[英]Compare two __m128i values for total order
I need a way to compare values of type __m128i
in C++ for a total order between any values of type __m128i
. 我需要一种方法来比较类型的值
__m128i
在C ++类型的任何值之间的总顺序__m128i
。 The type of order doesn't matter as long as it establishes a total order between all values of type __m128i
. 只要在
__m128i
类型的所有值之间建立总顺序,订单类型就__m128i
。 Hence the comparison might be less-than between 128-bit integers or something else entirely as long as is provides a total order. 因此,只要提供总订单,比较可能小于128位整数或其他整数。
I tried using the <
operator, but that didn't return a bool
, but instead seems to compare the vector components of __m128i
(ie SIMD): 我尝试使用
<
运算符,但是没有返回bool
,而是似乎比较了__m128i
(即SIMD)的向量组件:
#include <emmintrin.h>
inline bool isLessThan(__m128i a, __m128i b) noexcept {
// error: cannot convert '__vector(2) long int' to 'bool' in return
return a < b;
}
Another possibility would be to use memcmp
/ strcmp
or similar, but this would very likely not be optimal. 另一种可能性是使用
memcmp
/ strcmp
或类似的,但这很可能不是最佳的。 Targeting modern Intel x86-64 CPUs with at least SSE4.2 and AVX2, are there any intrinsics / instructions I could use for such comparisons? 针对至少具有SSE4.2和AVX2的现代Intel x86-64 CPU,我可以使用任何内在/指令进行此类比较吗? How to do it?
怎么做?
PS: Similar questions have been asked for checking equality but not for ordering: PS:已经要求类似的问题检查相等性,但不是为了订购:
Here you go. 干得好。
inline bool isLessThan( __m128i a, __m128i b )
{
/* Compare 8-bit lanes for ( a < b ), store the bits in the low 16 bits of the
scalar value: */
const int less = _mm_movemask_epi8( _mm_cmplt_epi8( a, b ) );
/* Compare 8-bit lanes for ( a > b ), store the bits in the low 16 bits of the
scalar value: */
const int greater = _mm_movemask_epi8( _mm_cmpgt_epi8( a, b ) );
/* It's counter-intuitive, but this scalar comparison does the right thing.
Essentially, integer comparison searches for the most significant bit that
differs... */
return less > greater;
}
The order is less than ideal 'coz pcmpgtb
treats these bytes as signed integers, but you said it's not important for your use case. 顺序不太理想'coz
pcmpgtb
将这些字节视为有符号整数,但是你说它对你的用例并不重要。
Update: here's a slightly slower version with uint128_t
sort order. 更新:这是一个稍微慢一点的
uint128_t
排序顺序版本。
// True if a < b, for unsigned 128 bit integers
inline bool cmplt_u128( __m128i a, __m128i b )
{
// Flip the sign bits in both arguments.
// Transforms 0 into -128 = minimum for signed bytes,
// 0xFF into +127 = maximum for signed bytes
const __m128i signBits = _mm_set1_epi8( (char)0x80 );
a = _mm_xor_si128( a, signBits );
b = _mm_xor_si128( b, signBits );
// Now the signed byte comparisons will give the correct order
const int less = _mm_movemask_epi8( _mm_cmplt_epi8( a, b ) );
const int greater = _mm_movemask_epi8( _mm_cmpgt_epi8( a, b ) );
return less > greater;
}
We build unsigned compares out of signed by range-shifting the unsigned inputs to signed (by flipping the high bit = subtract 128). 我们构建无符号比较,通过将无符号输入的范围转换为有符号(通过翻转高位=减去128)进行范围转换。
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