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swift-mirror/lib/SILPasses/SimplifyCFG.cpp
Erik Eckstein d44da6ecf0 Enable jump threading to combine two enum instructions. 
This is needed to simplify enum construction from integer raw values.



Swift SVN r23302
2014-11-13 10:25:12 +00:00

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//===--- SimplifyCFG.cpp - Clean up the SIL CFG ---------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2015 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See http://swift.org/LICENSE.txt for license information
// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "sil-simplify-cfg"
#include "swift/SILPasses/Passes.h"
#include "swift/SIL/Dominance.h"
#include "swift/SIL/SILArgument.h"
#include "swift/SIL/SILCloner.h"
#include "swift/SIL/SILModule.h"
#include "swift/SIL/SILUndef.h"
#include "swift/SILAnalysis/DominanceAnalysis.h"
#include "swift/SILAnalysis/SimplifyInstruction.h"
#include "swift/SILPasses/Transforms.h"
#include "swift/SILPasses/Utils/CFG.h"
#include "swift/SILPasses/Utils/Local.h"
#include "swift/SILPasses/Utils/SILInliner.h"
#include "swift/SILPasses/Utils/SILSSAUpdater.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/Debug.h"
using namespace swift;
STATISTIC(NumBlocksDeleted, "Number of unreachable blocks removed");
STATISTIC(NumBlocksMerged, "Number of blocks merged together");
STATISTIC(NumJumpThreads, "Number of jumps threaded");
STATISTIC(NumConstantFolded, "Number of terminators constant folded");
STATISTIC(NumDeadArguments, "Number of unused arguments removed");
//===----------------------------------------------------------------------===//
// alloc_box Promotion
//===----------------------------------------------------------------------===//
namespace {
class SimplifyCFG {
SILFunction &Fn;
SILPassManager *PM;
// WorklistList is the actual list that we iterate over (for determinism).
// Slots may be null, which should be ignored.
SmallVector<SILBasicBlock*, 32> WorklistList;
// WorklistMap keeps track of which slot a BB is in, allowing efficient
// containment query, and allows efficient removal.
llvm::SmallDenseMap<SILBasicBlock*, unsigned, 32> WorklistMap;
// Keep track of loop headers - we don't want to jump-thread through them.
SmallPtrSet<SILBasicBlock *, 32> LoopHeaders;
// Dominance and post-dominance info for the current function
DominanceInfo *DT;
PostDominanceInfo *PDT;
public:
SimplifyCFG(SILFunction &Fn, SILPassManager *PM) :
Fn(Fn), PM(PM) {}
bool run();
private:
/// popWorklist - Return the next basic block to look at, or null if the
/// worklist is empty. This handles skipping over null entries in the
/// worklist.
SILBasicBlock *popWorklist() {
while (!WorklistList.empty())
if (auto *BB = WorklistList.pop_back_val()) {
WorklistMap.erase(BB);
return BB;
}
return nullptr;
}
/// addToWorklist - Add the specified block to the work list if it isn't
/// already present.
void addToWorklist(SILBasicBlock *BB) {
unsigned &Entry = WorklistMap[BB];
if (Entry != 0) return;
WorklistList.push_back(BB);
Entry = WorklistList.size();
}
/// removeFromWorklist - Remove the specified block from the worklist if
/// present.
void removeFromWorklist(SILBasicBlock *BB) {
assert(BB && "Cannot add null pointer to the worklist");
auto It = WorklistMap.find(BB);
if (It == WorklistMap.end()) return;
// If the BB is in the worklist, null out its entry.
if (It->second) {
assert(WorklistList[It->second-1] == BB && "Consistency error");
WorklistList[It->second-1] = nullptr;
}
// Remove it from the map as well.
WorklistMap.erase(It);
if (LoopHeaders.count(BB))
LoopHeaders.erase(BB);
}
bool simplifyBlocks();
void canonicalizeSwitchEnums();
bool dominatorBasedSimplify(DominanceInfo *DT);
/// \brief Remove the basic block if it has no predecessors. Returns true
/// If the block was removed.
bool removeIfDead(SILBasicBlock *BB);
bool tryJumpThreading(BranchInst *BI);
bool simplifyAfterDroppingPredecessor(SILBasicBlock *BB);
bool simplifyBranchOperands(OperandValueArrayRef Operands);
bool simplifyBranchBlock(BranchInst *BI);
bool simplifyCondBrBlock(CondBranchInst *BI);
bool simplifySwitchEnumUnreachableBlocks(SwitchEnumInst *SEI);
bool simplifySwitchEnumBlock(SwitchEnumInst *SEI);
bool simplifyUnreachableBlock(UnreachableInst *UI);
bool simplifyCheckedCastBranchBlock(CheckedCastBranchInst *CCBI);
bool simplifyArgument(SILBasicBlock *BB, unsigned i);
bool simplifyArgs(SILBasicBlock *BB);
void findLoopHeaders();
};
class RemoveUnreachable {
SILFunction &Fn;
llvm::SmallSet<SILBasicBlock *, 8> Visited;
public:
RemoveUnreachable(SILFunction &Fn) : Fn(Fn) { }
void visit(SILBasicBlock *BB);
bool run();
};
} // end anonymous namespace
static bool isConditional(TermInst *I) {
switch (I->getKind()) {
case ValueKind::CondBranchInst:
case ValueKind::SwitchValueInst:
case ValueKind::SwitchEnumInst:
case ValueKind::SwitchEnumAddrInst:
case ValueKind::CheckedCastBranchInst:
return true;
default:
return false;
}
}
// Replace a SwitchEnumInst with an unconditional branch based on the
// assertion that it will select a particular element.
