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Wrong result when using gpuArray with multiple implicit singleton expansions

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Yunhui Zhou
Yunhui Zhou on 3 Oct 2018
Answered: Wentao Du on 8 Oct 2018
Hi, I am using Matlab R2017b Update 8, and recently I find Matlab produce wrong result when calculating expressions using gpuArray with multiple implicit singleton expansion.
A minimal test case:
Matrixc = rand(2,2) * 3;
Matrixg = gpuArray(Matrixc);
wc = -2:1:2;
w3c = reshape(wc,1,1,length(wc));
w3g = gpuArray(w3c);
GPUc = gather(Matrixg(1,:).*w3g + 0.5*(Matrixg(1,:).^2+Matrixg.^2));
CPU = Matrixc(1,:).*w3c + 0.5*(Matrixc(1,:).^2+Matrixc.^2);
GPUtemp1 = Matrixg(1,:).*w3g;
GPUTemp2 = 0.5*(Matrixg(1,:).^2+Matrixg.^2);
GPUc2 = gather(GPUtemp1 + GPUTemp2);
disp(max(Matrixg(:) - Matrixc(:)));
disp(max(w3g(:) - w3c(:)));
disp(max(GPUc(:) - CPU(:)));
disp(max(GPUc2(:) - CPU(:)));
Produce results:
0
0
0.635767744700676
0
And as you can see here the problem can be by-passed by splitting up the expression to multiple steps. I want to know is it a Matlab bug or is it designed in this way?
UPDATE:
This is indeed a bug. See this bug report.

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Answers (1)

Wentao Du
Wentao Du on 8 Oct 2018
As described in the bug report , this bug was fixed as of R2018b and R2018a Update 4.
If you would like to use R2017b, you may try the workaround mentioned in the report.

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