Automatic use of internal block RAMs

You will find recommended ways of describing memory in hdl in this document: https://www.intel.com/content/www/us/en/programmable/documentation/sbc1513987577203.html#mwh1409959579089

If you don't stick to the guide, sometimes it works and sometimes it doesn't. AFAIK shared variables are only used in Quartus to describe memories with dual clocks.