Wrong result when using gpuArray with multiple implicit singleton expansions

Question

Yunhui Zhou 2018-10-3

1
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/421944-wrong-result-when-using-gpuarray-with-multiple-implicit-singleton-expansions

回答： Wentao Du 2018-10-8

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.