[LLVMdev] [LoopVectorizer] Missed vectorization opportunities caused by sext/zext operations

[Sanjoy Das]

Hi Silviu,

>> void test0(unsigned short a, unsigned short *  in, unsigned short * out) {
>>   for (unsigned short w = 1; w < a - 1; w++) //this will never overflow
>>       out[w] = in[w+7] * 2;
>> }
>
> Turns out this can actually overflow for a = 0, and in this case would
> be an infinite loop (end condition would be w < MAX_USHORT). Run-time
> checks could still be added here:

Maybe I'm missing something here, but for a = 0 this will run exactly
(MAX_USHORT - 1) iterations, it will stop once w is MAX_USHORT.

In general, if your loop condition is "I u< N" for any N, I++ cannot
unsigned overflow since the only value of I for which I++ overflows is
MAX_USHORT (= -1) and MAX_USHORT will not be u< N for any N.  A
similar fact follows for "I s< N" and sign overflow in I++.

>> I think it will be sufficient to add a couple of new cases to
>> ScalarEvolution::HowManyLessThans --
>>
>>   zext(A) ult zext(B) == A ult B
>>   sext(A) slt sext(B) == A slt B
>
> I had a go at this but realized that the problem above would affect
> at least the ult case (we need to have B < max(type(B)) for it to hold),

Why?  AFAICT, the above two facts hold for all values of A and B,
including INT_MAX, INT_MIN, INT_UMAX etc.

-- Sanjoy