__builtin_bswap64 bug

clang.out

$ clang swap64.c
$ ./a.out
72057594037927936

emcc.out

$ emcc swap64.c
$ node a.out.js
72057594037927940

swap64.c

#include <stdio.h>
#include <stdint.h>

int main() {
  int64_t a = 1;
  a = __builtin_bswap64(a);

  printf("%lld\n", a);

  return 0;
}