AliasSetTracker: UnknownInsts should contribute to the refcount

AliasSetTracker::addUnknown may create an AliasSet devoid of pointers
just to contain an instruction if no suitable AliasSet already exists.
It will then AliasSet::addUnknownInst and we will be done.

However, it's possible for addUnknown to choose an existing AliasSet to
addUnknownInst.
If this were to occur, we are in a bit of a pickle: removing pointers
from the AliasSet can cause the entire AliasSet to become destroyed,
taking our unknown instructions out with them.

Instead, keep track whether or not our AliasSet has any unknown
instructions.

This fixes PR21582.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222338 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Analysis/AliasSetTracker.h

@@ -254,13 +254,16 @@ private:
                   const AAMDNodes &AAInfo,
                   bool KnownMustAlias = false);
   void addUnknownInst(Instruction *I, AliasAnalysis &AA);
-  void removeUnknownInst(Instruction *I) {
+  void removeUnknownInst(AliasSetTracker &AST, Instruction *I) {
+    bool WasEmpty = UnknownInsts.empty();
     for (size_t i = 0, e = UnknownInsts.size(); i != e; ++i)
       if (UnknownInsts[i] == I) {
         UnknownInsts[i] = UnknownInsts.back();
         UnknownInsts.pop_back();
         --i; --e;  // Revisit the moved entry.
       }
+    if (!WasEmpty && UnknownInsts.empty())
+      dropRef(AST);
   }
   void setVolatile() { Volatile = true; }
 

lib/Analysis/AliasSetTracker.cpp

@@ -55,12 +55,16 @@ void AliasSet::mergeSetIn(AliasSet &AS, AliasSetTracker &AST) {
       AliasTy = MayAlias;
   }
 
+  bool ASHadUnknownInsts = false;
   if (UnknownInsts.empty()) {            // Merge call sites...
-    if (!AS.UnknownInsts.empty())
+    if (!AS.UnknownInsts.empty()) {
       std::swap(UnknownInsts, AS.UnknownInsts);
+      addRef();
+    }
   } else if (!AS.UnknownInsts.empty()) {
     UnknownInsts.insert(UnknownInsts.end(), AS.UnknownInsts.begin(), AS.UnknownInsts.end());
     AS.UnknownInsts.clear();
+    ASHadUnknownInsts = true;
   }
 
   AS.Forward = this;  // Forward across AS now...
@@ -76,6 +80,8 @@ void AliasSet::mergeSetIn(AliasSet &AS, AliasSetTracker &AST) {
     AS.PtrListEnd = &AS.PtrList;
     assert(*AS.PtrListEnd == nullptr && "End of list is not null?");
   }
+  if (ASHadUnknownInsts)
+    AS.dropRef(AST);
 }
 
 void AliasSetTracker::removeAliasSet(AliasSet *AS) {
@@ -123,6 +129,8 @@ void AliasSet::addPointer(AliasSetTracker &AST, PointerRec &Entry,
 }
 
 void AliasSet::addUnknownInst(Instruction *I, AliasAnalysis &AA) {
+  if (UnknownInsts.empty())
+    addRef();
   UnknownInsts.push_back(I);
 
   if (!I->mayWriteToMemory()) {
@@ -218,13 +226,14 @@ AliasSet *AliasSetTracker::findAliasSetForPointer(const Value *Ptr,
                                                   uint64_t Size,
                                                   const AAMDNodes &AAInfo) {
   AliasSet *FoundSet = nullptr;
-  for (iterator I = begin(), E = end(); I != E; ++I) {
-    if (I->Forward || !I->aliasesPointer(Ptr, Size, AAInfo, AA)) continue;
+  for (iterator I = begin(), E = end(); I != E;) {
+    iterator Cur = I++;
+    if (Cur->Forward || !Cur->aliasesPointer(Ptr, Size, AAInfo, AA)) continue;
     
     if (!FoundSet) {      // If this is the first alias set ptr can go into.
-      FoundSet = I;       // Remember it.
+      FoundSet = Cur;     // Remember it.
     } else {              // Otherwise, we must merge the sets.
-      FoundSet->mergeSetIn(*I, *this);     // Merge in contents.
+      FoundSet->mergeSetIn(*Cur, *this);     // Merge in contents.
     }
   }
 
