Process Control Statements

Currently, there are two execution modes of a mainstream deep learning framework: a static graph mode (GRAPH_MODE) and a dynamic graph mode (PYNATIVE_MODE).

In PYNATIVE_MODE, MindSpore fully supports process control statements of the native Python syntax. In GRAPH_MODE, MindSpore performance is optimized during build. Therefore, there are some special constraints on using process control statements when during network definition. Other constraints are the same as those in the native Python syntax.

When switching the running mode from dynamic graph to static graph, pay attention to the static graph syntax support. The following describes how to use process control statements when defining a network in GRAPH_MODE.

Constant and Variable Conditions

When a network is defined in GRAPH_MODE, MindSpore classifies condition expressions in process control statements into constant and variable conditions. During graph build, a condition expression that can be determined to be either true or false is a constant condition, while a condition expression that cannot be determined to be true or false is a variable condition. MindSpore generates control flow operators on a network only when the condition expression is a variable condition.

It should be noted that, when a control flow operator exists in a network, the network is divided into multiple execution subgraphs, and process jumping and data transmission between the subgraphs cause performance loss to some extent.

Constant Conditions

Check methods:

The condition expression does not contain tensors or any list, tuple, or dict whose elements are of the tensor type.
The condition expression contains tensors or a list, tuple, or dict of the tensor type, but the condition expression result is not affected by the tensor value.

Examples:

for i in range(0,10): i is a scalar. The result of the potential condition expression i < 10 can be determined during graph build. Therefore, it is a constant condition.
self.flag: A scalar of the Boolean type. Its value is determined when the Cell object is built.
x + 1 < 10: x is a scalar: The value of x + 1 is uncertain when the Cell object is built. MindSpore computes the results of all scalar expressions during graph build. Therefore, the value of the expression is determined during build.
len(my_list) < 10: my_list is a list object whose element is of the tensor type. This condition expression contains tensors, but the expression result is not affected by the tensor value and is related only to the number of tensors in my_list.

Variable Conditions

Check method:

The condition expression contains tensors or a list, tuple, or dict of the tensor type, and the condition expression result is affected by the tensor value.

Examples:

x < y: x and y are operator outputs.
x in list: x is the operator output.

The operator output can be determined only when each step is executed. Therefore, the preceding two conditions are variable conditions.

if Statement

When defining a network in GRAPH_MODE using the if statement, pay attention to the following: When the condition expression is a variable condition, the same variable in different branches must be assigned the same data type.

if Statement Under a Variable Condition

In the following code, shapes of tensors assigned to the out variable in the if and else branches are () and (2,), respectively. The shape of the Tensor returned by the network is determined by the condition x < y. The result of x < y cannot be determined during graph build. Therefore, whether the out shape is () or (2,) cannot be determined during graph build.

import numpy as np
import mindspore as ms
from mindspore import nn

class SingleIfNet(nn.Cell):

    def construct(self, x, y, z):
        # Build an if statement whose condition expression is a variable condition.
        if x < y:
            out = x
        else:
            out = z
        out = out + 1
        return out

forward_net = SingleIfNet()

x = ms.Tensor(np.array(0), dtype=ms.int32)
y = ms.Tensor(np.array(1), dtype=ms.int32)
z = ms.Tensor(np.array([1, 2]), dtype=ms.int32)

output = forward_net(x, y, z)

if Statement Under a Constant Condition

When the condition expression in the if statement is a constant condition, the usage of the condition expression is the same as that of the native Python syntax, and there is no additional constraint. In the following code, the condition expression x < y + 1 of the if statement is a constant condition (because x and y are scalar constants). During graph build, the out variable is of the scalar int type. The network can be built and executed properly, and the correct result 1 is displayed.

import numpy as np
import mindspore as ms
from mindspore import nn

class SingleIfNet(nn.Cell):

    def construct(self, z):
        x = 0
        y = 1

        # Build an if statement whose condition expression is a constant condition.
        if x < y + 1:
            out = x
        else:
            out = z
        out = out + 1

        return out

z = ms.Tensor(np.array([0, 1]), dtype=ms.int32)
forward_net = SingleIfNet()

output = forward_net(z)
print("output:", output)

output: 1

for Statement

The for statement expands the loop body. Therefore, the number of subgraphs and operators of the network that uses the for statement depends on the number of loops of the for statement. If the number of operators or subgraphs is too large, more hardware resources are consumed.

