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//===--- DeadCodeElimination.cpp - Delete dead code ----------------------===//
// This source file is part of the open source project
// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
// See for license information
// See for the list of Swift project authors
#define DEBUG_TYPE "sil-dce"
#include "swift/SIL/SILArgument.h"
#include "swift/SIL/SILBasicBlock.h"
#include "swift/SIL/SILBuilder.h"
#include "swift/SIL/SILFunction.h"
#include "swift/SIL/SILUndef.h"
#include "swift/SIL/DebugUtils.h"
#include "swift/SILOptimizer/Analysis/DominanceAnalysis.h"
#include "swift/SILOptimizer/PassManager/Passes.h"
#include "swift/SILOptimizer/PassManager/Transforms.h"
#include "swift/SILOptimizer/Utils/Local.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace swift;
STATISTIC(NumBranchesPromoted, "Number of dead branches promoted to jumps");
STATISTIC(NumDeletedInsts, "Number of instructions deleted");
namespace {
// Without any complex analysis, does this instruction seem like
// something that we need to keep?
// FIXME: Reconcile the similarities between this and
// isInstructionTriviallyDead.
static bool seemsUseful(SILInstruction *I) {
if (I->mayHaveSideEffects())
return true;
if (auto *BI = dyn_cast<BuiltinInst>(I)) {
// Although the onFastPath builtin has no side-effects we don't want to
// remove it.
return BI->getBuiltinInfo().ID == BuiltinValueKind::OnFastPath;
if (isa<UnreachableInst>(I))
return true;
if (auto TI = dyn_cast<TermInst>(I)) {
if (TI->isFunctionExiting())
return true;
return false;
// We map from post-dominator tree node to a ControllingInfo struct
// which contains the post-dominator tree level of this node, along
// with the direct predecessors that this node is control-dependent
// on, and the minimum level number of any predecessor in the subtree
// below this node in the post-dominator tree.
struct ControllingInfo {
typedef std::pair<SILBasicBlock *, unsigned> PredInfo;
SILBasicBlock *Block;
// The post-dominator tree level for this node.
unsigned Level;
llvm::SmallVector<PredInfo, 2> ControllingPreds;
unsigned MinTreePredLevel;
class DCE : public SILFunctionTransform {
typedef llvm::DomTreeNodeBase<SILBasicBlock> PostDomTreeNode;
llvm::SmallPtrSet<SILNode *, 16> LiveValues;
llvm::SmallPtrSet<SILBasicBlock *, 16> LiveBlocks;
llvm::SmallVector<SILInstruction *, 64> Worklist;
PostDominanceInfo *PDT;
llvm::DenseMap<SILBasicBlock *, ControllingInfo> ControllingInfoMap;
// Maps instructions which produce a failing condition (like overflow
// builtins) to the actual cond_fail instructions which handle the failure.
// Dependencies which go in the reverse direction. Usually for a pair
// %1 = inst_a
// inst_b(%1)
// the dependency goes from inst_b to inst_a: if inst_b is alive then also
// inst_a is alive.
// For some instructions the dependency is exactly the other way round, e.g.
// %1 = inst_which_can_fail
// cond_fail(%1)
// In this case cond_fail is alive only if inst_which_can_fail is alive.
// The key of this map is the source of the dependency (inst_a), the
// value is the destination (inst_b).
llvm::DenseMap<SILInstruction *, SILInstruction *> ReverseDependencies;
/// Tracks if the pass changed branches.
bool BranchesChanged;
/// Tracks if the pass changed ApplyInsts.
bool CallsChanged;
/// The entry point to the transformation.
void run() override {
BranchesChanged = false;
CallsChanged = false;
SILFunction *F = getFunction();
auto *DA = PM->getAnalysis<PostDominanceAnalysis>();
PDT = DA->get(F);
// If we have a function that consists of nothing but a
// structurally infinite loop like:
// while true {}
// we'll have an empty post dominator tree.
