[PATCH] D121354: [SLP] Fix lookahead operand reordering for splat loads.

[Vasileios Porpodas via Phabricator via llvm-commits]

vporpo added inline comments.


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Comment at: llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp:1091
+    /// `ScoreSplat` because it takes only 1 instruction in x86 for a 2-lane
+    /// vector using `movddup (%reg), xmm0`.
+    static const int ScoreSplatLoads = 3;
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ABataev wrote:
> Thos is target specific for x86. What about other targets?
Please see the discussion below.


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Comment at: llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp:1121
+      if (V1 == V2) {
+        if (isa<LoadInst>(V1) && NumLanes == 2)
+          return VLOperands::ScoreSplatLoads;
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lebedev.ri wrote:
> This should be a lot more principled than this (i.e, query the actual costmodel).
> E.g., `movddup` is only for `double`, but i think this will also trigger for `float`?
> Also, what about broadcasting loads that are available in AVX/AVX2/AVX512?
I agree, this needs to be target specific. I could add a new `ShuffleKind`, something like `SK_BroadcastLoad`. But this is not strictly a shuffle pattern, it is a combined Load + Shuffle. So this might need its own function, like `TargetTransformInfo::getBroadcastLoadCost(VectorType *VecTy)`. What do you think?



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  https://reviews.llvm.org/D121354/new/

https://reviews.llvm.org/D121354