Teach globalopt to not nuke all stores to globals. Keep them around of they

might be deliberate "one time" leaks, so that leak checkers can find them.
This is a reapply of r160602 with the fix that this time I'm committing the
code I thought I was committing last time; the I->eraseFromParent() goes
*after* the break out of the loop.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@160664 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 177 insertions, 8 deletions

diff --git a/lib/Transforms/IPO/GlobalOpt.cpp b/lib/Transforms/IPO/GlobalOpt.cpp
index 4e1c23c198..60ce958de0 100644
--- a/lib/Transforms/IPO/GlobalOpt.cpp
+++ b/lib/Transforms/IPO/GlobalOpt.cpp
@@ -296,6 +296,165 @@ static bool AnalyzeGlobal(const Value *V, GlobalStatus &GS,
   return false;
 }
 
+/// isLeakCheckerRoot - Is this global variable possibly used by a leak checker
+/// as a root?  If so, we might not really want to eliminate the stores to it.
+static bool isLeakCheckerRoot(GlobalVariable *GV) {
+  // A global variable is a root if it is a pointer, or could plausibly contain
+  // a pointer.  There are two challenges; one is that we could have a struct
+  // the has an inner member which is a pointer.  We recurse through the type to
+  // detect these (up to a point).  The other is that we may actually be a union
+  // of a pointer and another type, and so our LLVM type is an integer which
+  // gets converted into a pointer, or our type is an [i8 x #] with a pointer
+  // potentially contained here.
+
+  if (GV->hasPrivateLinkage())
+    return false;
+
+  SmallVector<Type *, 4> Types;
+  Types.push_back(cast<PointerType>(GV->getType())->getElementType());
+
+  unsigned Limit = 20;
+  do {
+    Type *Ty = Types.pop_back_val();
+    switch (Ty->getTypeID()) {
+      default: break;
+      case Type::PointerTyID: return true;
+      case Type::ArrayTyID:
+      case Type::VectorTyID: {
+        SequentialType *STy = cast<SequentialType>(Ty);
+        Types.push_back(STy->getElementType());
+        break;
+      }
+      case Type::StructTyID: {
+        StructType *STy = cast<StructType>(Ty);
+        if (STy->isOpaque()) return true;
+        for (StructType::element_iterator I = STy->element_begin(),
+                 E = STy->element_end(); I != E; ++I) {
+          Type *InnerTy = *I;
+          if (isa<PointerType>(InnerTy)) return true;
+          if (isa<CompositeType>(InnerTy))
+            Types.push_back(InnerTy);
+        }
+        break;
+      }
+    }
+    if (--Limit == 0) return true;
+  } while (!Types.empty());
+  return false;
+}
+
+/// Given a value that is stored to a global but never read, determine whether
+/// it's safe to remove the store and the chain of computation that feeds the
+/// store.
+static bool IsSafeComputationToRemove(Value *V) {
+  do {
+    if (isa<Constant>(V))
+      return true;
+    if (!V->hasOneUse())
+      return false;
+    if (isa<LoadInst>(V) || isa<Argument>(V) || isa<GlobalValue>(V))
+      return false;
+    if (isAllocationFn(V))
+      return true;
+
+    Instruction *I = cast<Instruction>(V);
+    if (I->mayHaveSideEffects())
+      return false;
+    if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(I)) {
+      if (!GEP->hasAllConstantIndices())
+        return false;
+    } else if (I->getNumOperands() != 1) {
+      return false;
+    }
+
+    V = I->getOperand(0);
+  } while (1);
+}
+
+/// CleanupPointerRootUsers - This GV is a pointer root.  Loop over all users
+/// of the global and clean up any that obviously don't assign the global a
+/// value that isn't dynamically allocated.
+///
+static bool CleanupPointerRootUsers(GlobalVariable *GV) {
+  // A brief explanation of leak checkers.  The goal is to find bugs where
+  // pointers are forgotten, causing an accumulating growth in memory
+  // usage over time.  The common strategy for leak checkers is to whitelist the
+  // memory pointed to by globals at exit.  This is popular because it also
+  // solves another problem where the main thread of a C++ program may shut down
+  // before other threads that are still expecting to use those globals.  To
+  // handle that case, we expect the program may create a singleton and never
+  // destroy it.
+
+  bool Changed = false;
+
+  // If Dead[n].first is the only use of a malloc result, we can delete its
+  // chain of computation and the store to the global in Dead[n].second.
