it defaults to off for standard-compliant mode. Which in my mind was the default mode as that's what we use everywhere I have worked in the last 15 years. But of course that's not the case.
In any case, according to the sibling comment, the default is 'fast' even in std-compliant mode in C++, which I find very surprising. I'm not very familiar with that corner of the standard, but it must be looser than the equivalent wording in the C standard.
