[llvm-dev] LoopVectorizer with ifconversion

[Jonas Paulsson via llvm-dev]

On 2017-03-17 23:23, Renato Golin wrote:
> On 17 March 2017 at 16:34, Hal Finkel <hfinkel at anl.gov> wrote:
>> In general, this is true everywhere. In a large vectorized loop, this cost
>> may well be worthwhile. The idea is that the cost model should account for
>> all of these costs. If it doesn't properly, we should fix that.
> Isn't this only worth when the SIMD instructions can be
> conditionalised per lane? I believe AVX512 and SVE have such
> abilities, but not NEON or SSE.
>
> Does SystemZ support conditionalisation in SIMD?
>
No, z13 has no masked vector operations. I think this is checked with 
the isLegalMasked...() hooks. Is this implicitly assumed in 
LoopVectorizers if-conversion algorithm somewhere? I see the scalar 
costs are adjusted with 'ScalarCost /= getReciprocalPredBlockProb()'. It 
seems that this should be done for all blocks that remain after 
vectorization?

If not doing the simple thing and disabling store with predication 
vectorization, I agree with Hal that this could be worth modeling as an 
extra cost to not reject some large loop with just one conditional 
store, or so.

I think this extra cost should be added to the compare, but I am not 
sure if it would be better to instead add it to the branch, because 
there are also cases of e.g. (AND (COMPARE, COMPARE)). Adding a cost to 
a vectorized branch instead could be done by assuming that a conditional 
branch would have to be set up for each branch after the vector compare.

Does the loop vectorizer know which blocks that need predication in the 
scalar loop will remain after vectorization? SystemZ could check such 
blocks by looking for stores, but that seems like extra work.

thanks,

Jonas