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Questions tagged [simd]

Single instruction, multiple data (SIMD) is the concept of having each instruction operate on a small chunk or vector of data elements. CPU vector instruction sets include: x86 SSE and AVX, ARM NEON, and PowerPC AltiVec. To efficiently use SIMD instructions, data needs to be in structure-of-arrays form and should occur in longer streams. Naively "SIMD optimized" code frequently surprises by running slower than the original.

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How to compile Tensorflow with SSE4.2 and AVX instructions?

This is the message received from running a script to check if Tensorflow is working: I tensorflow/stream_executor/dso_loader.cc:125] successfully opened CUDA library libcublas.so.8.0 locally I tensorflow/stream_executor/dso_loader.cc:125]…
GabrielChu
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Header files for x86 SIMD intrinsics

Which header files provide the intrinsics for the different x86 SIMD instruction set extensions (MMX, SSE, AVX, ...)? It seems impossible to find such a list online. Correct me if I'm wrong.
fredoverflow
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Subtracting packed 8-bit integers in an 64-bit integer by 1 in parallel, SWAR without hardware SIMD

If I have a 64-bit integer that I'm interpreting as an array of packed 8-bit integers with 8 elements. I need to subtract the constant 1 from each packed integer while handling overflow without the result of one element affecting the result of…
cam-white
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Why is vectorization, faster in general, than loops?

Why, at the lowest level of the hardware performing operations and the general underlying operations involved (i.e.: things general to all programming languages' actual implementations when running code), is vectorization typically so dramatically…
Ben Sandeen
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Fastest way to do horizontal SSE vector sum (or other reduction)

Given a vector of three (or four) floats. What is the fastest way to sum them? Is SSE (movaps, shuffle, add, movd) always faster than x87? Are the horizontal-add instructions in SSE3 worth it? What's the cost to moving to the FPU, then faddp, faddp?…
FeepingCreature
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Parallel for vs omp simd: when to use each?

OpenMP 4.0 introduces a new construct called "omp simd". What is the benefit of using this construct over the old "parallel for"? When would each be a better choice over the other? EDIT: Here is an interesting paper related to the SIMD directive.
zr.
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SSE intrinsic functions reference

Does anyone know of a reference listing the operation of the SSE intrinsic functions for gcc, i.e. the functions in the <*mmintrin.h> header files? Thanks.
NGaffney
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How to choose AVX compare predicate variants

In the Advanced Vector Extensions (AVX) the compare instructions like _m256_cmp_ps, the last argument is a compare predicate. The choices for the predicate overwhelm me. They seem to be a tripple of type, ordering, signaling. E.g. _CMP_LE_OS is…
Bram
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Getting started with Intel x86 SSE SIMD instructions

I want to learn more about using the SSE. What ways are there to learn, besides the obvious reading the Intel® 64 and IA-32 Architectures Software Developer's Manuals? Mainly I'm interested to work with the GCC X86 Built-in Functions.
Liran Orevi
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ARM Cortex-A8: Whats the difference between VFP and NEON

In ARM Cortex-A8 processor, I understand what NEON is, it is an SIMD co-processor. But is VFP(Vector Floating Point) unit, which is also a co-processor, works as a SIMD processor? If so which one is better to use? I read few links such as…
HaggarTheHorrible
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How to determine if memory is aligned?

I am new to optimizing code with SSE/SSE2 instructions and until now I have not gotten very far. To my knowledge a common SSE-optimized function would look like this: void sse_func(const float* const ptr, int len){ if( ptr is aligned ) { …
user229898
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AVX2 what is the most efficient way to pack left based on a mask?

If you have an input array, and an output array, but you only want to write those elements which pass a certain condition, what would be the most efficient way to do this in AVX2? I've seen in SSE where it was done like…
Froglegs
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Why is strcmp not SIMD optimized?

I've tried to compile this program on an x64 computer: #include int main(int argc, char* argv[]) { return ::std::strcmp(argv[0], "really really really really really really really really really" "really really really really…
user1095108
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Why vectorizing the loop does not have performance improvement

I am investigating the effect of vectorization on the performance of the program. In this regard, I have written following code: #include #include #include #define LEN 10000000 int main(){ struct timeval…
Pouya
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Difference between MOVDQA and MOVAPS x86 instructions?

I'm looking Intel datasheet: Intel® 64 and IA-32 Architectures Software Developer’s Manual and I can't find the difference between MOVDQA: Move Aligned Double Quadword MOVAPS: Move Aligned Packed Single-Precision In Intel datasheet I can find…
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