[LLVMdev] nsw is still logically inconsistent

[Rafael Ávila de Espíndola]

> LICM on ToT hoists the div. Even speculatively.
> 
> The other thing is that div is just one example. The problem could
> also come up with an array load with an index could be safe to
> speculate if the index is known to be bounded. isDereferenceablePointer
> currently doesn't know how to do this, but it'll probably get
> smarter some day.

So, looking at just this example, it looks like semantics we assign to
poison values have to do one of

*) Avoid the udiv being moved, since a valid removal of nsw would create
a program that is now exposed to undefined behavior.

*) Say that use of poison values only produces other poison values, not
undefined behavior.

The second option would give us a lot of liberty to move instruction
around, but is a major change to what the IL looks like these days. In
particular:

*) removing a nsw would become invalid, as it would change a poison
value to a 0, which would cause udiv to have undefined behavior again.

*) Codegen would have to be very careful not to produce undefined
behavior when handling instructions that potentially see poison values.

If we really want the ability to expose instructions with side effects
to poison, maybe the best thing to do is to have poison resistant
variants. For example, we would say that moving the udiv is invalid, but
it could be replaced with a "udiv poison i1 1, %t5". The poison variant
has the same definition as udiv, put produces poison when t5 is 0 (or
poison) and therefore can be moved. The same goes for speculating a load.

> Dan
> 

Cheers,
Rafael