I'm not sure the exact conditions that trigger "small mode" (code reference here though). From looking around, on aarch64 it seems like capacity < 8 is the trigger, but I don't observe that with stdx::unordered_map<int, int>.

Our pretty printers don't handle this mode, and the code for our absl pretty printers (e.g., absl_get_nodes) is also shared in other pretty printers. This is partially the cause of mongod-dump-sessions not being able to work (at least if there are fewer than 8 sessions).

This can easily be reproduced by adding a test like this in unordered_map_test.cpp, breaking on the ASSERT_EQ, and then running p pre:

struct X { int a, b, c, d, e, f, g, h, i; };
TEST(StdxUnorderedMapTest, SmallMode) {
    const mongo::stdx::unordered_map<int, X> pre{
        {1, {}},
    };
    ASSERT_EQ(pre.size(), 1);
}

Results look like this:

(gdb) p pre
$1 = absl::node_hash_map<int, (anonymous namespace)::X> with 1 elems  = {
  [Cannot access memory at address 0x1