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

[Nikita Popov via llvm-commits]

================
@@ -2811,6 +2902,220 @@ class AllocaSliceRewriter : public InstVisitor<AllocaSliceRewriter, bool> {
     return CanSROA;
   }
 
+  /// Attempts to rewrite a partition using tree-structured merge optimization.
+  ///
+  /// This function analyzes a partition to determine if it can be optimized
+  /// using a tree-structured merge pattern, where multiple non-overlapping
+  /// stores completely fill an alloca. And there is no load from the alloca in
+  /// the middle of the stores. Such patterns can be optimized by eliminating
+  /// the intermediate stores and directly constructing the final vector by
+  /// using shufflevectors.
+  ///
+  /// Example transformation:
+  /// Before: (stores do not have to be in order)
+  ///   %alloca = alloca <8 x float>
+  ///   store <2 x float> %val0, ptr %alloca             ; offset 0-1
+  ///   store <2 x float> %val2, ptr %alloca+16          ; offset 4-5
+  ///   store <2 x float> %val1, ptr %alloca+8           ; offset 2-3
+  ///   store <2 x float> %val3, ptr %alloca+24          ; offset 6-7
+  ///
+  /// After:
+  ///   %alloca = alloca <8 x float>
+  ///   %shuffle0 = shufflevector %val0, %val1, <4 x i32> <i32 0, i32 1, i32 2,
+  ///                 i32 3>
+  ///   %shuffle1 = shufflevector %val2, %val3, <4 x i32> <i32 0, i32 1, i32 2,
+  ///                 i32 3>
+  ///   %shuffle2 = shufflevector %shuffle0, %shuffle1, <8 x i32> <i32 0, i32 1,
+  ///                 i32 2, i32 3, i32 4, i32 5, i32 6, i32 7>
+  ///   store %shuffle2, ptr %alloca
+  ///
+  /// The optimization looks for partitions that:
+  /// 1. Have no overlapping split slice tails
+  /// 2. Contain non-overlapping stores that cover the entire alloca
+  /// 3. Have exactly one load that reads the complete alloca structure and not
+  ///    in the middle of the stores (TODO: maybe we can relax the constraint
+  ///    about reading the entire alloca structure)
+  ///
+  /// \param P The partition to analyze and potentially rewrite
+  /// \return An optional vector of values that were deleted during the rewrite
+  ///         process, or std::nullopt if the partition cannot be optimized
+  ///         using tree-structured merge
+  std::optional<SmallVector<Value *, 4>>
+  rewriteTreeStructuredMerge(Partition &P) {
+    // No tail slices that overlap with the partition
+    if (P.splitSliceTails().size() > 0)
+      return std::nullopt;
+
+    SmallVector<Value *, 4> DeletedValues;
+    LoadInst *TheLoad = nullptr;
+
+    // Structure to hold store information
+    struct StoreInfo {
+      StoreInst *Store;
+      uint64_t BeginOffset;
+      uint64_t EndOffset;
+      Value *StoredValue;
+      StoreInfo(StoreInst *SI, uint64_t Begin, uint64_t End, Value *Val)
+          : Store(SI), BeginOffset(Begin), EndOffset(End), StoredValue(Val) {}
+    };
+
+    SmallVector<StoreInfo, 4> StoreInfos;
+
+    // The alloca must be a fixed vector type
+    Type *AllocatedEltTy = nullptr;
+    if (auto *FixedVecTy = dyn_cast<FixedVectorType>(NewAI.getAllocatedType()))
+      AllocatedEltTy = FixedVecTy->getElementType();
+    else
+      return std::nullopt;
+    // If the allocated element type is a pointer, we do not handle it
+    // TODO: handle this case by using inttoptr/ptrtoint
+    if (AllocatedEltTy->isPtrOrPtrVectorTy())
----------------
nikic wrote:

```suggestion
    if (AllocatedEltTy->isPointerTy())
```
It's already scalar. Same for checks below.

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