I've been curious, after various recent observations, about whether matrix multiplication always allocates fresh memory for its output. For example, suppose I do something like
Is this equivalent to
z=B*x; %memory allocated here A(:,1)=z;
Or, does the output of B*x get directly generated in the memory locations occupied by A(:,1)? Obviously, the latter would be more efficient, but I wasn't sure how it worked. I know for example that this
is equivalent to
z=B(1,:); %memory allocated here A(1,:)=z*x;
so obviously not everything is as well optimized as it could be.
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Interesting question. The answer will probably depend on MATLAB version and JIT settings. TMW likely keeps stuffing more & more optimization like this into their JIT code with each version release. (E.g., the in-place operations that can sometimes take place for certain function call syntaxes.) Probably the only way to get any insight into it would be to do some timing and/or memory tests using very large arrays.
(That reminds me ... I need to finish testing & upload the new version of MTIMESX which allows in-place operations)
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