[tialaramex]

These things aren't the default, the compiler may "let you" but it doesn't do it until you already know you've got a problem and you explicitly tell it you don't want standard behaviour. However lets ignore that for a moment:

My GCC offers to turn signed integer overflow into either an abort or wrapping, either of which is defined behaviour, but how do I get MSVC to do precisely the same thing?

Likewise for aliasing rules. GCC has a switch to have the optimiser not assume the language's aliasing rules are actually obeyed, but what switch in MSVC does exactly the same thing?

[legalcorrection]

MSVC doesn’t enforce strict aliasing to begin with. And passing /d2UndefIntOverflow makes signed integer overflow well-defined. Even if you don’t pass it, MSVC is very conservative in exploiting that UB, precisely to avoid breaking code that was valid before they started doing this optimization (2015 or so).