Optimization_1x4_5

Copy the contents of file MMult_1x4_4.c into a file named MMult_1x4_5.c and change the contents:

|| from || to || ||<^> -Include(HowToOptimizeGemm/Details/MMult_1x4_4)- ||<^> -Include(HowToOptimizeGemm/Details/MMult_1x4_5)- ||

Change the first lines in the makefile to {{{ OLD := MMult_1x4_4 NEW := MMult_1x4_5
}}}

• make run {{{ octave:3> PlotAll % this will create the plot }}}

This time the performance graph will look something like

ImageLink(http://www.cs.utexas.edu/users/rvdg/HowToOptimizeGemm/Graphs/compare_MMult-1x4-4_MMult-1x4-5.png,http://www.cs.utexas.edu/users/rvdg/HowToOptimizeGemm/Graphs/compare_MMult-1x4-4_MMult-1x4-5.png,width=40%)

Now we start seeing a performance benefit. The reason is that the four loops have been fused and therefore the four inner products are now being performed simultaneously. This has the following benefits:

• The index p needs only be updated once every eight floating point operations.

• Element A( 0, p ) needs only be brought in from memory once instead of four times. (This only becomes a benefit when the matrices no longer fit in the L2 cache.)