首先,您可以通过分解乘法并摆脱分支来稍微加快原始代码:
int whiteCount = 0;
for (int q = i; q < i + windowHeight; q++)
{
const bool * const row = &imageData[q * W];
for (int w = j; w < j + windowWidth; w++)
{
whiteCount += row[w];
}
}
(这假设它imageData[]
是真正的二进制,即每个元素只能是 0 或 1。)
这是一个简单的 NEON 实现:
#include <arm_neon.h>
// ...
int i, w;
int whiteCount = 0;
uint32x4_t v_count = { 0 };
for (q = i; q < i + windowHeight; q++)
{
const bool * const row = &imageData[q * W];
uint16x8_t vrow_count = { 0 };
for (w = j; w <= j + windowWidth - 16; w += 16) // SIMD loop
{
uint8x16_t v = vld1q_u8(&row[j]); // load 16 x 8 bit pixels
vrow_count = vpadalq_u8(vrow_count, v); // accumulate 16 bit row counts
}
for ( ; w < j + windowWidth; ++w) // scalar clean up loop
{
whiteCount += row[j];
}
v_count = vpadalq_u16(v_count, vrow_count); // update 32 bit image counts
} // from 16 bit row counts
// add 4 x 32 bit partial counts from SIMD loop to scalar total
whiteCount += vgetq_lane_s32(v_count, 0);
whiteCount += vgetq_lane_s32(v_count, 1);
whiteCount += vgetq_lane_s32(v_count, 2);
whiteCount += vgetq_lane_s32(v_count, 3);
// total is now in whiteCount
(这假设它imageData[]
是真正的二进制imageWidth <= 2^19
, 和sizeof(bool) == 1
。)
更新版本unsigned char
和值 255 为白色,0 为黑色:
#include <arm_neon.h>
// ...
int i, w;
int whiteCount = 0;
const uint8x16_t v_mask = { 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1 };
uint32x4_t v_count = { 0 };
for (q = i; q < i + windowHeight; q++)
{
const uint8_t * const row = &imageData[q * W];
uint16x8_t vrow_count = { 0 };
for (w = j; w <= j + windowWidth - 16; w += 16) // SIMD loop
{
uint8x16_t v = vld1q_u8(&row[j]); // load 16 x 8 bit pixels
v = vandq_u8(v, v_mask); // mask out all but LS bit
vrow_count = vpadalq_u8(vrow_count, v); // accumulate 16 bit row counts
}
for ( ; w < j + windowWidth; ++w) // scalar clean up loop
{
whiteCount += (row[j] == 255);
}
v_count = vpadalq_u16(v_count, vrow_count); // update 32 bit image counts
} // from 16 bit row counts
// add 4 x 32 bit partial counts from SIMD loop to scalar total
whiteCount += vgetq_lane_s32(v_count, 0);
whiteCount += vgetq_lane_s32(v_count, 1);
whiteCount += vgetq_lane_s32(v_count, 2);
whiteCount += vgetq_lane_s32(v_count, 3);
// total is now in whiteCount
(这假设imageData[]
白色的值为 255,黑色的值为 0,并且imageWidth <= 2^19
.)
请注意,以上所有代码都未经测试,可能需要进一步的工作。