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wheelsdong

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03-17-2010
08:30 PM

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How to sum up a large array of 32-bit integers in a fast way?

I want to sum up a large array of 32-bit integers, e.g 10M integers. How to do this in a very fast way?

I am now trying to use Intel SSE Intrinsics to do this.

Each time, I load 4 integers by one instruction, and then sum up. So I only need to loop for N/4 times,where N is the array size.

Steps are asfollows:

Loop N/4 times

1, load 4 32-bit int, a0 a1 a2 a3

2, pack the data as:

xmm1 = a0 0 a1 0

xmm2 = a2 0 a3 0

3, sum up

xmm3 = a0+a2 0 a1+a3 0

Loop End

The problem is: two 32-bit integers sum up may result in overflow.

So how to add two 32-bit integers and save them to a 64-bit integer by using SSE intrinsics functions?

Thank you very much.

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5 Replies

TimP

Black Belt

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03-17-2010
09:48 PM

525 Views

You will have to wait for AVX to see 4-wide parallel 64-bit adds. Then, if you trust Intel C, you get them from plain C source code:

long long int extsum(int *a, int n){

long long int sum = 0;

for(int i = 0; i < n; ++i)sum += a

...

vpxor xmm0, xmm0, xmm0

add eax, edx

xor ecx, ecx

.B1.4: ; Preds .B1.4 .B

vpmovsxdq xmm1, QWORD PTR [esi+ecx*4]

vpmovsxdq xmm2, QWORD PTR [8+esi+ecx*4]

vpmovsxdq xmm4, QWORD PTR [16+esi+ecx*4]

vpmovsxdq xmm6, QWORD PTR [24+esi+ecx*4]

vpaddq xmm0, xmm0, xmm1

vpaddq xmm3, xmm0, xmm2

add ecx, 8

vpaddq xmm5, xmm3, xmm4

vpaddq xmm0, xmm5, xmm6

cmp ecx, eax

jb .B1.4 ; Prob 82%

.B1.5: ; Preds .B1.4

vpshufd xmm1, xmm0, 14

vpaddq xmm0, xmm0, xmm1

vmovd ecx, xmm0

vpsrlq xmm2, xmm0, 32

vmovd esi, xmm2

wheelsdong

Beginner

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03-17-2010
10:26 PM

525 Views

Thanks a lot for your quick reply.

Of course, using Intel C++ Compiler is a good choice. But I want to do the vectorization by myself.

In my opinion, it costs a lot of time to access the memory to read out the integers one by one.

So I hope to load 4 32-bit integers by one instruction. Then do the pack and sum using Intrinsics. But I don't know how to cast 32-bit integer to 64-bit integer in the 128-bit registers.

My code is like:

__int32 arr32[4] = {INT_MIN, INT_MIN, INT_MIN, INT_MIN};

__m128i* wrd_ptr = (__m128i*)arr32; // cast the pointer type

xmm1 = _mm_loadu_si128(wrd_ptr); // a0 a1 a2 a3

xmm2 = _mm_unpacklo_epi32(xmm1, xmm0); // a0 0 a1 0

xmm3 = _mm_unpackhi_epi32(xmm1, xmm0); // a2 0 a3 0

Problem: how to cast xmm2(4 32-bit int) to xmm2(2 64-bit int)??

If it is possible, then I can use the 64-bit addition like:

xmm4 = _mm_add_epi64(xmm2, xmm3);

Thank you!

TimP

Black Belt

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03-18-2010
07:53 AM

525 Views

You could use the intrinsic to convert a pair of 32-bit ints to double, and sum the doubles by _mm_add_pd, if you like that alternate approach to protecting against overflow. Any good compiler ought to choose those intrinsics automatically:

pxor xmm1, xmm1

movaps xmm0, xmm1

.B1.4: ; Preds .B1.4 .B1

cvtdq2pd xmm2, QWORD PTR [esi+eax*4]

cvtdq2pd xmm3, QWORD PTR [8+esi+eax*4]

cvtdq2pd xmm4, QWORD PTR [16+esi+eax*4]

cvtdq2pd xmm5, QWORD PTR [24+esi+eax*4]

addpd xmm0, xmm2

addpd xmm1, xmm3

addpd xmm0, xmm4

addpd xmm1, xmm5

add eax, 8

cmp eax, ecx

jb .B1.4 ; Prob 82%

.B1.5: ; Preds .B1.4

mov esi, DWORD PTR [8+esp]

addpd xmm0, xmm1

movaps xmm1, xmm0

unpckhpd xmm1, xmm0

addsd xmm0, xmm1

Note that the compiler chose to sum 8 elements per pass, ending up with 4 partial sums, which are combined into a single sum after the loop.

If you're talking about speeding up a disk performance bound operation to read the big array, intrinsics won't solve the bottleneck.

wheelsdong

Beginner

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03-18-2010
07:15 PM

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I think I should try a good compiler to do the vectorization automatically.

Or, I may try the alternate approach to convert 32-bit int to double. But Iam afraidthis conversion might waste some time. So I am not sure whether the whole performance will increase.

TimP

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03-19-2010
06:51 AM

525 Views

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