For the record, on MSVC we can use __assume(0) to mark unreachable code.
It does the same as gcc's __builtin_unreachable(). I tested it with the
same Pavel's palloc-heavy test case that you used earlier, with the
one-shot plan commit temporarily reverted, and saw a similar speedup you
reported with gcc's __builtin_unreachable(). So, committed that.
- Heikki