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Bug in index output of max and min on gpuArray

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Kyle Watson
Kyle Watson on 26 Oct 2020
Answered: Srivardhan Gadila on 30 Oct 2020
There is a bug in the index output of max and min for gpuArrays. I am using 2018b and the bug also appears in 2019b. The index of the max (the 2nd output) is wrong for various lengths of arrays. For example:
x = randn(8680, 2, 'gpuArray');
[m, k] = max(x, [], 1);
disp(k)
[mCpu, kCpu] = max(gather(x), [], 1);
disp(kCpu)
isequal(k, kCpu) % different values!!
x = randn(8681, 2, 'gpuArray');
[m, k] = max(x, [], 1);
disp(k)
[mCpu, kCpu] = max(gather(x), [], 1);
disp(kCpu)
isequal(k, kCpu) % nagically now the same value!
OK let's do a test to see which lengths of x faile
n = 1 : 10e3;
bad = [];
for i = 1 : length(n)
x = randn(n(i), 2, 'single', 'gpuArray');
[m1, k1] = max(x, [], 1);
[m2, k2] = max(gather(x), [], 1);
if ~isequal(k1, k2)
bad(end+1) = n(i);
end
end
figure
plot(bad) % there are 410 values between 1 and 10e3 that fail!!
Now let's try again with different 2nd dim lengths of x
n = 1 : 10e3;
bad = [];
for i = 1 : length(n)
x = randn(n(i), 4, 'single', 'gpuArray');
[m1, k1] = max(x, [], 1);
[m2, k2] = max(gather(x), [], 1);
if ~isequal(k1, k2)
bad(end+1) = n(i);
end
end
figure
plot(bad) % now 147 that fail.

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Answers (1)

Srivardhan Gadila
Srivardhan Gadila on 30 Oct 2020
The issue has been reported to the concerned team.
The workaround would be to transpose the matrix and reducing along the columns:
[m1, k1] = max(x', [], 2);
m1 = m1';
k1 = k1';

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