Why does GCC store XMM registers into RAM then load them back instead of using them directly?

[Liu Hao <ltpmouse at gmail dot com>]

This can be observed from the following example:
(For your reference: https://godbolt.org/g/toFOVc )

```c++
#include <emmintrin.h>

double my_fmax_1(double x, double y){
    return _mm_cvtsd_f64(_mm_max_sd(_mm_set_sd(x), _mm_set_sd(y)));
}
double my_fmax_2(double x, double y){
    double r;
    __asm__ (
        "maxsd   %%xmm1, %%xmm0"
        : "=x"(r)
        : "0"(x), "x"(y)
    );
    return r;
}
```

After being compiled with `-O3`, this snippet results in the following 
assembly:

```assembly
my_fmax_1(double, double):
        movsd   %xmm0, -24(%rsp)
        movsd   %xmm1, -16(%rsp)
        movsd   -24(%rsp), %xmm0
        movsd   -16(%rsp), %xmm1
        maxsd   %xmm1, %xmm0
        ret
my_fmax_2(double, double):
        maxsd   %xmm1, %xmm0
        ret
```

The first function seems very inefficient. Are there any particular 
reasons why GCC doesn't optimize it well (like the second function)

--
Best regards,
ltpmouse