Eliminate Redundant Copies of Variables in Generated Code
When Redundant Copies Occur
During C/C++ code generation, the code generator checks for statements that attempt to access uninitialized memory. If it detects execution paths where a variable is used but is potentially not defined, it generates a compile-time error. To prevent these errors, define variables by assignment before using them in operations or returning them as function outputs.
Note, however, that variable assignments not only copy the properties of the assigned data to
the new variable, but also initialize the new variable to the assigned value. This forced
initialization sometimes results in redundant copies in C/C++ code. To eliminate redundant
copies, define uninitialized variables by using the
function, as described in How to Eliminate Redundant Copies by Defining Uninitialized Variables.
How to Eliminate Redundant Copies by Defining Uninitialized Variables
Define the variable with
Initialize the variable before reading it.
When the uninitialized variable is an array, you must initialize all of its elements before passing the array as an input to a function or operator — even if the function or operator does not read from the uninitialized portion of the array.
Defining Uninitialized Variables
In the following code, the assignment statement
X = zeros(1,N) not only
X to be a 1-by-5 vector of real doubles, but also initializes
each element of
X to zero.
function X = withoutNullcopy %#codegen N = 5; X = zeros(1,N); for i = 1:N if mod(i,2) == 0 X(i) = i; elseif mod(i,2) == 1 X(i) = 0; end end
This forced initialization creates an extra copy in the generated code. To eliminate this
coder.nullcopy in the definition of
function X = withNullcopy %#codegen N = 5; X = coder.nullcopy(zeros(1,N)); for i = 1:N if mod(i,2) == 0 X(i) = i; else X(i) = 0; end end