• 10/30/2018
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Avoid Spurious Operations in Kernels

As every line in kernel code is executed many times, make sure you have no spurious instructions in your kernel code.
Spurious instructions are not always evident. Consider the following kernel:
__kernel void foo(const __global int* data, const uint dataSize) {   size_t tid = get_global_id(0);   size_t gridSize = get_global_size(0);   size_t workPerItem = dataSize / gridSize;   size_t myStart = tid * workPerItem;   for (size_t i = myStart; i < myStart + workPerItem; ++i)   {     //actual work   } }  
In this kernel, the
for
loop is used to reduce the number of work-items and the overhead of keeping them. However, in this example, every work-item recalculates the amount of indices to iterate on while this number is identical for all work-items.
The size of the dataset and the NDRange dimensions is known before kernel launch. Calculate the amount of work per item on the host once and then pass the result as a constant parameter.
Using
size_t
for indices makes vectorization of indexing arithmetic less efficient. To improve performance, use the
int
data type, when your index fits the 32-bit integer range. Consider the following example:
__kernel void foo(const __global int* data, const uint workPerItem) {   int tid = get_global_id(0);   int gridSize = get_global_size(0);   //int workPerItem = dataSize / gridSize;   int myStart = tid * workPerItem;   for (int i = myStart; i < mystart +="" workperitem;="" ++i)="" …=""

See Also

OpenCL™ 1.2 Specification at
Overview Presentations of the OpenCL™ Standard at

Product and Performance Information

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Performance varies by use, configuration and other factors. Learn more at www.Intel.com/PerformanceIndex.