In the following sample code, the loop body in the for statement is executed for three times, and the output is 5.

import numpy as np
from mindspore import nn
import mindspore as ms

class IfInForNet(nn.Cell):

    def construct(self, x, y):
        out = 0

        # Build a for statement whose condition expression is a constant condition.
        for i in range(0, 3):
            # Build an if statement whose condition expression is a variable condition.
            if x + i < y:
                out = out + x
            else:
                out = out + y
            out = out + 1

        return out

forward_net = IfInForNet()

x = ms.Tensor(np.array(0), dtype=ms.int32)
y = ms.Tensor(np.array(1), dtype=ms.int32)

output = forward_net(x, y)
print("output:", output)

output: 5

The for statement expands the loop body. Therefore, the preceding code is equivalent to the following code:

import numpy as np
from mindspore import nn
import mindspore as ms

class IfInForNet(nn.Cell):
    def construct(self, x, y):
        out = 0

        # Loop: 0
        if x + 0 < y:
            out = out + x
        else:
            out = out + y
        out = out + 1
        # Loop: 1
        if x + 1 < y:
            out = out + x
        else:
            out = out + y
        out = out + 1
        # Loop: 2
        if x + 2 < y:
            out = out + x
        else:
            out = out + y
        out = out + 1

        return out

forward_net = IfInForNet()

x = ms.Tensor(np.array(0), dtype=ms.int32)
y = ms.Tensor(np.array(1), dtype=ms.int32)

output = forward_net(x, y)
print("output:", output)

output: 5

According to the preceding sample code, using the for statement may cause too many subgraphs in some scenarios. To reduce hardware resource overhead and improve network build performance, you can convert the for statement to the while statement whose condition expression is a variable condition.

while Statement

The while statement is more flexible than the for statement. When the condition of while is a constant, while processes and expands the loop body in a similar way as for.

When the condition expression of while is a variable condition, the while statement does not expand the loop body. Instead, a control flow operator is generated during graph execution. Therefore, the problem of too many subgraphs caused by the for loop can be avoided.

while Statement Under a Constant Condition

In the following sample code, the loop body in the for statement is executed for three times, and the output result is 5, which is essentially the same as the sample code in the for statement.

import numpy as np
from mindspore import nn
import mindspore as ms

class IfInWhileNet(nn.Cell):

    def construct(self, x, y):
        i = 0
        out = x
        # Build a while statement whose condition expression is a constant condition.
        while i < 3:
            # Build an if statement whose condition expression is a variable condition.
            if x + i < y:
                out = out + x
            else:
                out = out + y
            out = out + 1
            i = i + 1
        return out

forward_net = IfInWhileNet()
x = ms.Tensor(np.array(0), dtype=ms.int32)
y = ms.Tensor(np.array(1), dtype=ms.int32)

output = forward_net(x, y)
print("output:", output)

output: 5

while Statement Under a Variable Condition

Constraint: When the condition expression in the while statement is a variable condition, the input shape of the operator cannot be changed in the loop body. The data types of variables with the same name inside the loop body and outside the loop body should be the same, for example, Tensor type variables require the same shape and type.

MindSpore requires that the input shape of the same operator on the network be determined during graph build. However, changing the input shape of the operator in the while loop body takes effect during graph execution.

The following uses sample code as an example:

import numpy as np
from mindspore import nn
import mindspore as ms
from mindspore import ops

class IfInWhileNet(nn.Cell):

    def __init__(self):
        super().__init__()

    def construct(self, x, y, i):
        out = x
        # Build a while statement whose condition expression is a variable condition.
        while i < 3:
            if x + i < y:
                out = out + x
            else:
                out = out + y
            out = out + 1
            # Change the input shape of an operator.
            out = ops.expand_dims(out, -1)
            i = i + 1
        return out

forward_net = IfInWhileNet()
i = ms.Tensor(np.array(0), dtype=ms.int32)
x = ms.Tensor(np.array(0), dtype=ms.int32)
y = ms.Tensor(np.array(1), dtype=ms.int32)

output = forward_net(x, y, i)

In the preceding code, the ExpandDims operator in the while loop body of the condition expression i < 3 changes the input shape of the expression out = out + 1 in the next loop. As a result, an error occurs during graph build. The following error information is displayed during code execution:

RuntimeError: Exceed function call depth limit 1000, (function call depth: 1001, simulate call depth: 665).