riscv/bswap: use compiler builtins

av_bswapXX() are used in context that expect exact size types, notably
variable arguments to av_log(). On Linux RV64, uint_fast32_t is an
unsigned long, so the current inline assembler does not work properly.

Since GCC and Clang gained their byte-swap built-ins before they
supported RISC-V, we can simply defer to them. As an added bonus, the
compiler can do instruction scheduling, which it couldn't with the Zbb
inline assembler.

libavutil/riscv/bswap.h

@@ -23,52 +23,10 @@
 #include "config.h"
 #include "libavutil/attributes.h"
 
-#if defined (__riscv_zbb) && (__riscv_zbb > 0) && HAVE_INLINE_ASM
+#if defined (__GNUC__) || defined (__clang__)
+#define av_bswap16 __builtin_bswap16
+#define av_bswap32 __builtin_bswap32
+#define av_bswap64 __builtin_bswap64
+#endif
 
-static av_always_inline av_const uintptr_t av_bswap_xlen(uintptr_t x)
-{
-    uintptr_t y;
-
-    __asm__("rev8 %0, %1" : "=r" (y) : "r" (x));
-    return y;
-}
-
-#define av_bswap16 av_bswap16
-
-static av_always_inline av_const uint_fast16_t av_bswap16(uint_fast16_t x)
-{
-    return av_bswap_xlen(x) >> (__riscv_xlen - 16);
-}
-
-#if (__riscv_xlen == 32)
-#define av_bswap32 av_bswap_xlen
-#define av_bswap64 av_bswap64
-
-static av_always_inline av_const uint64_t av_bswap64(uint64_t x)
-{
-    return (((uint64_t)av_bswap32(x)) << 32) | av_bswap32(x >> 32);
-}
-
-#else
-#define av_bswap32 av_bswap32
-
-static av_always_inline av_const uint_fast32_t av_bswap32(uint_fast32_t x)
-{
-    return av_bswap_xlen(x) >> (__riscv_xlen - 32);
-}
-
-#if (__riscv_xlen == 64)
-#define av_bswap64 av_bswap_xlen
-
-#else
-#define av_bswap64 av_bswap64
-
-static av_always_inline av_const uint_fast64_t av_bswap64(uint_fast64_t x)
-{
-    return av_bswap_xlen(x) >> (__riscv_xlen - 64);
-}
-
-#endif /* __riscv_xlen > 64 */
-#endif /* __riscv_xlen > 32 */
-#endif /* __riscv_zbb */
 #endif /* AVUTIL_RISCV_BSWAP_H */