap64
+#elif __has_include(<byteswap.h>)
+#include <byteswap.h>
+#define bswap16 bswap_16
+#define bswap32 bswap_32
+#define bswap64 bswap_64
+#else
+#define bswap16 bswap16_impl
+#define bswap32 bswap32_impl
+#define bswap64 bswap64_impl
+#define IMPL_BSWAP
+#endif
+
+#if defined(IMPL_BSWAP) || !defined(NDEBUG)
+static inline uint16_t bswap16_impl(uint16_t x) {
+	return (x << 8) | (x >> 8);
+}
+
+static inline uint32_t bswap32_impl(uint32_t x) {
+	return (x << 24) |
+		((0x0000FF00U & x) << 8) |
+		((0x00FF0000U & x) >> 8) |
+		(x >> 24);
+}
+
+static inline uint64_t bswap64_impl(uint64_t x) {
+	return (x << 56) |
+		((0x000000000000FF00ULL & x) << 40) |
+		((0x0000000000FF0000ULL & x) << 24) |
+		((0x00000000FF000000ULL & x) << 8) |
+		((0x000000FF00000000ULL & x) >> 8) |
+		((0x0000FF0000000000ULL & x) >> 24) |
+		((0x00FF000000000000ULL & x) >> 40) |
+		(x >> 56);
+}
+#endif
+#undef IMPL_BSWAP