C++11 Memory model: the compiler is restricted to don't introduce new data races

Watching a CppCon talk by Michel Wong on 2015 (called C++1/14/17 atomics and memory model... about minute 33:00), he said two sentences that I didn't understand:

No compiler transformation is allowed to introduce a data race (more restrictions on invented writes and possibly fewer speculative stores and potentially loads)

What kind of "invented writes" and "speculative stores and loads" did compiler do, which of those were "common" before the C++11 memory model but now are forbidden? Were those important optimizations that are now lost? Or they had a tiny impact anyway?

There are atomic memory operations that don't cause races (or they race but are well-defined)

What does he mean here by "there are"? In the hardware or does he is refering to some C++ built-in operations? Which memory operations can "cause races but are well-defined"?