"set_instance_assignment -name GLOBAL_SIGNAL OFF" -> what does it do?

Translation: this prevents the compiler from automatically putting the referenced signal on one of the global routing channels in the device even if it might improve performance of the signal. The global routing channels are dedicated routing resources used to distribute clocks and other high fan-out control signals, like a reset, but they are a limited resource in older devices (not as much in newer devices like Stratix 10 and Agilex).

You use the GLOBAL_SIGNAL assignment to either force a signal to use one of these resources (when set to ON) or prevent a global routing resource's usage (when set to OFF). You might want to set this to OFF for a signal if you have limited global routing resources or if the "insertion delay" penalty for getting the signal onto the global resource through one of the dedicated buffers adds too much delay to the signal, potentially causing timing issues.

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