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14 May 2015 Whereas a gather operation reads elements from memory and packs them in an SIMD register, the scatter operation unpacks the data and then writes to individual memory locations. Typical coding for this will result in the non-optimal use of the SIMD instructions on an Intel Xeon Phi coprocessor. Gathers
It is the vector-equivalent of register indirect addressing, with gather involving indexed reads and scatter indexed writes. Vector processors (and some SIMD units in CPUs) have hardware support for gather-scatter operations, providing instructions such as Load Vector Indexed for gather and Store Vector Indexed for scatter.
VEX.128 version: For dword indices, the instruction will gather four single-precision floating-point values. For qword indices, the instruction will gather two values and zero the upper 64 bits of the destination. VEX.256 version: Memory ordering with other instructions follows the Intel-64 memory-ordering model. Faults are
17 Jun 2016 I did some benchmarking of the AVX gather instructions and it seems to be a fairly simple brute force implementation - even when the elements to be loaded are contiguous it seems that there is still one read cycle per element, so performance is really no better than just doing scalar loads.
As Casey mentioned on stream for newer instruction set AVX2 which is available starting with Intel Haswell CPU's there are gather instructions that can fetch memory from multiple locations with single instruction and store all results in one SSE/AVX register. This is exactly what this loop does (which is last
Intel® AVX2 also provides enhanced functionality for broadcast/permute operations on data elements, vector shift instructions with variable-shift count per data element, GATHER instructions: The Intel® AVX2 vector GATHER instructions are used for fetching non-contiguous data elements from memory using vector-index
11 Jun 2011 These are extremely interesting instructions. It's worth noting, however, that things like gather/scatter live and die by the quality of their implementation. I would hope that these are considered worth delivering high-quality implementations, but let's just say that Intel has occasionally been known to deliver
6 Jan 2014 Extending the concept to scatter instructions: The operation of the scatter instructions is very similar to that of the corresponding gather instructions. The only difference being that the instruction stores data elements into the memory instead of loading them into an output vector register.
29 Jan 2014 vector gather implementation on Intel Haswell and Knights Corner microarchitectures. Finally we discuss why GPU implementations perform much better for this specific algorithm. Keywords SIMD, Intel MIC, gather, computed tomography, back projection, performance. 1. Introduction. Single Instruction
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