[PATCH 1/6] bswap.h: add support for built-in bswap functions

["Dennis Ameling via GitGitGadget" <gitgitgadget@gmail.com>]

From: Dennis Ameling <dennis@dennisameling.com>

Newer compiler versions, like GCC 10 and Clang 12, have built-in
functions for bswap32 and bswap64. This comes in handy, for example,
when targeting CLANGARM64 on Windows, which would not be supported
without this logic.

Signed-off-by: Dennis Ameling <dennis@dennisameling.com>
---
 compat/bswap.h | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/compat/bswap.h b/compat/bswap.h
index b34054f2bd7..9e0f98e00b9 100644
--- a/compat/bswap.h
+++ b/compat/bswap.h
@@ -35,7 +35,19 @@ static inline uint64_t default_bswap64(uint64_t val)
 #undef bswap32
 #undef bswap64
 
-#if defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
+/**
+ * __has_builtin is available since Clang 10 and GCC 10.
+ * Below is a fallback for older compilers.
+ */
+#ifndef __has_builtin
+	#define __has_builtin(x) 0
+#endif
+
+#if __has_builtin(__builtin_bswap32) && __has_builtin(__builtin_bswap64)
+#define bswap32(x) __builtin_bswap32((x))
+#define bswap64(x) __builtin_bswap64((x))
+
+#elif defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
 
 #define bswap32 git_bswap32
 static inline uint32_t git_bswap32(uint32_t x)
-- 
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