Just keep in mind that the FPRs and vector registers are now aliased together (in VMX-only CPUs this wasn't necessarily the case). What is particularly stupid about my example is that it may have to spill to memory to move the uint64_t (a GPR) into the VSX register (an FPR) and then move it back because PowerPC famously had no direct GPR-FPR moves for quite a while. Since I didn't specify -mcpu=power8 (or higher), gcc doesn't issue the new instructions and I'm not sure it would know how to.

A better way would be to explicitly use the newer mtvsrd (mtfprd) and mfvsrd (mffprd) instructions and avoid the spill. So here's a revision 2.

  #include <errno.h>
  #include <stdio.h>
  #include <stdint.h>
  #include <stdlib.h>
 
  int main(int argc, char **argv) {
        uint64_t v = 0;
 
        if (argc != 2) {
                fprintf(stderr, "usage: %s quantity\n", argv[0]);
                return 1;
        }
 
        v = strtoull(argv[1], NULL, 0);
        if (errno == EINVAL || errno == ERANGE) {
                perror("strtoull");
                return 1;
        }
 
        __asm__(
                "mtfprd %1, %0\n"
                "xxbrd %1, %1\n"
                "mffprd %1, %0\n"
                :"=r"(v)
                :"r"(v)
        );
        fprintf(stdout, "0x%lx\n", v);
        return 0;
  }

If v is already in a register, then it can just stay there.