[llvm] Add SD matchers and unit test coverage for ISD::VECTOR_SHUFFLE (PR #119592)

[Min-Yih Hsu via llvm-commits]

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@@ -540,6 +549,25 @@ struct BinaryOpc_match {
   }
 };
 
+/// Matches shuffle.
+template <typename T0, typename T1> struct SDShuffle_match {
+  T0 Op1;
+  T1 Op2;
+  ArrayRef<int> Mask;
+
+  SDShuffle_match(const T0 &Op1, const T1 &Op2, const ArrayRef<int> &Mask)
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mshockwave wrote:

First of all, ArrayRef tries to model a reference and it's more like a pointer than an object, so you should pass it by value according to what you try to do here.

Second, I would like to invite you to step back a little bit and think about what you want to achieve here first -- there are two possible scenarios where you need `m_Shuffle` w.r.t masks: either you want to _obtain_ the mask from a shuffle, or you want to match against a shuffle with a _specific_ mask. In the first scenario, you'll store a reference to the output mask variable inside `SDShuffle_match` and write the mask value into it upon matching the shuffle. What you do here is more like the second scenraio, where the user provide a mask and `SDPattern_match` matches a shuffle with that mask.

However, it seems like your unit test is expecting the _first_ scenario (which is also what you did in the previous version of the patch). Therefore, I think this is wrong.

I'll say you can implement either one of the two scenarios in this patch, or implement both with two different matching functions / classes. For instance, `m_Shuffle` for the first scenario, and something like `m_ShuffleSpecificMask` for the second scenario.

https://github.com/llvm/llvm-project/pull/119592