[LLVMdev] nsw is still logically inconsistent

[Eli Friedman]

2011/12/14 Rafael Ávila de Espíndola <rafael.espindola at gmail.com>:
>> 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.

Note that this matches existing practice: IIRC, for a testcase like
the given testcase, LICM will hoist everything except the div
instruction.

-Eli