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//
// This confidential and proprietary software may be used only as
// authorised by a licensing agreement from ARM Limited
// (C) COPYRIGHT 2020-2021 ARM Limited
// ALL RIGHTS RESERVED
// The entire notice above must be reproduced on all authorised
// copies and copies may only be made to the extent permitted
// by a licensing agreement from ARM Limited.
=== Scatter/Gather Operators
==== GATHER
Generate a tensor for which each element in the output is a subtensor of the values tensor based on the indices.
N is the number of batches, W the number of indices in each batch, K the range of each index and C the number data channels for each index.
include::{generated}/operators/GATHER.adoc[]
[source,c++]
----
for_each(0 <= n < N, 0 <= w < W, 0 <= c < C) {
index_t k = tensor_read<index_t>(indices, [N,W], [n,w]);
REQUIRE(0 <= k && k < K);
in_out_t value = tensor_read<in_out_t>(values, [N,K,C], [n,k,c]);
tensor_write<in_out_t>(output, [N,W,C], [n,w,c], value);
}
----
==== SCATTER
The values_out tensor is set to the values_in tensor with data modified as follows: data from the input tensor is inserted at the positions specified by the indices tensor.
N is the number of batches, W the number of indices in each batch, K the range of each index and C the number data channels for each index.
It is not permitted to repeat the same output index within a single SCATTER operation and so each output index occurs at most once.
It follows that K >= W.
In use cases that require multiple updates to the same output position, these must be decomposed into multiple SCATTER operations.
include::{generated}/operators/SCATTER.adoc[]
[source,c++]
----
// The following array is used to check compliance that an output position
// is modified at most once.
bool_t output_modified[N,K,C];
// Copy the values_in tensor to the values_out tensor.
// Values not written by the scatter operation are unchanged in the output.
for_each(0 <= n < N, 0 <= k < K, 0 <= c < C) {
in_out_t value = tensor_read<in_out_t>(values_in, [N,K,C], [n,k,c]);
tensor_write<in_out_t>(values_out, [N,K,C], [n, k, c], value);
output_modified[n,k,c]=false;
}
// Now perform the SCATTER operation, modifying the positions from the indices tensor
for_each(0 <= n < N, 0 <= w < W, 0 <= c < C) {
index_t k = tensor_read<index_t>(indices, [N,W], [n,w]);
REQUIRE(0 <= k && k < K);
REQUIRE(output_modified[n,k,c] == false);
in_out_t value = tensor_read<in_out_t>(input, [N,W,C], [n,w,c]);
tensor_write<in_out_t>(values_out, [N,K,C], [n, k, c], value);
output_modified[n,k,c] = true;
}
----
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