Re: [Qemu-devel] [v3 1/3] cutils: add avx2 instruction optimization

[Paolo Bonzini <pbonzini@redhat.com>]

On 10/12/2015 10:22, Li, Liang Z wrote:
>>>>   without the ' -mavx2' option for gcc, there are compiling error:
>>>> '__m256i undeclared', the __attribute__((target("avx2"))) can't solve
>>>> this issue.  Any idea?
>>>
>>> You're right that you can't use the normal __m256i, as it doesn't get
>>> declared.
>>
>> It should be declared.  *intrin.h uses #pragma GCC target and always defines
>> all vector types.
>>
>> In fact, the following compiles for me with just "gcc foo.c" under GCC 5.x:
>>
>> #include <immintrin.h>
>>
>> // #if defined CONFIG_IFUNC && defined CONFIG_AVX2 #pragma GCC
>> push_options #pragma GCC target("avx2")
>> #define AVX2_VECTYPE        __m256i
>> #define AVX2_SPLAT(p)       _mm256_set1_epi8(*(p))
>> #define AVX2_ALL_EQ(v1, v2) \
>>     (_mm256_movemask_epi8(_mm256_cmpeq_epi8(v1, v2)) == 0xFFFFFFFF)
>> #define AVX2_VEC_OR(v1, v2) (_mm256_or_si256(v1, v2))
>>
>> size_t buffer_find_nonzero_offset_avx2(const void *buf, size_t len) {
>>     const AVX2_VECTYPE *p = buf;
>>     const AVX2_VECTYPE zero = (AVX2_VECTYPE){0};
>>     size_t i;
>>
>>     if (!len) {
>>         return 0;
>>     }
>>
>>     for (i = 0; i < 4; i++) {
>>         if (!AVX2_ALL_EQ(p[i], zero)) {
>>             return i * sizeof(AVX2_VECTYPE);
>>         }
>>     }
>>
>>     for (i = 4; i < len / sizeof(AVX2_VECTYPE); i += 4) {
>>         AVX2_VECTYPE tmp0 = AVX2_VEC_OR(p[i + 0], p[i + 1]);
>>         AVX2_VECTYPE tmp1 = AVX2_VEC_OR(p[i + 2], p[i + 3]);
>>         AVX2_VECTYPE tmp2 = AVX2_VEC_OR(p[i + 4], p[i + 5]);
>>         AVX2_VECTYPE tmp3 = AVX2_VEC_OR(p[i + 6], p[i + 7]);
>>         AVX2_VECTYPE tmp01 = AVX2_VEC_OR(tmp0, tmp1);
>>         AVX2_VECTYPE tmp23 = AVX2_VEC_OR(tmp2, tmp3);
>>         if (!AVX2_ALL_EQ(AVX2_VEC_OR(tmp01, tmp23), zero)) {
>>             break;
>>         }
>>     }
>>
>>     return i * sizeof(AVX2_VECTYPE);
>> }
>>
>> #pragma GCC pop_options
>> // #endif
>>
>> so perhaps the configure test is testing the wrong thing?
>>
>> Paolo
> 
> Hi Paolo,
> 
> what's your opinion?  putting the AVX2 related code to util/cutils.c and use the "#pragma ..." you referred?

Yes, that's best.  And you can keep using __m256i if you prefer that.

Paolo