[nfc] Expose canReturn from FunctionAttrs (llvm#135650)

This is a fairly light-weight traversal and is needed in instrumentation. No need to run the whole `FunctionAttrs` pass at this stage. To avoid layering issues, this patch factors `canRun` and related under Analysis/CFG.

Author: mtrofin

llvm/include/llvm/Analysis/CFG.h

@@ -174,6 +174,7 @@ bool containsIrreducibleCFG(RPOTraversalT &RPOTraversal, const LoopInfoT &LI) {
 
   return false;
 }
+bool canReturn(const Function &F);
 } // End llvm namespace
 
 #endif

llvm/lib/Analysis/CFG.cpp

@@ -322,3 +322,37 @@ bool llvm::isPotentiallyReachable(
   return isPotentiallyReachable(
       A->getParent(), B->getParent(), ExclusionSet, DT, LI);
 }
+
+static bool instructionDoesNotReturn(const Instruction &I) {
+  if (auto *CB = dyn_cast<CallBase>(&I))
+    return CB->hasFnAttr(Attribute::NoReturn);
+  return false;
+}
+
+// A basic block can only return if it terminates with a ReturnInst and does not
+// contain calls to noreturn functions.
+static bool basicBlockCanReturn(const BasicBlock &BB) {
+  if (!isa<ReturnInst>(BB.getTerminator()))
+    return false;
+  return none_of(BB, instructionDoesNotReturn);
+}
+
+// FIXME: this doesn't handle recursion.
+bool llvm::canReturn(const Function &F) {
+  SmallVector<const BasicBlock *, 16> Worklist;
+  SmallPtrSet<const BasicBlock *, 16> Visited;
+
+  Visited.insert(&F.front());
+  Worklist.push_back(&F.front());
+
+  do {
+    const BasicBlock *BB = Worklist.pop_back_val();
+    if (basicBlockCanReturn(*BB))
+      return true;
+    for (const BasicBlock *Succ : successors(BB))
+      if (Visited.insert(Succ).second)
+        Worklist.push_back(Succ);
+  } while (!Worklist.empty());
+
+  return false;
+}

llvm/lib/Transforms/IPO/FunctionAttrs.cpp

@@ -2090,41 +2090,6 @@ static void addNoRecurseAttrs(const SCCNodeSet &SCCNodes,
   Changed.insert(F);
 }
 
-static bool instructionDoesNotReturn(Instruction &I) {
-  if (auto *CB = dyn_cast<CallBase>(&I))
-    return CB->hasFnAttr(Attribute::NoReturn);
-  return false;
-}
-
-// A basic block can only return if it terminates with a ReturnInst and does not
-// contain calls to noreturn functions.
-static bool basicBlockCanReturn(BasicBlock &BB) {
-  if (!isa<ReturnInst>(BB.getTerminator()))
-    return false;
-  return none_of(BB, instructionDoesNotReturn);
-}
-
-// FIXME: this doesn't handle recursion.
-static bool canReturn(Function &F) {
-  SmallVector<BasicBlock *, 16> Worklist;
-  SmallPtrSet<BasicBlock *, 16> Visited;
-
-  Visited.insert(&F.front());
-  Worklist.push_back(&F.front());
-
-  do {
-    BasicBlock *BB = Worklist.pop_back_val();
-    if (basicBlockCanReturn(*BB))
-      return true;
-    for (BasicBlock *Succ : successors(BB))
-      if (Visited.insert(Succ).second)
-        Worklist.push_back(Succ);
-  } while (!Worklist.empty());
-
-  return false;
-}
-
-
 // Set the noreturn function attribute if possible.
 static void addNoReturnAttrs(const SCCNodeSet &SCCNodes,
                              SmallSet<Function *, 8> &Changed) {