[llvm] [LLVM] Add flatten function attribute to LLVM IR and implement recursive inlining in AlwaysInliner (PR #174899)

[Nikita Popov via llvm-commits]

================
@@ -99,11 +110,78 @@ bool AlwaysInlineImpl(
     }
   }
 
+  // Flatten functions with the flatten attribute using a local worklist.
+  for (Function *F : NeedFlattening) {
+    SmallVector<std::pair<CallBase *, int>, 16> Worklist;
+    SmallVector<std::pair<Function *, int>, 16> InlineHistory;
+    SmallVector<CallBase *> NewCallSites;
+    OptimizationRemarkEmitter ORE(F);
+
+    // Collect initial calls.
+    for (BasicBlock &BB : *F) {
+      for (Instruction &I : BB) {
+        if (auto *CB = dyn_cast<CallBase>(&I)) {
+          if (CB->getAttributes().hasFnAttr(Attribute::NoInline))
+            continue;
+          Function *Callee = CB->getCalledFunction();
+          if (!Callee || Callee->isDeclaration())
+            continue;
+          Worklist.push_back({CB, -1});
+        }
+      }
+    }
+
+    while (!Worklist.empty()) {
+      auto Item = Worklist.pop_back_val();
+      CallBase *CB = Item.first;
+      int InlineHistoryID = Item.second;
----------------
nikic wrote:

```suggestion
      auto [CB, InlineHistoryID] = Worklist.pop_back_val();
```

https://github.com/llvm/llvm-project/pull/174899