if (!PDT->getRootNode())
assert(Worklist.empty() && LiveValues.empty() && LiveBlocks.empty() &&
ControllingInfoMap.empty() && ReverseDependencies.empty() &&
"Expected to start with empty data structures!");
if (!precomputeControlInfo(*F)) {
if (removeDead(*F)) {
using InvalidationKind = SILAnalysis::InvalidationKind;
unsigned Inv = InvalidationKind::Instructions;
if (CallsChanged) Inv |= (unsigned) InvalidationKind::Calls;
if (BranchesChanged) Inv |= (unsigned) InvalidationKind::Branches;
bool precomputeControlInfo(SILFunction &F);
void markLive(SILFunction &F);
void addReverseDependency(SILInstruction *From, SILInstruction *To);
bool removeDead(SILFunction &F);
void computeLevelNumbers(PostDomTreeNode *Node, unsigned Level);
bool hasInfiniteLoops(SILFunction &F);
void computePredecessorDependence(SILFunction &F);
unsigned computeMinPredecessorLevels(PostDomTreeNode *Node);
void insertControllingInfo(SILBasicBlock *Block, unsigned Level);
void markValueLive(SILNode *V);
void markTerminatorArgsLive(SILBasicBlock *Pred, SILBasicBlock *Succ,
size_t ArgIndex);
void markControllingTerminatorsLive(SILBasicBlock *Block);
void propagateLiveBlockArgument(SILArgument *Arg);
void propagateLiveness(SILInstruction *I);
void collectControllingBlocksInTree(ControllingInfo &QueryInfo,
PostDomTreeNode *Node,
llvm::SmallPtrSetImpl<SILBasicBlock *> &Controlling);
void collectControllingBlocks(SILBasicBlock *Block,
llvm::SmallPtrSetImpl<SILBasicBlock *> &);
SILBasicBlock *nearestUsefulPostDominator(SILBasicBlock *Block);
void replaceBranchWithJump(SILInstruction *Inst, SILBasicBlock *Block);
// Keep track of the fact that V is live and add it to our worklist
// so that we can process the values it depends on.
void DCE::markValueLive(SILNode *V) {
V = V->getRepresentativeSILNodeInObject();
if (LiveValues.count(V) || isa<SILUndef>(V))
LLVM_DEBUG(llvm::dbgs() << "Marking as live:\n");
if (auto *Def = dyn_cast<SILInstruction>(V)) {
// TODO: MultiValueInstruction
assert(isa<SILArgument>(V) &&
"Only expected instructions and arguments!");
auto *Arg = cast<SILArgument>(V);
/// Gets the producing instruction of a cond_fail condition. Currently these
/// are overflow builtins but may be extended to other instructions in the
/// future.
static SILInstruction *getProducer(CondFailInst *CFI) {
// Check for the pattern:
// %1 = builtin "some_operation_with_overflow"
// %2 = tuple_extract %1
// %3 = cond_fail %2
SILValue FailCond = CFI->getOperand();
if (auto *TEI = dyn_cast<TupleExtractInst>(FailCond)) {
if (auto *BI = dyn_cast<BuiltinInst>(TEI->getOperand())) {
return BI;
return nullptr;
// Determine which instructions from this function we need to keep.
void DCE::markLive(SILFunction &F) {
// Find the initial set of instructions in this function that appear
// to be live in the sense that they are not trivially something we
// can delete by examining only that instruction.
for (auto &BB : F) {
for (auto &I : BB) {
if (auto *CFI = dyn_cast<CondFailInst>(&I)) {
// A cond_fail is only alive if its (identifiable) producer is alive.
if (SILInstruction *Prod = getProducer(CFI)) {
addReverseDependency(Prod, CFI);
} else {
if (auto *FLI = dyn_cast<FixLifetimeInst>(&I)) {
// A fix_lifetime (with a non-address type) is only alive if it's
// definition is alive.
SILValue Op = FLI->getOperand();
auto *OpInst = Op->getDefiningInstruction();
if (OpInst && !Op->getType().isAddress()) {
addReverseDependency(OpInst, FLI);
} else {
if (seemsUseful(&I))
// Now propagate liveness backwards from each instruction in our
// worklist, adding new instructions to the worklist as we discover
// more that we need to keep.
while (!Worklist.empty()) {
auto *I = Worklist.pop_back_val();
// Records a reverse dependency. See DCE::ReverseDependencies.
void DCE::addReverseDependency(SILInstruction *From, SILInstruction *To) {
assert(!ReverseDependencies.lookup(To) &&
"Target of reverse dependency already in map");
SILInstruction *&Target = ReverseDependencies[From];
SILInstruction *ExistingTarget = Target;
Target = To;
if (ExistingTarget) {
// Create a single linked chain if From has multiple targets.
