如何将两个_pd转换为一个_ps

How to convert two _pd into one _ps?

本文关键字：ps 一个 转换 两个 pd      更新时间：2023-10-16

我正在循环一些数据，计算一些双精度，每 2 个__m128d操作，我想将数据存储在__m128浮点数上。

所以 64+64 + 64+64(2 __m128d (存储到 1 32+32+32+32 __m128 中。

我做这样的事情：

__m128d v_result;
__m128 v_result_float;
...
// some operations on v_result
// store the first two "slot" on float
v_result_float = _mm_cvtpd_ps(v_result);
// some operations on v_result
// I need to store the last two "slot" on float
v_result_float = _mm_cvtpd_ps(v_result); ?!?

但它每次都会覆盖(显然(前 2 个浮动"插槽"。

如何第二次"间隔"_mm_cvtpd_ps以开始将值插入 3° 和 4° "槽"？

下面是完整的代码：

__m128d v_pA;
__m128d v_pB;
__m128d v_result;
__m128 v_result_float;
float *pCEnd = pTest + roundintup8(blockSize);
for (; pTest < pCEnd; pA += 8, pB += 8, pTest += 8) {
    v_pA = _mm_load_pd(pA);
    v_pB = _mm_load_pd(pB);
    v_result = _mm_add_pd(v_pA, v_pB);
    v_result = _mm_max_pd(v_boundLower, v_result);
    v_result = _mm_min_pd(v_boundUpper, v_result);
    v_result = _mm_mul_pd(v_rangeLn2per12, v_result);
    v_result = _mm_add_pd(v_minLn2per12, v_result);
    // two double processed: store in 1° and 2° float slot
    v_result_float = _mm_cvtpd_ps(v_result);
    v_pA = _mm_load_pd(pA + 2);
    v_pB = _mm_load_pd(pB + 2);
    v_result = _mm_add_pd(v_pA, v_pB);
    v_result = _mm_max_pd(v_boundLower, v_result);
    v_result = _mm_min_pd(v_boundUpper, v_result);
    v_result = _mm_mul_pd(v_rangeLn2per12, v_result);
    v_result = _mm_add_pd(v_minLn2per12, v_result);
    // another two double processed: store in 3° and 4° float slot
    v_result_float = _mm_cvtpd_ps(v_result); // fail
    v_result_float = someFunction(v_result_float);
    _mm_store_ps(pTest, v_result_float);
    v_pA = _mm_load_pd(pA + 4);
    v_pB = _mm_load_pd(pB + 4);
    v_result = _mm_add_pd(v_pA, v_pB);
    v_result = _mm_max_pd(v_boundLower, v_result);
    v_result = _mm_min_pd(v_boundUpper, v_result);
    v_result = _mm_mul_pd(v_rangeLn2per12, v_result);
    v_result = _mm_add_pd(v_minLn2per12, v_result);
    // two double processed: store in 1° and 2° float slot
    v_result_float = _mm_cvtpd_ps(v_result);
    v_pA = _mm_load_pd(pA + 6);
    v_pB = _mm_load_pd(pB + 6);
    v_result = _mm_add_pd(v_pA, v_pB);
    v_result = _mm_max_pd(v_boundLower, v_result);
    v_result = _mm_min_pd(v_boundUpper, v_result);
    v_result = _mm_mul_pd(v_rangeLn2per12, v_result);
    v_result = _mm_add_pd(v_minLn2per12, v_result);
    // another two double processed: store in 3° and 4° float slot
    v_result_float = _mm_cvtpd_ps(v_result); // fail
    v_result_float = someFunction(v_result_float);      
    _mm_store_ps(pTest + 4, v_result_float);
}
您需要

使用 movlhps ( _mm_movelh_ps 将第二次转换的低词移动到第一次转换结果的高词。简化示例：

#include <immintrin.h>
__m128d some_double_operation(__m128d);
__m128 some_float_operation(__m128);
void foo(double const* input, float* output, int size)
{
    // assuming everything is already nicely aligned ...
    for(int i=0; i<size; i+=4, input+=4, output+=4)
    {
        __m128d res_lo = some_double_operation(_mm_load_pd(input));
        __m128d res_hi = some_double_operation(_mm_load_pd(input+2));
        __m128 res_float = _mm_movelh_ps(_mm_cvtpd_ps(res_lo), _mm_cvtpd_ps(res_hi));
        __m128 res_final = some_float_operation(res_float);
        _mm_store_ps(output, res_final);
    }
}

神弩箭演示：https://godbolt.org/z/wgKjxN。

如果some_double_operation是内联的，编译器可能会将第一个双精度操作的结果保存在第二次调用函数不使用的寄存器中，因此不需要将任何内容存储到内存中。

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