When using the ALTSYNCRAM Megafunction with Dual-Port Access enabled, what is the output of the second port if the first port is writing data to the same address being read from the second port?

The M4K hardware only supports the old_data mode. The main reason for providing the dont_care is for the portability between MRAM and M4K. The Stratix® II M-RAM does not support old_data mode. 

For users that don't care about the mixed port read-during-write behavior, they can set the mode to don't care and the RAM block can have more flexibilities in placement. They can be implemented in M4K or MRAM. The resulting SOF/POF won't have differences since hardware only support 1 mode. However, simulation should show that we don't guarantee the read-during-write behavior if the don't_care mode is picked.