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

[Nikita Popov via llvm-commits]

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@@ -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;
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nikic wrote:

If I'm understanding correctly, this currently checks that at least one predecessor folds, rather than that all but one folds, so we could end up inserting extra insertvalues in *many* predecessors. Shouldn't we be limiting this such that we only insert into at most one predecessor?

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