static void simplifySwitchEnumInst(SwitchEnumInst *SEI,
EnumElementDecl *Element,
SILBasicBlock *BB) {
auto *Dest = SEI->getCaseDestination(Element);
if (Dest->bbarg_empty()) {
SILBuilderWithScope<1>(SEI).createBranch(SEI->getLoc(), Dest);
SEI->eraseFromParent();
return;
}
SILValue Arg;
if (BB->bbarg_empty()) {
auto &Mod = SEI->getModule();
auto OpndTy = SEI->getOperand()->getType(0);
auto Ty = OpndTy.getEnumElementType(Element, Mod);
auto *UED = SILBuilderWithScope<1>(SEI)
.createUncheckedEnumData(SEI->getLoc(), SEI->getOperand(), Element, Ty);
Arg = SILValue(UED);
} else {
Arg = BB->getBBArg(0);
}
ArrayRef<SILValue> Args = { Arg };
SILBuilderWithScope<1>(SEI).createBranch(SEI->getLoc(), Dest, Args);
SEI->eraseFromParent();
}
static void simplifyCheckedCastBranchInst(CheckedCastBranchInst *CCBI,
bool SuccessTaken,
SILBasicBlock *DomBB) {
if (SuccessTaken)
SILBuilderWithScope<1>(CCBI).createBranch(CCBI->getLoc(),
CCBI->getSuccessBB(),
SILValue(DomBB->getBBArg(0)));
else
SILBuilderWithScope<1>(CCBI).createBranch(CCBI->getLoc(),
CCBI->getFailureBB());
CCBI->eraseFromParent();
}
/// Returns true if BB is the basic block jumped to when CondBr's condition
/// evaluates to true.
static bool getBranchTaken(CondBranchInst *CondBr, SILBasicBlock *BB) {
if (CondBr->getTrueBB() == BB)
return true;
else
return false;
}
static EnumElementDecl *
getOtherElementOfTwoElementEnum(EnumDecl *E, EnumElementDecl *Element) {
assert(Element && "This should never be null");
if (!Element)
return nullptr;
EnumElementDecl *OtherElt = nullptr;
for (EnumElementDecl *Elt : E->getAllElements()) {
// Skip the case where we find the select_enum element
if (Elt == Element)
continue;
// If we find another element, then we must have more than 2, so bail.
if (OtherElt)
return nullptr;
OtherElt = Elt;
}
return OtherElt;
}
/// If PredTerm is a (cond_br (select_enum)) or a (switch_enum), return the decl
/// that will yield DomBB.
static EnumElementDecl *
getEnumEltTaken(TermInst *PredTerm, SILBasicBlock *DomBB) {
// First check if we have a (cond_br (select_enum)).
if (auto *CBI = dyn_cast<CondBranchInst>(PredTerm)) {
auto *SEI = dyn_cast<SelectEnumInst>(CBI->getCondition());
if (!SEI)
return nullptr;
// Try to find a single literal "true" case.
// TODO: More general conditions in which we can relate the BB to a single
// case, such as when there's a single literal "false" case.
EnumElementDecl *TrueElement = SEI->getSingleTrueElement();
if (!TrueElement)
return nullptr;
// If DomBB is the taken branch, we know that the EnumElementDecl is the one
// checked for by enum is tag. Return it.
if (getBranchTaken(CBI, DomBB)) {
return TrueElement;
}
// Ok, DomBB is not the taken branch. If we have an enum with only two
// cases, we can still infer the other case for the current branch.
EnumDecl *E = SEI->getEnumOperand().getType().getEnumOrBoundGenericEnum();
// This will return nullptr if we have more than two cases in our decl.
return getOtherElementOfTwoElementEnum(E, TrueElement);
}
auto *SWEI = dyn_cast<SwitchEnumInst>(PredTerm);
if (!SWEI)
return nullptr;
return SWEI->getUniqueCaseForDestination(DomBB);
}
static void simplifyCondBranchInst(CondBranchInst *BI, bool BranchTaken) {
auto LiveArgs = BranchTaken ? BI->getTrueArgs(): BI->getFalseArgs();
auto *LiveBlock = BranchTaken ? BI->getTrueBB() : BI->getFalseBB();
SILBuilderWithScope<1>(BI).createBranch(BI->getLoc(), LiveBlock, LiveArgs);
BI->dropAllReferences();
BI->eraseFromParent();
}
/// Returns true if C1, C2 represent equivalent conditions in the
/// sense that each is eventually based on the same value.
