[llvm] [SLP] Reject 2-element vectorization when vector inst count exceeds scalar (PR #190414)

[Ryan Buchner via llvm-commits]

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@@ -12817,6 +12829,112 @@ bool BoUpSLP::areAllUsersVectorized(
          });
 }
 
+unsigned BoUpSLP::getNumScalarInsts() const {
+  unsigned Count = 0;
+  for (const std::unique_ptr<TreeEntry> &Ptr : VectorizableTree) {
+    const TreeEntry &TE = *Ptr;
+    if (DeletedNodes.contains(&TE))
+      continue;
+    if (TE.isGather() || TransformedToGatherNodes.contains(&TE)) {
+      // Count instruction scalars in gathers — they exist in the scalar
+      // code regardless of vectorization. ExtractElement instructions
+      // become free when the vector input is used directly.
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bababuck wrote:

If they exist regardless why count them?

https://github.com/llvm/llvm-project/pull/190414