ReverseDependencies[To] = ExistingTarget;
// Mark as live the terminator argument at index ArgIndex in Pred that
// targets Succ.
void DCE::markTerminatorArgsLive(SILBasicBlock *Pred,
SILBasicBlock *Succ,
size_t ArgIndex) {
auto *Term = Pred->getTerminator();
// If the arguments are live, we need to keep the terminator that
// delivers those arguments.
switch (Term->getTermKind()) {
case TermKind::ReturnInst:
case TermKind::ThrowInst:
case TermKind::UnwindInst:
case TermKind::YieldInst:
case TermKind::UnreachableInst:
case TermKind::SwitchValueInst:
case TermKind::SwitchEnumAddrInst:
case TermKind::CheckedCastAddrBranchInst:
llvm_unreachable("Unexpected argument for terminator kind!");
case TermKind::DynamicMethodBranchInst:
case TermKind::SwitchEnumInst:
case TermKind::CheckedCastBranchInst:
case TermKind::CheckedCastValueBranchInst:
assert(ArgIndex == 0 && "Expected a single argument!");
// We do not need to do anything with these. If the resulting
// argument is used at the destination these terminators will end
// up live, and then our normal liveness propagation will mark the
// single operand of these instructions as live.
case TermKind::BranchInst:
case TermKind::CondBranchInst: {
auto *CondBr = cast<CondBranchInst>(Term);
if (CondBr->getTrueBB() == Succ) {
auto TrueArgs = CondBr->getTrueArgs();
if (CondBr->getFalseBB() == Succ) {
auto FalseArgs = CondBr->getFalseArgs();
case TermKind::TryApplyInst: {
assert(ArgIndex == 0 && "Expect a single argument!");
// Propagate liveness back from Arg to the terminator arguments that
// supply its value.
void DCE::propagateLiveBlockArgument(SILArgument *Arg) {
// Conceptually, the dependency from a debug instruction to its definition
// is in reverse direction: Only if its definition (the Arg) is alive, also
// the debug_value instruction is alive.
for (Operand *DU : getDebugUses(Arg))
if (isa<SILFunctionArgument>(Arg))
auto *Block = Arg->getParent();
auto ArgIndex = Arg->getIndex();
for (auto Pred : Block->getPredecessorBlocks())
markTerminatorArgsLive(Pred, Block, ArgIndex);
// Given an instruction which is considered live, propagate that liveness
// back to the instructions that produce values it consumes.
void DCE::propagateLiveness(SILInstruction *I) {
if (!isa<TermInst>(I)) {
for (auto &O : I->getAllOperands())
// Conceptually, the dependency from a debug instruction to its definition
// is in reverse direction: Only if its definition is alive, also the
// debug_value instruction is alive.
for (auto result : I->getResults())
for (Operand *DU : getDebugUses(result))
// Handle all other reverse-dependency instructions, like cond_fail and
// fix_lifetime. Only if the definition is alive, the user itself is alive.
if (SILInstruction *DepInst = ReverseDependencies.lookup(I)) {
switch (cast<TermInst>(I)->getTermKind()) {
case TermKind::BranchInst:
case TermKind::UnreachableInst:
case TermKind::UnwindInst:
case TermKind::ReturnInst:
case TermKind::ThrowInst:
case TermKind::CondBranchInst:
case TermKind::SwitchEnumInst:
case TermKind::SwitchEnumAddrInst:
case TermKind::DynamicMethodBranchInst:
case TermKind::CheckedCastBranchInst:
case TermKind::CheckedCastValueBranchInst:
case TermKind::TryApplyInst:
case TermKind::SwitchValueInst:
case TermKind::YieldInst:
for (auto &O : I->getAllOperands())
case TermKind::CheckedCastAddrBranchInst:
llvm_unreachable("corrupt instruction!");
SILBasicBlock *DCE::nearestUsefulPostDominator(SILBasicBlock *Block) {
// Find the nearest post-dominator that has useful instructions.
