比较SSE内在函数中的符号位

Question

如何使用SSE内在函数创建一个掩码,指示两个打包浮点(__m128)的符号是否相同,例如比较a和b,其中a为[1.0 -1.0 0.0 2.0],b为[1.0 1.0 1.0 1.0]我们得到的所需面具是[true false true true].

Answer 1

这是一个解决方案:

const __m128i MASK = _mm_set1_epi32(0xffffffff);

__m128 a = _mm_setr_ps(1,-1,0,2);
__m128 b = _mm_setr_ps(1,1,1,1);

__m128  f = _mm_xor_ps(a,b);
__m128i i = _mm_castps_si128(f);

i = _mm_srai_epi32(i,31);
i = _mm_xor_si128(i,MASK);

f = _mm_castsi128_ps(i);

//  i = (0xffffffff, 0, 0xffffffff, 0xffffffff)
//  f = (0xffffffff, 0, 0xffffffff, 0xffffffff)

在这个片段中,双方i并f具有相同的位掩码.我假设你想要它在__m128类型中所以我添加了f = _mm_castsi128_ps(i);从一个转换回来__m128i.

请注意,此代码对零符号敏感.所以0.0并-0.0会影响结果.

说明:

代码的工作方式如下:

f = _mm_xor_ps(a,b);       //  xor the sign bits (well all the bits actually)

i = _mm_castps_si128(f);   //  Convert it to an integer. There's no instruction here.

i = _mm_srai_epi32(i,31);  //  Arithmetic shift that sign bit into all the bits.

i = _mm_xor_si128(i,MASK); //  Invert all the bits

f = _mm_castsi128_ps(i);   //  Convert back. Again, there's no instruction here.