Double check that passes correctly set their Modified status

The approach is simple: if a pass reports that it's not modifying a
Function/Module, compute a loose hash of that Function/Module and compare it
with the original one. If we report no change but there's a hash change, then we
have an error.

This approach misses a lot of change but it's not super intrusive and can
detect most of the simple mistakes.

Differential Revision: https://reviews.llvm.org/D80916

llvm/lib/IR/LegacyPassManager.cpp

@@ -1475,6 +1475,74 @@ void FPPassManager::dumpPassStructure(unsigned Offset) {
   }
 }
 
+#ifdef EXPENSIVE_CHECKS
+namespace {
+namespace details {
+
+// Basic hashing mechanism to detect structural change to the IR, used to verify
+// pass return status consistency with actual change. Loosely copied from
+// llvm/lib/Transforms/Utils/FunctionComparator.cpp
+
+class StructuralHash {
+  uint64_t Hash = 0x6acaa36bef8325c5ULL;
+
+  void update(uint64_t V) { Hash = hashing::detail::hash_16_bytes(Hash, V); }
+
+public:
+  StructuralHash() = default;
+
+  void update(Function &F) {
+    if (F.empty())
+      return;
+
+    update(F.isVarArg());
+    update(F.arg_size());
+
+    SmallVector<const BasicBlock *, 8> BBs;
+    SmallPtrSet<const BasicBlock *, 16> VisitedBBs;
+
+    BBs.push_back(&F.getEntryBlock());
+    VisitedBBs.insert(BBs[0]);
+    while (!BBs.empty()) {
+      const BasicBlock *BB = BBs.pop_back_val();
+      update(45798); // Block header
+      for (auto &Inst : *BB)
+        update(Inst.getOpcode());
+
+      const Instruction *Term = BB->getTerminator();
+      for (unsigned i = 0, e = Term->getNumSuccessors(); i != e; ++i) {
+        if (!VisitedBBs.insert(Term->getSuccessor(i)).second)
+          continue;
+        BBs.push_back(Term->getSuccessor(i));
+      }
+    }
+  }
+
+  void update(Module &M) {
+    for (Function &F : M)
+      update(F);
+  }
+
+  uint64_t getHash() const { return Hash; }
+};
+
+} // namespace details
+
+uint64_t StructuralHash(Function &F) {
+  details::StructuralHash H;
+  H.update(F);
+  return H.getHash();
+}
+
+uint64_t StructuralHash(Module &M) {
+  details::StructuralHash H;
+  H.update(M);
+  return H.getHash();
+}
+
+} // end anonymous namespace
+
+#endif
 
 /// Execute all of the passes scheduled for execution by invoking
 /// runOnFunction method.  Keep track of whether any of the passes modifies
@@ -1513,7 +1581,16 @@ bool FPPassManager::runOnFunction(Function &F) {
     {
       PassManagerPrettyStackEntry X(FP, F);
       TimeRegion PassTimer(getPassTimer(FP));
+#ifdef EXPENSIVE_CHECKS
+      uint64_t RefHash = StructuralHash(F);
+#endif
       LocalChanged |= FP->runOnFunction(F);
+
+#ifdef EXPENSIVE_CHECKS
+      assert((LocalChanged || (RefHash == StructuralHash(F))) &&
+             "Pass modifies its input and doesn't report it.");
+#endif
+
       if (EmitICRemark) {
         unsigned NewSize = F.getInstructionCount();
 
@@ -1614,7 +1691,17 @@ MPPassManager::runOnModule(Module &M) {
       PassManagerPrettyStackEntry X(MP, M);
       TimeRegion PassTimer(getPassTimer(MP));
 
+#ifdef EXPENSIVE_CHECKS
+      uint64_t RefHash = StructuralHash(M);
+#endif
+
       LocalChanged |= MP->runOnModule(M);
+
+#ifdef EXPENSIVE_CHECKS
+      assert((LocalChanged || (RefHash == StructuralHash(M))) &&
+             "Pass modifies its input and doesn't report it.");
+#endif
+
       if (EmitICRemark) {
         // Update the size of the module.
         unsigned ModuleCount = M.getInstructionCount();

llvm/unittests/IR/LegacyPassManagerTest.cpp

@@ -680,7 +680,7 @@ namespace llvm {
       ASSERT_EQ(M->getFunctionList().size(), 4U);
       Function *F = M->getFunction("test2");
       Function *SF = splitSimpleFunction(*F);
-      CallInst::Create(F, "", &SF->getEntryBlock());
+      CallInst::Create(F, "", &*SF->getEntryBlock().getFirstInsertionPt());
       ASSERT_EQ(M->getFunctionList().size(), 5U);
       CGModifierPass *P = new CGModifierPass();
       legacy::PassManager Passes;