auto *PostDomNode = PDT->getNode(Block)->getIDom();
while (PostDomNode && !LiveBlocks.count(PostDomNode->getBlock()))
PostDomNode = PostDomNode->getIDom();
if (PostDomNode)
return PostDomNode->getBlock();
return nullptr;
// Replace the given conditional branching instruction with a plain
// jump (aka unconditional branch) to the destination block.
void DCE::replaceBranchWithJump(SILInstruction *Inst, SILBasicBlock *Block) {
assert(Block && "Expected a destination block!");
assert((isa<CondBranchInst>(Inst) ||
isa<SwitchValueInst>(Inst) ||
isa<SwitchEnumInst>(Inst) ||
isa<SwitchEnumAddrInst>(Inst) ||
isa<DynamicMethodBranchInst>(Inst) ||
isa<CheckedCastBranchInst>(Inst)) &&
"Unexpected dead terminator kind!");
SILInstruction *Branch;
if (!Block->args_empty()) {
std::vector<SILValue> Args;
auto E = Block->args_end();
for (auto A = Block->args_begin(); A != E; ++A) {
assert(!LiveValues.count(*A) && "Unexpected live block argument!");
Args.push_back(SILUndef::get((*A)->getType(), (*A)->getModule()));
Branch =
SILBuilderWithScope(Inst).createBranch(Inst->getLoc(), Block, Args);
} else {
Branch = SILBuilderWithScope(Inst).createBranch(Inst->getLoc(), Block);
LLVM_DEBUG(llvm::dbgs() << "Inserted unconditional branch:\n");
// Remove the instructions that are not potentially useful.
bool DCE::removeDead(SILFunction &F) {
bool Changed = false;
for (auto &BB : F) {
for (auto I = BB.args_begin(), E = BB.args_end(); I != E;) {
auto Inst = *I++;
if (LiveValues.count(Inst))
LLVM_DEBUG(llvm::dbgs() << "Removing dead argument:\n");
Changed = true;
for (auto I = BB.begin(), E = BB.end(); I != E; ) {
auto *Inst = &*I;
if (LiveValues.count(Inst) || isa<BranchInst>(Inst))
// We want to replace dead terminators with unconditional branches to
// the nearest post-dominator that has useful instructions.
if (isa<TermInst>(Inst)) {
SILBasicBlock *postDom = nearestUsefulPostDominator(Inst->getParent());
if (!postDom)
replaceBranchWithJump(Inst, postDom);
BranchesChanged = true;
Changed = true;
LLVM_DEBUG(llvm::dbgs() << "Removing dead instruction:\n");
if (isa<ApplyInst>(Inst))
CallsChanged = true;
Changed = true;
return Changed;
// Precompute some information from the post-dominator tree to aid us
// in determining control dependence without generating a complete
// control dependence graph. Inspired by:
// Optimal control dependence and the Roman chariots problem
// TOPLAS, v19, issue 3, 1997
// For each node in the post-dominator tree we will compute:
// -- A level number.
// -- The list of immediate predecessors that this block is
// control-dependent on along with the level number in the
// post-dominator tree of each of those predecessors.
// -- The lowest level number of any predecessor below the given node
// in the post-dominator tree. This will be used to exit early in
// later control-dependence queries.
// Returns true upon success, false if nodes that are not present in the
// post-dominator tree are detected.
bool DCE::precomputeControlInfo(SILFunction &F) {
computeLevelNumbers(PDT->getRootNode(), 0);
if (hasInfiniteLoops(F))
return false;
return true;
void DCE::insertControllingInfo(SILBasicBlock *Block, unsigned Level) {
assert(ControllingInfoMap.find(Block) == ControllingInfoMap.end() &&
"Unexpected map entry for node!");
ControllingInfo Info;
Info.Block = Block;
Info.Level = Level;
Info.MinTreePredLevel = -1;
ControllingInfoMap[Block] = Info;
// Assign a level number to each node in the post-dominator tree, and
void DCE::computeLevelNumbers(PostDomTreeNode *Node, unsigned Level) {
insertControllingInfo(Node->getBlock(), Level);
for (auto Child = Node->begin(), End = Node->end(); Child != End; ++Child)
computeLevelNumbers(*Child, Level + 1);
// Structurally infinite loops like:
// bb1:
// br bb1
// are not present in the post-dominator tree. Their presence
// requires significant modifications to the way the rest of the
// algorithm works. They should be rare, so for now we'll do the most
// conservative thing and completely bail out, doing no dead code
// elimination. Note this will also hit for unreachable code, but
// presumably we'll run DCE at some point after removing unreachable
// code.
