[Attributor] Detect possibly unbounded cycles in functions

This patch add mayContainUnboundedCycle helper function which checks whether a function has any cycle which we don't know if it is bounded or not.
Loops with maximum trip count are considered bounded, any other cycle not.
It also contains some fixed tests and some added tests contain bounded and
unbounded loops and non-loop cycles.

Reviewed By: jdoerfert, uenoku, baziotis

Differential Revision: https://reviews.llvm.org/D74691

Author: omarahmed1111

llvm/include/llvm/Analysis/MustExecute.h

@@ -176,6 +176,8 @@ class ICFLoopSafetyInfo: public LoopSafetyInfo {
   virtual ~ICFLoopSafetyInfo() {};
 };
 
+bool mayContainIrreducibleControl(const Function &F, const LoopInfo *LI);
+
 struct MustBeExecutedContextExplorer;
 
 /// Enum that allows us to spell out the direction.

llvm/lib/Analysis/MustExecute.cpp

@@ -484,13 +484,13 @@ static bool maybeEndlessLoop(const Loop &L) {
   return true;
 }
 
-static bool mayContainIrreducibleControl(const Function &F, const LoopInfo *LI) {
+bool llvm::mayContainIrreducibleControl(const Function &F, const LoopInfo *LI) {
   if (!LI)
     return false;
   using RPOTraversal = ReversePostOrderTraversal<const Function *>;
   RPOTraversal FuncRPOT(&F);
-  return !containsIrreducibleCFG<const BasicBlock *, const RPOTraversal,
-                                 const LoopInfo>(FuncRPOT, *LI);
+  return containsIrreducibleCFG<const BasicBlock *, const RPOTraversal,
+                                const LoopInfo>(FuncRPOT, *LI);
 }
 
 /// Lookup \p Key in \p Map and return the result, potentially after

llvm/lib/Transforms/IPO/Attributor.cpp

@@ -28,6 +28,7 @@
 #include "llvm/Analysis/LazyValueInfo.h"
 #include "llvm/Analysis/Loads.h"
 #include "llvm/Analysis/MemoryBuiltins.h"
+#include "llvm/Analysis/MustExecute.h"
 #include "llvm/Analysis/ScalarEvolution.h"
 #include "llvm/Analysis/ValueTracking.h"
 #include "llvm/IR/Argument.h"
@@ -36,9 +37,9 @@
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/InstIterator.h"
 #include "llvm/IR/IntrinsicInst.h"
+#include "llvm/IR/NoFolder.h"
 #include "llvm/IR/Verifier.h"
 #include "llvm/InitializePasses.h"
-#include "llvm/IR/NoFolder.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
@@ -2491,28 +2492,34 @@ struct AAUndefinedBehaviorFunction final : AAUndefinedBehaviorImpl {
 
 /// ------------------------ Will-Return Attributes ----------------------------
 
-// Helper function that checks whether a function has any cycle.
-// TODO: Replace with more efficent code
-static bool containsCycle(Function &F) {
-  SmallPtrSet<BasicBlock *, 32> Visited;
-
-  // Traverse BB by dfs and check whether successor is already visited.
-  for (BasicBlock *BB : depth_first(&F)) {
-    Visited.insert(BB);
-    for (auto *SuccBB : successors(BB)) {
-      if (Visited.count(SuccBB))
+// Helper function that checks whether a function has any cycle which we don't
+// know if it is bounded or not.
+// Loops with maximum trip count are considered bounded, any other cycle not.
+static bool mayContainUnboundedCycle(Function &F, Attributor &A) {
+  ScalarEvolution *SE =
+      A.getInfoCache().getAnalysisResultForFunction<ScalarEvolutionAnalysis>(F);
+  LoopInfo *LI = A.getInfoCache().getAnalysisResultForFunction<LoopAnalysis>(F);
+  // If either SCEV or LoopInfo is not available for the function then we assume
+  // any cycle to be unbounded cycle.
+  // We use scc_iterator which uses Tarjan algorithm to find all the maximal
+  // SCCs.To detect if there's a cycle, we only need to find the maximal ones.
+  if (!SE || !LI) {
+    for (scc_iterator<Function *> SCCI = scc_begin(&F); !SCCI.isAtEnd(); ++SCCI)
+      if (SCCI.hasCycle())
         return true;
-    }
+    return false;
   }
-  return false;
-}
 
-// Helper function that checks the function have a loop which might become an
-// endless loop
-// FIXME: Any cycle is regarded as endless loop for now.
-//        We have to allow some patterns.
-static bool containsPossiblyEndlessLoop(Function *F) {
-  return containsCycle(*F);
+  // If there's irreducible control, the function may contain non-loop cycles.
+  if (mayContainIrreducibleControl(F, LI))
+    return true;
+
+  // Any loop that does not have a max trip count is considered unbounded cycle.
+  for (auto *L : LI->getLoopsInPreorder()) {
+    if (!SE->getSmallConstantMaxTripCount(L))
+      return true;
+  }
+  return false;
 }
 
 struct AAWillReturnImpl : public AAWillReturn {
@@ -2523,7 +2530,7 @@ struct AAWillReturnImpl : public AAWillReturn {
     AAWillReturn::initialize(A);
 
     Function *F = getAssociatedFunction();
-    if (!F || !A.isFunctionIPOAmendable(*F) || containsPossiblyEndlessLoop(F))
+    if (!F || !A.isFunctionIPOAmendable(*F) || mayContainUnboundedCycle(*F, A))
       indicatePessimisticFixpoint();
   }
 

llvm/test/Transforms/Attributor/read_write_returned_arguments_scc.ll

@@ -101,7 +101,7 @@ return:                                           ; preds = %if.end, %if.then
   ret i32* %retval.0
 }
 
-; CHECK: Function Attrs: argmemonly nofree norecurse nosync nounwind
+; CHECK: Function Attrs: argmemonly nofree norecurse nosync nounwind willreturn
 ; CHECK-NEXT: define i32* @external_sink_ret2_nrw(i32* nofree readnone %n0, i32* nocapture nofree readonly %r0, i32* nofree returned writeonly "no-capture-maybe-returned" %w0)
 define i32* @external_sink_ret2_nrw(i32* %n0, i32* %r0, i32* %w0) {
 entry:
@@ -160,6 +160,7 @@ entry:
 ;
 ; CHECK-NOT: attributes #
 ; CHECK: attributes #{{.*}} = { argmemonly nofree nosync nounwind }
-; CHECK: attributes #{{.*}} = { argmemonly nofree norecurse nosync nounwind }
+; CHECK: attributes #{{.*}} = { argmemonly nofree norecurse nosync nounwind willreturn }
 ; CHECK: attributes #{{.*}} = { nosync nounwind }
+; CHECK: attributes #{{.*}} = { norecurse nosync nounwind willreturn }
 ; CHECK-NOT: attributes #