[llvm] [MergeFunctions] Preserve instruction-level profile metadata during merging (PR #208009)

[Alok Kumar Sharma via llvm-commits]

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@@ -858,28 +889,235 @@ void MergeFunctions::writeAlias(Function *F, Function *G) {
 
 // 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)
+    mergeInstrProfMetadataInto(F, G);
+
+  if (ShouldErase) {
     G->eraseFromParent();
     return true;
   }
-  if (canCreateAliasFor(G)) {
+
+  if (ShouldAlias) {
     writeAlias(F, G);
     return true;
   }
-  if (canCreateThunkFor(F)) {
+  if (ShouldThunk) {
     writeThunk(F, G);
     return true;
   }
-  return false;
+
+  llvm_unreachable("Erase, alias or thunk must apply");
 }
 
 /// Returns true if \p F is either weak_odr or linkonce_odr.
 static bool isODR(const Function *F) {
   return F->hasWeakODRLinkage() || F->hasLinkOnceODRLinkage();
 }
 
+static uint64_t getBlockCountForMerging(const BlockFrequencyInfo *BFI,
+                                        const BasicBlock *BB,
+                                        std::optional<uint64_t> EC) {
+  if (BFI) {
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alokkrsharma wrote:

Agreed, I will change BFI to be passed by reference instead of pointer in the next revision, since it's always available from FAM.getResult<BlockFrequencyAnalysis>.

https://github.com/llvm/llvm-project/pull/208009