[LLVMdev] nsw is still logically inconsistent

[Rafael Ávila de Espíndola]

> We first perform a speculation transformation, hoisting all of the
> code above the %overflow_check branch:
> 
>   %t0 = add nsw i32 %a, %b
>   %t1 = sext i32 %t0 to i64
>   %t2 = ashr i64 %t1, 31
>   %t3 = add i64 %t2, 1
>   %t5 = icmp ult %t3, 2
>   %t6 = udiv i1 1, %t5
>   br i1 %overflow_check, label %no_overflow, label %end
> 
> no_overflow:
> 
> Was this valid?
> 
> If nsw overflow is immediate undefined behavior, this transformation
> would break the program, because the overflow is no longer guarded
> by %overflow_check. But a premise of this exercise is that we want
> to be able to speculate add nsw instructions. For now, let's assume
> that there's a way to define nsw which permits this, with some kind
> of deferred undefined behavior semantics.

Could we say that moving the udiv was invalid? It is a instruction
witch can cause undefined behavior and doing the move makes it exposed
to poison values in conditions it was not before.

> Dan
> 

Cheers,
Rafael