[vyodaiken]

It is completely useless to allow compilers to assume false things about the code they generate.

[fooker]

It’s not useless. The assumption is not false if the program doesn’t have undefined behavior. The assumption allows the code to be a few times faster. To disallow this assumption would inhibit these optimizations.

[vyodaiken]

a) the assumption is not false if it is not false! b) the speedup is not shown anywhere

[fooker]

The speedup is more or less the difference between O1 and O3 optimization levels.