chapters/control_flow.adoc

//
// 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.

=== Control Flow Operators

TOSA implements two control flow operators, for conditional branching and loop based control. Both have attributes that are TOSA sub-graphs.

==== COND_IF

Evaluates a Boolean condition and then takes one of two distinct execution paths. This implements the semantic if-then-else structure.

*Arguments:*

|===
|Argument|Type|Name|Description

|Input |tensor_list_t  |input_list |List of input tensors
|Input |bool_t         |condition  |Input condition as rank-0 tensor
|Attribute|tosa_graph_t|then_graph |TOSA graph to execute if condition is true
|Attribute|tosa_graph_t|else_graph |TOSA graph to execute if condition is false
|Output|tensor_list_t  |output_list|List of output tensors
|===

*Operation Function:*

[source,c++]
----
ERROR_IF(tensor_list_shape(input_list) != tosa_input_shape(then_graph));
ERROR_IF(tensor_list_shape(input_list) != tosa_input_shape(else_graph));
ERROR_IF(tensor_list_shape(output_list) != tosa_output_shape(then_graph));
ERROR_IF(tensor_list_shape(output_list) != tosa_output_shape(else_graph));

if (condition) {
    tosa_execute_graph(then_graph, input_list, output_list);
} else {
    tosa_execute_graph(else_graph, input_list, output_list);
}
----

==== WHILE_LOOP

Generates and evaluates a Bool condition and either executes a loop body or exits the loop. This action is performed repeatedly after updating and re-evaluating the Boolean condition every iteration. This implements the semantic foreach or while iterative loop structure.

*Arguments:*

|===
|Argument|Type|Name|Description

|Input |tensor_list_t  |input_list |List of input tensors
|Attribute|tosa_graph_t|cond_graph |TOSA graph to evaluate the condition
|Attribute|tosa_graph_t|body_graph |TOSA graph to execute the loop body
|Output|tensor_list_t  |output_list|List of output tensors
|===

*Operation Function:*

[source,c++]
----
ERROR_IF(tensor_list_shape(input_list) != tosa_list_shape(output_list));
ERROR_IF(tensor_list_shape(input_list) != tosa_input_shape(cond_graph));
ERROR_IF(tensor_list_shape(input_list) != tosa_input_shape(body_graph));
ERROR_IF(tensor_list_shape(input_list) != tosa_output_shape(body_graph));
ERROR_IF(tosa_output_shape(cond_graph) != tosa_list_shape([bool_t]));

// The iteration number 'i' is included to give unique names to variables
// in each iteration of the loop and is not required by implementations
int i=0;                 // iteration number
list[i] = input_list;    // copy input data as list[0]
tosa_execute_graph(cond_graph, list[i], [condition[i]]);   // initial condition
while (condition[i]) {
    tosa_execute_graph(body_graph, list[i], list[i+1]);
    i = i+1;
    tosa_execute_graph(cond_graph, list[i], [condition[i]]);
}
output_list = list[i];
----