[llvm-dev] MatchLoadCombine(): handling for vectorized loop.

Friedman, Eli via llvm-dev llvm-dev at lists.llvm.org
Fri Dec 7 14:02:03 PST 2018

On 12/4/2018 1:01 AM, Jonas Paulsson wrote:
> Hi Eli,
> On 2018-12-04 00:37, Friedman, Eli wrote:
>> On 12/3/2018 8:20 AM, Jonas Paulsson wrote:
>>> Hi,
>>> I have noticed some loops that build a wider element by loading 
>>> small elements, zero-extending them, shifting them (with different 
>>> amounts) to then 'or' them all together. They are either equivalent 
>>> of a wider load, or to that of a byte-swapped one.
>>> DAGCombiner::MatchLoadCombine() will combine this to a single wide 
>>> load, but only in the scalar cases of i16, i32 and i64. The result 
>>> is that these loops (I have seen a dozen or so on SPEC) get 
>>> vectorized with a lot of ugly code.
>>> I have begun to experiment with handling the vectorized loop also, 
>>> and would like to know if people think this would be a good idea? 
>>> Also, am I right to assume that it probably should be run before 
>>> type legalization?
>> You mean, trying to merge some combination of vector loads and 
>> shuffles into a single vector load in DAGCombine?  That seems sort of 
>> late, given the cost modeling involved in vectorization.
> What happens specifically is that a scalar loop that is written (most 
> likely at source level) like
> ///  i8 *a = ...
> ///  i32 val = a[0] | (a[1] << 8) | (a[2] << 16) | (a[3] << 24)
> becomes during DAGCombine (this is from the comment in 
> DAGCombiner.cpp:5750)
> /// =>
> ///  i32 val = *((i32)a)
> I also wondered why this is done so late but found in the commit 
> message (b52af07 / r293036) that there had been a discussion where 
> this had intentionally been addressed late: "...We came to the 
> conclusion that we want to do this transformation late in the pipeline 
> because in presence of atomic loads load widening is irreversible 
> transformation and it might hinder other optimizations..."

Yes.  (I'm not completely convinced that we need to delay the transform 
all the way to DAGCombine, though...)

> The loop vectorizer looks at such a loop and thinks the scalar loop 
> costs 13 and VF=4 costs 5. The cost for the vectorized loop is about 
> right, but the scalar loop becomes much better with the help of 
> MatchLoadCombine(): just 2 instructions (load + store). The vector 
> loop could also have been just 2 instructions: Vector Load + Vector 
> Store, but instead it does Vector Load + vector shuffles, vector zero 
> extensions, vector element shifts, vector ors and the Vector Store :-(
> It would be very far-fetched to correct the cost for the scalar loop 
> in the LoopVectorizer. It seems like a better idea to implement the 
> handling for the vectorized loop also in DAGCombiner. As an 
> alternative, we could just ignore this I guess...

The current vectorizer doesn't really have infrastructure for this, at 

> My patch does a recursive search starting at an ISD::Or node and then 
> tries to prove that the sub-DAG with all the ors, shifts and extends 
> is equivalent to the original Vector Load, possibly in reversed order. 
> I think that's a somewhat simpler problem than what the scalar loop 
> handling of MatchLoadCombine() is doing.

Oh, it's simpler because it doesn't actually have to deal with memory?  
Makes sense.  Still seems kind of awkward to generate the terrible 
vector loop and recover it later, though; it's fragile based on the 
shuffle costs.

If you are going to do this, probably simpler to do in instcombine?

>> See also 
>> http://lists.llvm.org/pipermail/llvm-dev/2018-February/121000.html ?
> + at Florian:
> I think SLP awareness in LoopVectorizer would be great and wonder now 
> what happened with this? (It seems however SLP is not able to handle 
> this type of loop).

The existing SLP pass actually could handle this sort of sequence, but 
we currently suppress it if the result would be too narrow.  See also 
https://reviews.llvm.org/D48725 .


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