[foxfluff]

> Can you still compile gcc code with -O0 (gcc option to turn optimization off) to get completely defined behavior?

No. The standard specifies what's undefined, optimization levels don't change it (though there are compiler flags such as -fwrapv which make undefined things defined).

However, turning off optimizations will make behavior easier to predict.

[lanstin]

it used to be passed around never to use -O0 because it was much less used/tested and would generate wrong code more often. Not sure id that was folklore or true.

[saagarjha]

-O0 is "well tested" because everyone uses it for debug builds. If it produced incorrect results people would be fairly upset (and they are when it does…but it's not particularly common, because incorrect optimizations are generally the problem when codegen is incorrect.)