gistfile1.c

#include <stdint.h>
#include <assert.h>
#include <emmintrin.h>

uint32_t foo(uint32_t a)
{
    __m128i xa = _mm_cvtsi32_si128(a);
    return _mm_cvtsi128_si32(_mm_packus_epi16(xa, xa));
}

__m128i bar(__m128i xa)
{
    return _mm_packus_epi16(xa, xa);
}

int main(void)
{
    assert(foo(0x12345678) == 0xFFFF);
    return 0;
}