FWIW, I recently measured the overhead of -ftrivial-auto-var-init=zero for postgres, and the overhead for *some* workloads could not fairly be described with "nearly no overhead". For read-only OLTP the overhead was in the 1-3% range, but for OLAP style queries it was higher, between 0-20% for the different queries in TPC-H.
FWIW, I recently measured the overhead of -ftrivial- auto-var- init=zero for postgres, and the overhead for *some* workloads could not fairly be described with "nearly no overhead". For read-only OLTP the overhead was in the 1-3% range, but for OLAP style queries it was higher, between 0-20% for the different queries in TPC-H.