Optimization_1x4_9

Copy the contents of file MMult_1x4_8.c into a file named MMult_1x4_9.c and change the contents.

Change the first lines in the makefile to

OLD  := MMult_1x4_8
NEW  := MMult_1x4_9

octave:3> PlotAll        % this will create the plot

This time the performance graph will look something like

We now use something called 'indirect addressing'. Notice, for example, the line

    c_00_reg += a_0p_reg * *(bp0_pntr+1);

Here

*a0p_reg holds the element A( 0, p+1 ) (yes, this is a bit confusing. A better name for the variable would be good...)

We want to bp0_pntr points to element B( p, 0 ). Hence bp0_pntr+1 addresses the element B( p+1, 0 ). There is a special machine instruction to then access the element at bp0_pntr+1 that does not require the pointer to be updated.
As a result, the pointers that address the elements in the columns of B only need to be updated once every fourth iteration of the loop.

Interestingly, it appears that the compiler did this optimization automatically, and hence we see no performance improvement...

Provide feedback