[llvm-dev] Is trapping allowed when an add with nsw flag overflows?

[David Majnemer via llvm-dev]

It comes down to observability.  It's fine to store poison into a memory
location and load it back; however, the optimizer is free to delete stores
of poison.  What is not ok is depending on the value of a load of a poison
value in code which may have observable results (volatile, system calls,
etc.).

On Mon, Apr 18, 2016 at 11:18 AM, Manuel Jacob <me at manueljacob.de> wrote:

> [This mail could be an answer to the other responses as well, as they
> basically are the same.]
>
> Ah, I think I understand now what poison is for.  Adds are defined to not
> have side-effects, so the dependence rule is needed so the optimizer is
> allowed to exploit undefined behavior.  Is this correct?
>
> I forgot to mention in my original mail that our trapping arithmetic
> operations are fully speculable.  This means that the trap won't happen
> until the result is e.g. stored to memory.  A bit like poison values in
> hardware.
>
> Something is still unclear to me: while, according to the examples in
> LangRef, a volatile store has undefined behavior when a poison value is
> stored, this seems to not be true in case of non-volatile stores.  Can
> someone clarify please?
>
>
> On 2016-04-15 19:44, John Regehr via llvm-dev wrote:
>
>> No, trapping is not allowed, since an overflowing add nsw is defined
>> to produce a poison value, which sort of explodes into undefined
>> behavior if it reaches a side-effect.  This is to support speculative
>> execution.
>>
>> If you emit trapping adds for nsw, you'll see spurious traps every now
>> and then.
>>
>> There's a stronger form of undefined behavior, exhibited by things
>> like divide by zero, that permits traps.
>>
>> John
>>
>>
>>
>> On 4/15/16 7:28 PM, Manuel Jacob via llvm-dev wrote:
>>
>>> Hi,
>>>
>>> In our backend, we currently emit add operations that trap on overflow
>>> if the IR operation has the nsw flag set.  Is this allowed?
>>>
>>> According to the documentation about poison values, overflowing a nsw
>>> add is undefined behavior.  However I didn't find a formal definition of
>>> undefined behavior in LLVM.  Judging from previous discussions on the
>>> mailing list, there seems to be a vague line of what LLVM is allowed to
>>> do in case of undefined behavior.  Is trapping allowed?
>>>
>>> -Manuel
>>>
>>
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