genmake_local must be a file copied directly in your build/ diretory. Placing it in the build via a soft link, e.g. ln -s path-to-file/genmake_local build/, will not be read by genmake2
MITgcm assigns a cost function accumulation term for each type of run (1) forward, (2) adjoint, and (3) tangent linear forward.
We see these defined in pkg/cost header files (1) cost.h, (2) adcost.h, and (3) g_cost.h.
Over time, TAF changed their naming convention, which prompted the creation of cost_ad.flow, which maps the common block variable names to the corresponding TAF generated variable names.
This means, for TAF generated code, g_fc = fc_tl and adfc = fc_ad. If you are tracking how the cost function changes within the TAF subroutines, search for the TAF variable names! Since these variable names a cummutative, you will never find one being set to the other, i.e. you will never see g_fc = fc_tl written in the source code. This is handled via the TAF flow directives.