Sum all elements in a quadword vector in ARM assembly with NEON
Im rather new to assembly and although the arm information center is often helpful sometimes the instructions can be a little confusing to a newbie. Basically what I need to do is sum 4 float values in a quadword register and store the result in a single precision r开发者_Python百科egister. I think the instruction VPADD can do what I need but I'm not quite sure.
You might try this (it's not in ASM, but you should be able to convert it easily):
float32x2_t r = vadd_f32(vget_high_f32(m_type), vget_low_f32(m_type));
return vget_lane_f32(vpadd_f32(r, r), 0);
In ASM it would be probably only VADD and VPADD.
I'm not sure if this is only one method to do this (and most optimal), but I haven't figured/found better one...
PS. I'm new to NEON too
It seems that you want to get the sum of a certain length of array, and not only four float values.
In that case, your code will work, but is far from optimized :
many many pipeline interlocks
unnecessary 32bit addition per iteration
Assuming the length of the array is a multiple of 8 and at least 16 :
vldmia {q0-q1}, [pSrc]!
sub count, count, #8
loop:
pld [pSrc, #32]
vldmia {q3-q4}, [pSrc]!
subs count, count, #8
vadd.f32 q0, q0, q3
vadd.f32 q1, q1, q4
bgt loop
vadd.f32 q0, q0, q1
vpadd.f32 d0, d0, d1
vadd.f32 s0, s0, s1
- pld - while being an ARM instruction and not NEON - is crucial for performance. It drastically increases cache hit rate.
I hope the rest of the code above is self explanatory.
You will notice that this version is many times faster than your initial one.
Here is the code in ASM:
vpadd.f32 d1,d6,d7 @ q3 is register that needs all of its contents summed
vadd.f32 s1,s2,s3 @ now we add the contents of d1 together (the sum)
vadd.f32 s0,s0,s1 @ sum += s1;
I may have forgotten to mention that in C the code would look like this:
float sum = 1.0f;
sum += number1 * number2;
I have omitted the multiplication from this little piece asm of code.
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