[llvm] [InstCombine] Extend folding of aggregate construction to cases when source aggregates are partially available (PR #100828)

[via llvm-commits]

================
@@ -1117,9 +1124,68 @@ Instruction *InstCombinerImpl::foldAggregateConstructionIntoAggregateReuse(
     // aggregate produced by OrigIVI must have been originally extracted from
     // the same aggregate. Is that so? Can we find said original aggregate?
     SourceAggregate = FindCommonSourceAggregate(UseBB, Pred);
-    if (Describe(SourceAggregate) != AggregateDescription::Found)
-      return nullptr; // Give up.
-    IV.first->second = *SourceAggregate;
+    if (Describe(SourceAggregate) == AggregateDescription::Found) {
+      FoundSrcAgg = true;
+      IV.first->second = *SourceAggregate;
+    } else {
+      // If UseBB is the single successor of Pred, we can add InsertValue to
+      // Pred.
+      auto *BI = dyn_cast<BranchInst>(Pred->getTerminator());
+      if (!BI || !BI->isUnconditional())
+        return nullptr;
+    }
+  }
+
+  if (!FoundSrcAgg)
+    return nullptr;
+
+  // Do some sanity check if we need to add insertvalue into predecessors.
+  auto OrigBB = OrigIVI.getParent();
+  for (auto &It : SourceAggregates) {
+    if (Describe(It.second) == AggregateDescription::Found)
+      continue;
+
+    // Element is defined in UseBB, so it can't be used in predecessors.
+    if (EltDefinedInUseBB)
+      return nullptr;
+
+    // Do this transformation cross loop boundary may cause dead loop. So we
+    // should avoid this situation. But LoopInfo is not generally available, we
+    // must be conservative here.
+    // If OrigIVI is in UseBB and it's the only successor of PredBB, PredBB
+    // can't be in inner loop.
+    if (UseBB != OrigBB)
+      return nullptr;
----------------
weiguozhi wrote:

It limits the insertvalue can only be inserted into immediate predecessors. When combining the previous condition that UseBB is the only successor of PredBB, the insertvalue can never be moved across any BB with multiple successors, such as a loop exit node. So it prevents moving insertvalue
* across loop iterations
* to inner loop from outside

https://github.com/llvm/llvm-project/pull/100828