[LLVMdev] loop vectorizer

[Andrew Trick]

On Nov 5, 2013, at 7:39 PM, Frank Winter <fwinter at jlab.org> wrote:

> Good that you bring this up. I still have no solution to this vectorization problem.
> 
> However, I can rewrite the code and insert a second loop which eliminates the 'urem' and 'div' instructions in the index calculations. In this case, the inner loop's trip count would be equal to the SIMD length and the loop vectorizer ignores the loop. Unrolling the loop and SLP is not an option, since the loop body can get lengthy.
> 
> What would be a quicker to implement: 
> 
> a) Teach the loop vectorizer the 'urem' and 'div' instructions, or 

If I’m correct assuming that this means teaching SCEV about (or (and…), (shl…)), then it seems like a worthwhile thing to do, but a fundamental improvement that is not easy and could have unknown impact.

> b) have the loop vectorizer process loops with trip count equal to the vector length ?

Seems to me like we should handle this and it’s just a matter of fixing the driver/heuristics. I would file a PR with this test case.

-Andy

> One of both solutions will be needed, I guess.
> 
> Frank
> 
> 
> 
> On 05/11/13 22:12, Andrew Trick wrote:
>> 
>> On Oct 30, 2013, at 11:21 PM, Renato Golin <renato.golin at linaro.org> wrote:
>> 
>>> On 30 October 2013 18:40, Frank Winter <fwinter at jlab.org> wrote:
>>>       const std::uint64_t ir0 = (i+0)%4;  // not working
>>> 
>>> I thought this would be the case when I saw the original expression. Maybe we need to teach module arithmetic to SCEV?
>> 
>> I let this thread get stale, so here’s the background again:
>> 
>> source:
>> 
>>       const std::uint64_t ir0 = i%4 + 8*(i/4);
>>       c[ ir0 ]         = a[ ir0 ]         + b[ ir0 ];
>> 
>> before instcombine:
>> 
>>   %4 = urem i64 %i.0, 4
>>   %5 = udiv i64 %i.0, 4
>>   %6 = mul i64 8, %5
>>   %7 = add i64 %4, %6
>>   %8 = getelementptr inbounds float* %a, i64 %7
>> 
>> after instcombine:
>> 
>>   %2 = and i64 %i.04, 3
>>   %3 = lshr i64 %i.04, 2
>>   %4 = shl i64 %3, 3
>>   %5 = or i64 %4, %2
>>   %11 = getelementptr inbounds float* %c, i64 %5
>>   store float %10, float* %11, align 4, !tbaa !0
>> 
>> Honestly, I don't understand why InstCombine "anti-canonicalizes" add->or. I think that transformation should be deferred into we begin target-specific lower (e.g. InstOptimize pass).
>> 
>> Given, that we aren't going to change that any time soon, SCEV could probably be taught to recognize the specific pattern:
>> 
>> Instructions (or (and %a, C1), (shl %b, C2)) -> SCEV (add %a, %b)
>> 
>> -Andy
> 
> 

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