[llvm] [SROA] Use tree-structure merge to remove alloca (PR #152793)

[Nikita Popov via llvm-commits]

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@@ -2667,6 +2668,96 @@ static Value *insertVector(IRBuilderTy &IRB, Value *Old, Value *V,
   return V;
 }
 
+/// This function takes two vector values and combines them into a single vector
+/// by concatenating their elements. The function handles:
+///
+/// 1. Element type mismatch: If either vector's element type differs from
+///    NewAIEltType, the function bitcasts the vector to use NewAIEltType while
+///    preserving the total bit width (adjusting the number of elements
+///    accordingly).
+///
+/// 2. Size mismatch: After transforming the vectors to have the desired element
+///    type, if the two vectors have different numbers of elements, the smaller
+///    vector is extended with poison values to match the size of the larger
+///    vector before concatenation.
+///
+/// 3. Concatenation: The vectors are merged using a shuffle operation that
+///    places all elements of V0 first, followed by all elements of V1.
+///
+/// \param V0 The first vector to merge (must be a vector type)
+/// \param V1 The second vector to merge (must be a vector type)
+/// \param DL The data layout for size calculations
+/// \param NewAIEltTy The desired element type for the result vector
+/// \param Builder IRBuilder for creating new instructions
+/// \return A new vector containing all elements from V0 followed by all
+/// elements from V1
+static Value *mergeTwoVectors(Value *V0, Value *V1, const DataLayout &DL,
+                              Type *NewAIEltTy, IRBuilder<> &Builder) {
+  assert(V0->getType()->isVectorTy() && V1->getType()->isVectorTy() &&
+         "Can not merge two non-vector values");
+
+  // V0 and V1 are vectors
+  // Create a new vector type with combined elements
+  // Use ShuffleVector to concatenate the vectors
+  auto *VecType0 = cast<FixedVectorType>(V0->getType());
+  auto *VecType1 = cast<FixedVectorType>(V1->getType());
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nikic wrote:

You can just drop the assert, as `cast<>` is an assert by itself.

https://github.com/llvm/llvm-project/pull/152793