+bool TailCallElim::runOnFunction(Function &F) {
+ if (skipOptnoneFunction(F))
+ return false;
+
+ DL = F.getParent()->getDataLayout();
+
+ bool AllCallsAreTailCalls = false;
+ bool Modified = markTails(F, AllCallsAreTailCalls);
+ if (AllCallsAreTailCalls)
+ Modified |= runTRE(F);
+ return Modified;
+}
+
+namespace {
+struct AllocaDerivedValueTracker {
+ // Start at a root value and walk its use-def chain to mark calls that use the
+ // value or a derived value in AllocaUsers, and places where it may escape in
+ // EscapePoints.
+ void walk(Value *Root) {
+ SmallVector<Use *, 32> Worklist;
+ SmallPtrSet<Use *, 32> Visited;
+
+ auto AddUsesToWorklist = [&](Value *V) {
+ for (auto &U : V->uses()) {
+ if (!Visited.insert(&U).second)
+ continue;
+ Worklist.push_back(&U);
+ }
+ };
+
+ AddUsesToWorklist(Root);
+
+ while (!Worklist.empty()) {
+ Use *U = Worklist.pop_back_val();
+ Instruction *I = cast<Instruction>(U->getUser());
+
+ switch (I->getOpcode()) {
+ case Instruction::Call:
+ case Instruction::Invoke: {
+ CallSite CS(I);
+ bool IsNocapture = !CS.isCallee(U) &&
+ CS.doesNotCapture(CS.getArgumentNo(U));
+ callUsesLocalStack(CS, IsNocapture);
+ if (IsNocapture) {
+ // If the alloca-derived argument is passed in as nocapture, then it
+ // can't propagate to the call's return. That would be capturing.
+ continue;
+ }
+ break;
+ }
+ case Instruction::Load: {
+ // The result of a load is not alloca-derived (unless an alloca has
+ // otherwise escaped, but this is a local analysis).
+ continue;
+ }
+ case Instruction::Store: {
+ if (U->getOperandNo() == 0)
+ EscapePoints.insert(I);
+ continue; // Stores have no users to analyze.
+ }
+ case Instruction::BitCast:
+ case Instruction::GetElementPtr:
+ case Instruction::PHI:
+ case Instruction::Select:
+ case Instruction::AddrSpaceCast:
+ break;
+ default:
+ EscapePoints.insert(I);
+ break;
+ }
+
+ AddUsesToWorklist(I);
+ }
+ }
+
+ void callUsesLocalStack(CallSite CS, bool IsNocapture) {
+ // Add it to the list of alloca users.
+ AllocaUsers.insert(CS.getInstruction());
+
+ // If it's nocapture then it can't capture this alloca.
+ if (IsNocapture)
+ return;
+
+ // If it can write to memory, it can leak the alloca value.
+ if (!CS.onlyReadsMemory())
+ EscapePoints.insert(CS.getInstruction());
+ }
+
+ SmallPtrSet<Instruction *, 32> AllocaUsers;
+ SmallPtrSet<Instruction *, 32> EscapePoints;
+};
+}
+
+bool TailCallElim::markTails(Function &F, bool &AllCallsAreTailCalls) {
+ if (F.callsFunctionThatReturnsTwice())
+ return false;
+ AllCallsAreTailCalls = true;
+
+ // The local stack holds all alloca instructions and all byval arguments.
+ AllocaDerivedValueTracker Tracker;
+ for (Argument &Arg : F.args()) {
+ if (Arg.hasByValAttr())
+ Tracker.walk(&Arg);
+ }
+ for (auto &BB : F) {
+ for (auto &I : BB)
+ if (AllocaInst *AI = dyn_cast<AllocaInst>(&I))
+ Tracker.walk(AI);
+ }
+
+ bool Modified = false;
+
+ // Track whether a block is reachable after an alloca has escaped. Blocks that
+ // contain the escaping instruction will be marked as being visited without an
+ // escaped alloca, since that is how the block began.
+ enum VisitType {
+ UNVISITED,
+ UNESCAPED,
+ ESCAPED
+ };
+ DenseMap<BasicBlock *, VisitType> Visited;
+
+ // We propagate the fact that an alloca has escaped from block to successor.
+ // Visit the blocks that are propagating the escapedness first. To do this, we
+ // maintain two worklists.
+ SmallVector<BasicBlock *, 32> WorklistUnescaped, WorklistEscaped;
+
+ // We may enter a block and visit it thinking that no alloca has escaped yet,
+ // then see an escape point and go back around a loop edge and come back to
+ // the same block twice. Because of this, we defer setting tail on calls when
+ // we first encounter them in a block. Every entry in this list does not
+ // statically use an alloca via use-def chain analysis, but may find an alloca
+ // through other means if the block turns out to be reachable after an escape
+ // point.
+ SmallVector<CallInst *, 32> DeferredTails;
+
+ BasicBlock *BB = &F.getEntryBlock();
+ VisitType Escaped = UNESCAPED;
+ do {
+ for (auto &I : *BB) {
+ if (Tracker.EscapePoints.count(&I))
+ Escaped = ESCAPED;
+
+ CallInst *CI = dyn_cast<CallInst>(&I);
+ if (!CI || CI->isTailCall())
+ continue;
+
+ if (CI->doesNotAccessMemory()) {
+ // A call to a readnone function whose arguments are all things computed
+ // outside this function can be marked tail. Even if you stored the
+ // alloca address into a global, a readnone function can't load the
+ // global anyhow.
+ //
+ // Note that this runs whether we know an alloca has escaped or not. If
+ // it has, then we can't trust Tracker.AllocaUsers to be accurate.
+ bool SafeToTail = true;
+ for (auto &Arg : CI->arg_operands()) {
+ if (isa<Constant>(Arg.getUser()))
+ continue;
+ if (Argument *A = dyn_cast<Argument>(Arg.getUser()))
+ if (!A->hasByValAttr())
+ continue;
+ SafeToTail = false;
+ break;
+ }
+ if (SafeToTail) {
+ emitOptimizationRemark(
+ F.getContext(), "tailcallelim", F, CI->getDebugLoc(),
+ "marked this readnone call a tail call candidate");
+ CI->setTailCall();
+ Modified = true;
+ continue;
+ }
+ }
+
+ if (Escaped == UNESCAPED && !Tracker.AllocaUsers.count(CI)) {
+ DeferredTails.push_back(CI);
+ } else {
+ AllCallsAreTailCalls = false;
+ }
+ }
+
+ for (auto *SuccBB : make_range(succ_begin(BB), succ_end(BB))) {
+ auto &State = Visited[SuccBB];
+ if (State < Escaped) {
+ State = Escaped;
+ if (State == ESCAPED)
+ WorklistEscaped.push_back(SuccBB);
+ else
+ WorklistUnescaped.push_back(SuccBB);
+ }