[llvm-dev] Speculation and control dependent no wrap flags

[Sanjoy Das via llvm-dev]

On Fri, Feb 3, 2017 at 5:49 AM, Hal Finkel via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> On 02/03/2017 07:28 AM, Artur Pilipenko wrote:
>>
>> I'm looking at the bug (https://llvm.org/bugs/show_bug.cgi?id=31181) which
>> was triggered by my change to make CVP mark adds as no wrap
>> (https://reviews.llvm.org/rL278220) and I'd like to have some broader
>> discussion of the problem. In this bug CVP correctly marks an add as nuw
>> basing on the loop latch check, but later loop rotation pass moves the add
>> to a point before the check. In the new context nuw is no longer valid and
>> leads to an incorrect transformation of the loop. See
>> https://llvm.org/bugs/show_bug.cgi?id=31181#c5 comment in the bug for more
>> details.
>>
>> Since nsw, nuw flags can be control dependent, it seems like we should be
>> treating them as metadata, i.e. we should be stripping them when we
>> speculate the instruction. I don’t think that we are doing this now
>> anywhere. The problem was noticed on loop rotation, but I expect any other
>> pass which speculates overflowing operations is suffering from the same
>> problem.
>>
>> Thoughts?
>
>
> We generally don't strip these because violating the wrapping constraint
> does not immediately cause UB. Instead, it generates a poison value. So long
> as that poison value is not used in way which causes UB, then everything is
> fine. In this case, I suspect that we want to fix IndVars to strip the flag,
> or not do the transformation, when it might introduce this kind of issue
> (i.e. a situation where we might branch on a poison value).

Yes, this looks like a straight-forward IndVarSimplify bug to me since
IndVars is introducing a branch on poison.  It needs to strip no-wrap
flags from the post inc value (or create a new post-inc value that
does not have those) before introducing a branch on it.

-- Sanjoy