[llvm] [MergeFunctions] Preserve import GUIDs when folding functions (PR #207003)

[Teresa Johnson via llvm-commits]

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@@ -856,43 +859,75 @@ void MergeFunctions::writeAlias(Function *F, Function *G) {
   ++NumAliasesWritten;
 }
 
+static DenseSet<GlobalValue::GUID> unionImportGUIDs(const Function &F,
+                                                    const Function &G) {
+  DenseSet<GlobalValue::GUID> AllImports = F.getImportGUIDs();
+  DenseSet<GlobalValue::GUID> GImports = G.getImportGUIDs();
+  AllImports.insert(GImports.begin(), GImports.end());
+  return AllImports;
+}
+
+static void mergeEntryCountsAndImportsInto(Function &F, Function &G) {
+  std::optional<uint64_t> FEntryCount = F.getEntryCount();
+  std::optional<uint64_t> GEntryCount = G.getEntryCount();
+  DenseSet<GlobalValue::GUID> AllImports = unionImportGUIDs(F, G);
+  if (!FEntryCount && !GEntryCount && AllImports.empty())
+    return;
+
+  // -1 is a safe placeholder here, getEntryCount() already treats it as
+  // "unknown" (same sentinel SamplePGO uses for no-sample functions), so
+  // it won't look hot to anyone reading the count back.
+  uint64_t Sum = static_cast<uint64_t>(-1);
+  if (FEntryCount || GEntryCount)
+    Sum = SaturatingAdd(FEntryCount ? *FEntryCount : uint64_t{0},
+                        GEntryCount ? *GEntryCount : uint64_t{0});
+  F.setEntryCount(Sum, AllImports.empty() ? nullptr : &AllImports);
+}
+
 // If needed, replace G with an alias to F if possible, or a thunk to F if
 // profitable. Returns false if neither is the case. If \p G is not needed (i.e.
-// it is discardable and unused), \p G is removed directly.
-bool MergeFunctions::writeThunkOrAliasIfNeeded(Function *F, Function *G) {
-  if (G->isDiscardableIfUnused() && G->use_empty() && !MergeFunctionsPDI) {
+// it is discardable and unused), \p G is removed directly. If \p MergeProfile
+// is set, G's profile metadata is merged into F.
+bool MergeFunctions::writeThunkOrAliasIfNeeded(Function *F, Function *G,
+                                               bool MergeProfile) {
+  bool ShouldErase =
+      G->isDiscardableIfUnused() && G->use_empty() && !MergeFunctionsPDI;
+  bool ShouldAlias = canCreateAliasFor(G);
+  bool ShouldThunk = canCreateThunkFor(F);
+
+  if (!ShouldErase && !ShouldAlias && !ShouldThunk)
+    return false;
+
+  if (MergeProfile) {
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teresajohnson wrote:

Nit: no braces around single line if body

https://github.com/llvm/llvm-project/pull/207003