bool DCE::hasInfiniteLoops(SILFunction &F) {
for (auto &BB : F)
if (ControllingInfoMap.find(&BB) == ControllingInfoMap.end())
return true;
return false;
// For each block, create a list of the direct predecessors that the
// block is control-dependent on. With each predecessor, also keep the
// level number of the predecessor in the post-dominator tree.
void DCE::computePredecessorDependence(SILFunction &F) {
for (auto &BB : F) {
assert(ControllingInfoMap.find(&BB) != ControllingInfoMap.end()
&& "Expected to already have a map entry for block!");
for (auto Pred : BB.getPredecessorBlocks())
if (!PDT->properlyDominates(&BB, Pred)) {
assert(ControllingInfoMap.find(Pred) != ControllingInfoMap.end() &&
"Expected to already have a map entry for block!");
auto PredLevel = ControllingInfoMap[Pred].Level;
auto PredInfo = std::make_pair(Pred, PredLevel);
auto &MapElement = ControllingInfoMap[&BB];
// Return the minimum post-dominator tree level of any of the
// direct controlling predecessors of this node or any child.
unsigned DCE::computeMinPredecessorLevels(PostDomTreeNode *Node) {
unsigned Min = -1;
auto *Block = Node->getBlock();
assert(ControllingInfoMap.find(Block) != ControllingInfoMap.end() &&
"Expected to have map entry for node!");
auto &MapElement = ControllingInfoMap[Block];
for (auto &PredInfo : MapElement.ControllingPreds)
Min = std::min(Min, PredInfo.second);
for (auto Child = Node->begin(), End = Node->end(); Child != End; ++Child)
Min = std::min(Min, computeMinPredecessorLevels(*Child));
MapElement.MinTreePredLevel = Min;
return Min;
void DCE::collectControllingBlocksInTree(ControllingInfo &QueryInfo,
PostDomTreeNode *Node,
llvm::SmallPtrSetImpl<SILBasicBlock *> &Controlling) {
auto *Block = Node->getBlock();
assert(ControllingInfoMap.find(Block) != ControllingInfoMap.end() &&
"Expected to have map entry for node!");
auto &MapEntry = ControllingInfoMap[Block];
if (MapEntry.MinTreePredLevel > QueryInfo.Level)
for (auto &PredInfo : MapEntry.ControllingPreds)
if (PredInfo.second <= QueryInfo.Level) {
QueryInfo.Block) &&
"Expected predecessor's post-dominator to post-dominate node.");
for (auto Child = Node->begin(), End = Node->end(); Child != End; ++Child)
collectControllingBlocksInTree(QueryInfo, (*Child), Controlling);
// Walk the post-dominator tree from the query block down, building
// the set of blocks that the given block is control-dependent on. To
// determine control dependence we use some precomputed information
// about the direct predecessors that control each block, along with
// the level numbers in the post-dominator tree of those controlling
// predecessors. We can use the latter to terminate the walk down the
// dominator tree early.
void DCE::collectControllingBlocks(SILBasicBlock *Block,
llvm::SmallPtrSetImpl<SILBasicBlock *> &Controlling) {
// First add the blocks that QueryNode is directly control-dependent on.
assert(ControllingInfoMap.find(Block) != ControllingInfoMap.end() &&
"Expected map entry for node!");
auto &MapEntry = ControllingInfoMap[Block];
// Now walk the children looking for nodes that have controlling
// predecessors that have the same or lower level number in the
// post-dominator tree.
collectControllingBlocksInTree(MapEntry, PDT->getNode(Block), Controlling);
void DCE::markControllingTerminatorsLive(SILBasicBlock *Block) {
if (LiveBlocks.count(Block))
llvm::SmallPtrSet<SILBasicBlock *, 4> ControllingBlocks;
collectControllingBlocks(Block, ControllingBlocks);
for (auto BB : ControllingBlocks)
} // end anonymous namespace
SILTransform *swift::createDCE() {
return new DCE();