late-gc-lowering: Prevent infinite recursion (#42445)

Phi nodes are allowed to recursively depend on themselves, so
we need to keep track of which ones we have seen to avoid an
infinite recursion here. Apparently this doesn't usually come
up, but I did see if with tsan enabled.

Author: Keno

src/llvm-final-gc-lowering.cpp

@@ -218,10 +218,11 @@ bool FinalLowerGC::doInitialization(Module &M) {
 
 bool FinalLowerGC::doFinalization(Module &M)
 {
+    GlobalValue *functionList[] = {queueRootFunc, poolAllocFunc, bigAllocFunc};
+    queueRootFunc = poolAllocFunc = bigAllocFunc = nullptr;
     auto used = M.getGlobalVariable("llvm.compiler.used");
     if (!used)
         return false;
-    GlobalValue *functionList[] = {queueRootFunc, poolAllocFunc, bigAllocFunc};
     SmallPtrSet<Constant*, 16> InitAsSet(
         functionList,
         functionList + sizeof(functionList) / sizeof(void*));

src/llvm-late-gc-lowering.cpp

@@ -1153,12 +1153,14 @@ static bool isConstGV(GlobalVariable *gv)
     return gv->isConstant() || gv->getMetadata("julia.constgv");
 }
 
-static bool isLoadFromConstGV(LoadInst *LI, bool &task_local);
-static bool isLoadFromConstGV(Value *v, bool &task_local)
+typedef llvm::SmallPtrSet<PHINode*, 1> PhiSet;
+
+static bool isLoadFromConstGV(LoadInst *LI, bool &task_local, PhiSet *seen = nullptr);
+static bool isLoadFromConstGV(Value *v, bool &task_local, PhiSet *seen = nullptr)
 {
     v = v->stripInBoundsOffsets();
     if (auto LI = dyn_cast<LoadInst>(v))
-        return isLoadFromConstGV(LI, task_local);
+        return isLoadFromConstGV(LI, task_local, seen);
     if (auto gv = dyn_cast<GlobalVariable>(v))
         return isConstGV(gv);
     // null pointer
@@ -1169,12 +1171,19 @@ static bool isLoadFromConstGV(Value *v, bool &task_local)
         return (CE->getOpcode() == Instruction::IntToPtr &&
                 isa<ConstantData>(CE->getOperand(0)));
     if (auto SL = dyn_cast<SelectInst>(v))
-        return (isLoadFromConstGV(SL->getTrueValue(), task_local) &&
-                isLoadFromConstGV(SL->getFalseValue(), task_local));
+        return (isLoadFromConstGV(SL->getTrueValue(), task_local, seen) &&
+                isLoadFromConstGV(SL->getFalseValue(), task_local, seen));
     if (auto Phi = dyn_cast<PHINode>(v)) {
+        PhiSet ThisSet(&Phi, &Phi);
+        if (!seen)
+            seen = &ThisSet;
+        else if (seen->count(Phi))
+            return true;
+        else
+            seen->insert(Phi);
         auto n = Phi->getNumIncomingValues();
         for (unsigned i = 0; i < n; ++i) {
-            if (!isLoadFromConstGV(Phi->getIncomingValue(i), task_local)) {
+            if (!isLoadFromConstGV(Phi->getIncomingValue(i), task_local, seen)) {
                 return false;
             }
         }
@@ -1206,7 +1215,7 @@ static bool isLoadFromConstGV(Value *v, bool &task_local)
 //
 // The white list implemented here and above in `isLoadFromConstGV(Value*)` should
 // cover all the cases we and LLVM generates.
-static bool isLoadFromConstGV(LoadInst *LI, bool &task_local)
+static bool isLoadFromConstGV(LoadInst *LI, bool &task_local, PhiSet *seen)
 {
     // We only emit single slot GV in codegen
     // but LLVM global merging can change the pointer operands to GEPs/bitcasts
@@ -1216,7 +1225,7 @@ static bool isLoadFromConstGV(LoadInst *LI, bool &task_local)
                {"jtbaa_immut", "jtbaa_const", "jtbaa_datatype"})) {
         if (gv)
             return true;
-        return isLoadFromConstGV(load_base, task_local);
+        return isLoadFromConstGV(load_base, task_local, seen);
     }
     if (gv)
         return isConstGV(gv);