[LLVMdev] scev questions

[Nick Lewycky]

On 22 August 2013 16:00, Eugene Toder <eltoder at gmail.com> wrote:

> On Thu, Aug 22, 2013 at 6:40 PM, Nick Lewycky <nlewycky at google.com> wrote:
>
>> It's a known limitation, see ScalarEvolution.cpp:5568.
>>
>> The fundamental problem is that len in your example could be (unsigned)
>> -1, -2 or -3, in which case your loop is infinite.
>>
>
> Unless I'm missing something, if len is -1 (or otherwise less than 0) the
> loop has 0 trip count. Did you mean INT_MAX-1, INT_MAX-2 etc? In this case,
> I believe, the behavior is undefined, because adds are marked with "nsw".
>

Doh, 's' was unsigned, 'i' was signed. My mistake.

Regardless, this part of SCEV isn't looking for NSW bits yet and
consequently is optimizing as-if the function were written with 'i' and
'len' as unsigned.

But I'm not sure that we should use nsw to fix this, in spite of the fact
that this is exactly what nsw was designed for. We know the answer even for
the unsigned case, and we should just use that. Add a new SCEV node which
lets us return that it's infinite in this case, and trip count X in the
other; along the lines of "SCEVPotentiallyInfinite(m > int_max-3, sdiv(m,
4))".

I've proposed this previously:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20120917/151250.html

Nick
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