It's not quite just disabling compilation of code, but also disables some runtime checks (eg. "else if (iomap_use_buffer_heads(srcmap))" branches will be false

It's not quite just disabling compilation of code, but also disables some runtime checks (eg. "else if (iomap_use_buffer_heads(srcmap))" branches will be false at compile time and optimized out). But the difference is probably unmeasurably small.