@@ -251,14 +260,14 @@ bool AliasSetTracker::containsUnknown(Instruction *Inst) const {
 
 AliasSet *AliasSetTracker::findAliasSetForUnknownInst(Instruction *Inst) {
   AliasSet *FoundSet = nullptr;
-  for (iterator I = begin(), E = end(); I != E; ++I) {
-    if (I->Forward || !I->aliasesUnknownInst(Inst, AA))
+  for (iterator I = begin(), E = end(); I != E;) {
+    iterator Cur = I++;
+    if (Cur->Forward || !Cur->aliasesUnknownInst(Inst, AA))
       continue;
-    
     if (!FoundSet)            // If this is the first alias set ptr can go into.
-      FoundSet = I;           // Remember it.
-    else if (!I->Forward)     // Otherwise, we must merge the sets.
-      FoundSet->mergeSetIn(*I, *this);     // Merge in contents.
+      FoundSet = Cur;         // Remember it.
+    else if (!Cur->Forward)   // Otherwise, we must merge the sets.
+      FoundSet->mergeSetIn(*Cur, *this);     // Merge in contents.
   }
   return FoundSet;
 }
@@ -406,6 +415,8 @@ void AliasSetTracker::add(const AliasSetTracker &AST) {
 /// tracker.
 void AliasSetTracker::remove(AliasSet &AS) {
   // Drop all call sites.
+  if (!AS.UnknownInsts.empty())
+    AS.dropRef(*this);
   AS.UnknownInsts.clear();
   
   // Clear the alias set.
@@ -510,10 +521,10 @@ void AliasSetTracker::deleteValue(Value *PtrVal) {
   if (Instruction *Inst = dyn_cast<Instruction>(PtrVal)) {
     if (Inst->mayReadOrWriteMemory()) {
       // Scan all the alias sets to see if this call site is contained.
-      for (iterator I = begin(), E = end(); I != E; ++I) {
-        if (I->Forward) continue;
-        
-        I->removeUnknownInst(Inst);
+      for (iterator I = begin(), E = end(); I != E;) {
+        iterator Cur = I++;
+        if (!Cur->Forward)
+          Cur->removeUnknownInst(*this, Inst);
       }
     }
   }

test/Transforms/LICM/PR21582.ll

@@ -0,0 +1,40 @@
+; RUN: opt < %s -basicaa -licm -S | FileCheck %s
+@b = external global i32, align 4
+@fn3.i = external global i32, align 4
+
+declare i32 @g() nounwind
+
+define i32 @f() {
+entry:
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.end, %entry
+; CHECK-LABEL: for.cond:
+; CHECK: store i32 0, i32* @b
+  store i32 0, i32* @b, align 4
+  br i1 true, label %for.body.preheader, label %for.end
+
+for.body.preheader:                               ; preds = %for.cond
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %for.body.preheader
+  %g.15 = phi i32 [ undef, %for.body ], [ 0, %for.body.preheader ]
+  %arrayidx2 = getelementptr inbounds i32* @fn3.i, i64 0
+  %0 = load i32* %arrayidx2, align 4
+  %call = call i32 @g()
+  br i1 false, label %for.body, label %for.end.loopexit
+
+for.end.loopexit:                                 ; preds = %for.body
+  br label %for.end
+
+for.end:                                          ; preds = %for.end.loopexit, %for.cond
+  %whatever = phi i32 [ %call, %for.end.loopexit ], [ undef, %for.cond ]
+  br i1 false, label %for.cond, label %if.then
+
+if.then:                                          ; preds = %for.end
+; CHECK-LABEL: if.then:
+; CHECK: phi i32 [ {{.*}}, %for.end ]
+; CHECK-NOT: store i32 0, i32* @b
+; CHECK: ret i32
+  ret i32 %whatever
+}