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Warnings from Conversion to Single-Precision C/C++ Code

When you generate single-precision C/C++ code by using the MATLAB® Coder™ app or codegen with the -singleC option, you can receive the following warnings.

Function Uses Double-Precision in the C89/C90 Standard

If the standard math library is C89/C90, the conversion process warns you when a function uses double-precision code in the C89/C90 standard.

Consider the function mysine.

function c = mysine(a)
c = sin(a);
end

Generate single-precision code for mysine using the C89/C90 standard.

x = -pi:0.01:pi;
cfg = coder.config('lib');
cfg.TargetLangStandard = 'C89/C90 (ANSI)';
codegen -singleC -config cfg mysine -args {x} -report

codegen warns that sin uses double-precision in the C89/C90 (ANSI) standard.

Warning: The function sin uses double-precision in the C89/C90 (ANSI) standard. For single-precision
code, consider using the C99 (ISO) standard or use your own function.

To open the code generation report, click the View Report link.

To see that double-precision operations remain in the converted code, click the Code Insights tab. Expand Potential data type issues and then expand Double-precision operations. The report indicates that mysine has a double-precision operation at line 2 c = sin(a).

To address this warning, use the default standard math library, C99 (ISO).

  • At the command line:

    cfg.TargetLangStandard = 'C99 (ISO)';
    

  • In the app, in the project build settings, on the Custom Code tab, set Standard math library to C99 (ISO).

Built-In Function Is Implemented in Double-Precision

Some built-in MATLAB functions are implemented using double-precision operations. The conversion process warns that the code generated for these functions contains double-precision operations.

Consider the function geterf that calls the built-in function erf.

function y = geterf(x)
y = erf(x);
end

Generate single-precision code for geterf.

codegen -singleC -config:lib -args {1} geterf -report

codegen warns that erf is implemented in double precision.

Warning: The builtin function erf is implemented in double-precision. Code generated for this
function will contain doubles.

To open the code generation report, click the View Report link.

To see that double-precision operations remain in the converted code, click the Code Insights tab. Expand Potential data type issues and then expand Double-precision operations. The report indicates that geterf has a double-precision operation at line 2 y = erf(x) .

To address this warning, rewrite your code so that it does not use the function that is implemented in double precision.

Built-In Function Returns Double-Precision

If a built-in MATLAB function returns a double-precision output, the conversion process generates a warning.

Consider the function mysum that calls the built-in function sum.

function y = mysum(x)
y = sum(int32(x));
end

Generate single-precision code formysum.

A = 1:10;
codegen -singleC -config:lib -args {A} mysum -report

codegen warns that mysum is implemented in double precision.

Warning: The output of builtin function sum is double-precision and has been cast to
single-precision. The code generated for the builtin function may still contain doubles.

To open the code generation report, click the View Report link.

To see that double-precision operations remain in the converted code, click the Code Insights tab. Expand Potential data type issues and then expand Double-precision operations. The report indicates that mysum has a double-precision operation at line 2 y = sum(int32(x)).

To address this warning, specify that you want the function to return the 'native' class.

(sum(int32(1), 'native')

Using this option causes the function to return the same type as the input.

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