[llvm-branch-commits] [llvm] [PatternMatch] match splats from an all-zeroes shufflevector (PR #226190)

[Graham Hunter via llvm-branch-commits]

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@@ -218,14 +218,29 @@ template <typename SubPattern_t> struct Splat_match {
       auto *Splat = C->getSplatValue();
       return Splat ? SubPattern.match(Splat) : false;
     }
-    // TODO: Extend to other cases (e.g. shufflevectors).
-    return false;
+
+    auto *Shuffle = dyn_cast<ShuffleVectorInst>(V);
+    if (!Shuffle || !Shuffle->isZeroEltSplat())
+      return false;
+
+    // Look for an insertelement.
+    auto *Insert = dyn_cast<InsertElementInst>(Shuffle->getOperand(0));
+    if (!Insert)
+      return false;
+
+    Value *SplatElt = Insert->getOperand(1);
+    ConstantInt *Idx = dyn_cast<ConstantInt>(Insert->getOperand(2));
+    if (!Idx || Idx->getZExtValue() != 0)
+      return false;
+
+    return SubPattern.match(SplatElt);
+    // TODO: Handle other splat patterns.
----------------
huntergr-arm wrote:

It's possible that we could splat an arbitrary element from an existing vector, e.g. from a vector load.

I don't think that's worth worrying about right now, and it's possible that the canonical form for that would be an extractelement -> insertelement at 0 -> zeroeltsplat anyway.

https://github.com/llvm/llvm-project/pull/226190