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

[via llvm-commits]

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@@ -4981,13 +5273,20 @@ AllocaInst *SROA::rewritePartition(AllocaInst &AI, AllocaSlices &AS,
                                P.endOffset(), IsIntegerPromotable, VecTy,
                                PHIUsers, SelectUsers);
   bool Promotable = true;
-  for (Slice *S : P.splitSliceTails()) {
-    Promotable &= Rewriter.visit(S);
-    ++NumUses;
-  }
-  for (Slice &S : P) {
-    Promotable &= Rewriter.visit(&S);
-    ++NumUses;
+  // Check whether we can have tree-structured merge.
+  if (auto DeletedValues = Rewriter.rewriteTreeStructuredMerge(P)) {
+    NumUses += DeletedValues->size() + 1;
+    for (Value *V : *DeletedValues)
+      DeadInsts.push_back(V);
+  } else {
+    for (Slice *S : P.splitSliceTails()) {
+      Promotable &= Rewriter.visit(S);
+      ++NumUses;
+    }
+    for (Slice &S : P) {
+      Promotable &= Rewriter.visit(&S);
+      ++NumUses;
+    }
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Chengjunp wrote:

I have verified that the impact of falling back to the old method is very small. Here is the result I had on the llvm compiler time tracker: https://llvm-compile-time-tracker.com/compare.php?from=f9cb95c9b0d52c11c7fbf11535cbd88039312ed3&to=74908e3a4491aa51b141a4d1de900c67b240cdc8&stat=instructions%3Au

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