mex file giving wrong outputs

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Bhushan Ravindra Attarde
Bhushan Ravindra Attarde am 8 Mai 2020
Hi, I am generating a mex file for my State Space equation and running it on GPU. When I am running the mex file, I am getting different outputs than expected.
My State Space Equation is:
function out = stateequation(a,b,c,d,x,u) %#codegen
n = size(u,2);
coder.gpu.kernel()
for i = 1:n
x(:,i+1) = a*x(:,i)+ b*u(:,i); %% State Equation
end
out = c*x(:,1:n)+ d*u(:,1:n); %% Output equation
end
On running this code, I am getting answer as :
24 156 948
24 156 948
24 156 948
I have generated mex file using codegen method as follows:
A = double(zeros(3,3));
B = double(zeros(3,3));
C = double(zeros(3,3));
D = double(zeros(3,3));
X = double(zeros(3,1));
U = double(zeros(3,3));
cfg = coder.gpuConfig('mex');
codegen -args {A,B,C,D,X,U} -config cfg stateequation
But on running this mex file I am getting result as :
156 156 156
156 156 156
156 156 156
Why it is giving different answer? Can Someone help me with this?
Thank you in advance

Akzeptierte Antwort

Stefanie Schwarz
Stefanie Schwarz am 13 Mai 2020
Bearbeitet: Stefanie Schwarz am 13 Mai 2020
If you work with MATLAB Coder or GPU Coder and experience runtime issues (e.g. wrong results or crashes), it is always a good idea to use the MATLAB/GPU Coder App. The Apps have a useful feature that helps detect runtime issues.
To do this, you first need to write a testbench script (say stateequation_tb.m) that calls stateequation.m and produces the output that you show above:
%% content of stateequation_tb.m
a = [2 2 2;2 2 2;2 2 2];
b = [2 2 2;2 2 2;2 2 2];
c = [2 2 2;2 2 2;2 2 2];
d = [2 2 2;2 2 2;2 2 2];
x = [2;2;2];
u = [2 2 2;2 2 2;2 2 2];
% call function
out = stateequation(a,b,c,d,x,u)
Open GPU Coder App, specify the function stateequation.m and its associated testbench stateequation_tb.m, and select "Check for issues on CPU". The following runtime error should be thrown:
Index exceeds array dimensions. Index value 2 exceeds valid range [1-1] for array 'x'.
This happens because the size of the input array X is modified during runtime. While MATLAB is able to handle this (which is why MATLAB is so cool and easy to use! :-) ) this does not go well if your aim is to go into C/CUDA code generation.
This can be fixed by using a correctly sized array that copies 'x' like:
function out = stateequation(a,b,c,d,x,u) %#codegen
n = size(u,2);
coder.gpu.kernel()
z = zeros(size(x,1),n+1); %% Define a local variable of the right size
z(:,1:size(x,2)) = x; %% Copy x into the corresponding locations of z
for i = 1:n
z(:,i+1) = a*z(:,i)+ b*u(:,i); %% State Equation
end
out = c*z(:,1:n)+ d*u(:,1:n); %% Output equation
end
If you regenerate the MEX file now, you should get correct results.
  1 Kommentar
Bhushan Ravindra Attarde
Bhushan Ravindra Attarde am 14 Mai 2020
Thank you Stepganie. The code works fine now

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