[SAI_Peregrinus]

> implementor (compiler writer) has two choices: (a) assume that the undefined behavior doesn’t occur, and implement optimizations under that assumption, or (b) nevertheless implement a defined behavior for it, which in many cases amounts to a pessimization.

No, they have two choices: (a) assume that the undefined behavior doesn't occur, and implement any output code generation whatsoever under that assumption, or (b) define a behavior for it, and implement output code generation based on that assumption which in many cases amounts to a pessimization.

Optimization isn't relevant. Assuming it can't happen and then continuing to generate code as though it can't happen is all that matters. You can't make any assumptions, including that disabling optimization will change the output code.