我需要改进循环,因为我的应用程序调用了数千次。我想我需要用 Neon 来做这件事,但我不知道从哪里开始。
假设/先决条件:
-
w
始终为 320(16/32 的倍数)。
-
pa
and pb
16 字节对齐
-
ma
and mb
是积极的。
int whileInstruction (const unsigned char *pa,const unsigned char *pb,int ma,int mb,int w)
{
int sum=0;
do {
sum += ((*pa++)-ma)*((*pb++)-mb);
} while(--w);
return sum;
}
这种矢量化的尝试效果不佳,而且不安全(缺少破坏),但演示了我正在尝试做的事情:
int whileInstruction (const unsigned char *pa,const unsigned char *pb,int ma,int mb,int w)
{
asm volatile("lsr %2, %2, #3 \n"
".loop: \n"
"# load 8 elements: \n"
"vld4.8 {d0-d3}, [%1]! \n"
"vld4.8 {d4-d7}, [%2]! \n"
"# do the operation: \n"
"vaddl.u8 q7, d0, r7 \n"
"vaddl.u8 q8, d1, d8 \n"
"vmlal.u8 q7, q7, q8 \n"
"# Sum the vector a save in sum (this is wrong):\n"
"vaddl.u8 q7, d0, r7 \n"
"subs %2, %2, #1 \n" // Decrement iteration count
"bne .loop \n" // Repeat unil iteration count is not zero
:
: "r"(pa), "r"(pb), "r"(w),"r"(ma),"r"(mb),"r"(sum)
: "r4", "r5", "r6","r7","r8","r9"
);
return sum;
}