+  SmallVector<std::pair<Instruction *, Instruction *>, 32> Dead;
+
+  // Constants can't be pointers to dynamically allocated memory.
+  for (Value::use_iterator UI = GV->use_begin(), E = GV->use_end();
+       UI != E;) {
+    User *U = *UI++;
+    if (StoreInst *SI = dyn_cast<StoreInst>(U)) {
+      Value *V = SI->getValueOperand();
+      if (isa<Constant>(V)) {
+        Changed = true;
+        SI->eraseFromParent();
+      } else if (Instruction *I = dyn_cast<Instruction>(V)) {
+        if (I->hasOneUse())
+          Dead.push_back(std::make_pair(I, SI));
+      }
+    } else if (MemSetInst *MSI = dyn_cast<MemSetInst>(U)) {
+      if (isa<Constant>(MSI->getValue())) {
+        Changed = true;
+        MSI->eraseFromParent();
+      } else if (Instruction *I = dyn_cast<Instruction>(MSI->getValue())) {
+        if (I->hasOneUse())
+          Dead.push_back(std::make_pair(I, MSI));
+      }
+    } else if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(U)) {
+      GlobalVariable *MemSrc = dyn_cast<GlobalVariable>(MTI->getSource());
+      if (MemSrc && MemSrc->isConstant()) {
+        Changed = true;
+        MTI->eraseFromParent();
+      } else if (Instruction *I = dyn_cast<Instruction>(MemSrc)) {
+        if (I->hasOneUse())
+          Dead.push_back(std::make_pair(I, MTI));
+      }
+    } else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(U)) {
+      if (CE->use_empty()) {
+        CE->destroyConstant();
+        Changed = true;
+      }
+    } else if (Constant *C = dyn_cast<Constant>(U)) {
+      if (SafeToDestroyConstant(C)) {
+        C->destroyConstant();
+        // This could have invalidated UI, start over from scratch.
+        Dead.clear();
+        CleanupPointerRootUsers(GV);
+        return true;
+      }
+    }
+  }
+
+  for (int i = 0, e = Dead.size(); i != e; ++i) {
+    if (IsSafeComputationToRemove(Dead[i].first)) {
+      Dead[i].second->eraseFromParent();
+      Instruction *I = Dead[i].first;
+      do {
+        Instruction *J = dyn_cast<Instruction>(I->getOperand(0));
+        if (!J)
+          break;
+        I->eraseFromParent();
+        I = J;
+      } while (!isAllocationFn(I));
+      I->eraseFromParent();
+    }
+  }
+
+  return Changed;
+}
+
 /// CleanupConstantGlobalUsers - We just marked GV constant.  Loop over all
 /// users of the global, cleaning up the obvious ones.  This is largely just a
 /// quick scan over the use list to clean up the easy and obvious cruft.  This
@@ -812,13 +971,18 @@ static bool OptimizeAwayTrappingUsesOfLoads(GlobalVariable *GV, Constant *LV,
   // If we nuked all of the loads, then none of the stores are needed either,
   // nor is the global.
   if (AllNonStoreUsesGone) {
-    DEBUG(dbgs() << "  *** GLOBAL NOW DEAD!\n");
-    CleanupConstantGlobalUsers(GV, 0, TD, TLI);
+    if (isLeakCheckerRoot(GV)) {
+      Changed |= CleanupPointerRootUsers(GV);
+    } else {
+      Changed = true;
+      CleanupConstantGlobalUsers(GV, 0, TD, TLI);
+    }
     if (GV->use_empty()) {
+      DEBUG(dbgs() << "  *** GLOBAL NOW DEAD!\n");
+      Changed = true;
       GV->eraseFromParent();
       ++NumDeleted;
     }
-    Changed = true;
   }
   return Changed;
 }
@@ -1794,10 +1958,15 @@ bool GlobalOpt::ProcessInternalGlobal(GlobalVariable *GV,
   if (!GS.isLoaded) {
     DEBUG(dbgs() << "GLOBAL NEVER LOADED: " << *GV);
 
-    // Delete any stores we can find to the global.  We may not be able to
-    // make it completely dead though.
-    bool Changed = CleanupConstantGlobalUsers(GV, GV->getInitializer(),
-                                              TD, TLI);
+    bool Changed;
+    if (isLeakCheckerRoot(GV)) {
+      // Delete any constant stores to the global.
+      Changed = CleanupPointerRootUsers(GV);
+    } else {
+      // Delete any stores we can find to the global.  We may not be able to
+      // make it completely dead though.
+      Changed = CleanupConstantGlobalUsers(GV, GV->getInitializer(), TD, TLI);
+    }
 
     // If the global is dead now, delete it.
     if (GV->use_empty()) {
@@ -1845,7 +2014,7 @@ bool GlobalOpt::ProcessInternalGlobal(GlobalVariable *GV,
 
         if (GV->use_empty()) {
           DEBUG(dbgs() << "   *** Substituting initializer allowed us to "
-                << "simplify all users and delete global!\n");
+                       << "simplify all users and delete global!\n");
           GV->eraseFromParent();
           ++NumDeleted;
         } else {