static bool areEquivalentConditions(SILValue C1, SILValue C2) {
if (auto *SEI = dyn_cast<SelectEnumInst>(C1))
C1 = SEI->getEnumOperand().stripCasts();
if (auto *SEI = dyn_cast<SelectEnumInst>(C2))
C2 = SEI->getEnumOperand().stripCasts();
return C1 == C2;
}
static bool trySimplifyConditional(TermInst *Term, DominanceInfo *DT) {
assert(isConditional(Term) && "Expected conditional terminator!");
auto *BB = Term->getParent();
auto Condition = Term->getOperand(0);
auto Kind = Term->getKind();
for (auto *Node = DT->getNode(BB); Node; Node = Node->getIDom()) {
auto *DomBB = Node->getBlock();
auto *Pred = DomBB->getSinglePredecessor();
if (!Pred)
continue;
// We assume that our predecessor terminator has operands like Term since
// otherwise it can not be "conditional".
auto *PredTerm = Pred->getTerminator();
if (!PredTerm->getNumOperands() ||
!areEquivalentConditions(PredTerm->getOperand(0), Condition))
continue;
// Okay, DomBB dominates Term, has a single predecessor, and that
// predecessor conditionally branches on the same condition. So we
// know that DomBB are control-dependent on the edge that takes us
// from Pred to DomBB. Since the terminator kind and condition are
// the same, we can use the knowledge of which edge gets us to
// Inst to optimize Inst.
switch (Kind) {
case ValueKind::SwitchEnumInst: {
if (EnumElementDecl *EltDecl = getEnumEltTaken(PredTerm, DomBB)) {
simplifySwitchEnumInst(cast<SwitchEnumInst>(Term), EltDecl, DomBB);
return true;
}
// FIXME: We could also simplify things in some cases when we
// reach this switch_enum_inst from another
// switch_enum_inst that is branching on the same value
// and taking the default path.
continue;
}
case ValueKind::CondBranchInst: {
auto *CBI = cast<CondBranchInst>(Term);
// If this CBI has an
if (auto *SEI = dyn_cast<SelectEnumInst>(CBI->getCondition())) {
if (auto TrueElement = SEI->getSingleTrueElement()) {
if (EnumElementDecl *Element = getEnumEltTaken(PredTerm, DomBB)) {
simplifyCondBranchInst(CBI, Element == TrueElement);
return true;
}
}
}
// Ok, we failed to determine an enum element decl.
auto *CondBrInst = dyn_cast<CondBranchInst>(PredTerm);
if (!CondBrInst)
continue;
bool BranchTaken = getBranchTaken(CondBrInst, DomBB);
simplifyCondBranchInst(CBI, BranchTaken);
return true;
}
case ValueKind::SwitchValueInst:
case ValueKind::SwitchEnumAddrInst:
// FIXME: Handle these.
return false;
case ValueKind::CheckedCastBranchInst: {
// We need to verify that the result type is the same in the
// dominating checked_cast_br.
auto *PredCCBI = dyn_cast<CheckedCastBranchInst>(PredTerm);
if (!PredCCBI)
continue;
auto *CCBI = cast<CheckedCastBranchInst>(Term);
if (PredCCBI->getCastType() != CCBI->getCastType())
continue;
assert((DomBB == PredCCBI->getSuccessBB() ||
DomBB == PredCCBI->getFailureBB()) &&
"Dominating block is not a successor of predecessor checked_cast_br");
simplifyCheckedCastBranchInst(CCBI, DomBB == PredCCBI->getSuccessBB(),
DomBB);
return true;
}
default:
llvm_unreachable("Should only see conditional terminators here!");
}
}
return false;
}
// Simplifications that walk the dominator tree to prove redundancy in
// conditional branching.
bool SimplifyCFG::dominatorBasedSimplify(DominanceInfo *DT) {
bool Changed = false;
for (auto &BB : Fn) {
if (isConditional(BB.getTerminator()))
Changed |= trySimplifyConditional(BB.getTerminator(), DT);
// Simplify the block argument list.
Changed |= simplifyArgs(&BB);
}
return Changed;
}
// If BB is trivially unreachable, remove it from the worklist, add its
// successors to the worklist, and then remove the block.
bool SimplifyCFG::removeIfDead(SILBasicBlock *BB) {
if (!BB->pred_empty() || BB == &*Fn.begin())
return false;
removeFromWorklist(BB);
// Add successor blocks to the worklist since their predecessor list is about
// to change.
for (auto &S : BB->getSuccs())
addToWorklist(S);
removeDeadBlock(BB);
++NumBlocksDeleted;
return true;
}
/// This is called when a predecessor of a block is dropped, to simplify the
/// block and add it to the worklist.
bool SimplifyCFG::simplifyAfterDroppingPredecessor(SILBasicBlock *BB) {
// TODO: If BB has only one predecessor and has bb args, fold them away, then
// use instsimplify on all the users of those values - even ones outside that
// block.
// Make sure that DestBB is in the worklist, as well as its remaining
// predecessors, since they may not be able to be simplified.
addToWorklist(BB);
for (auto *P : BB->getPreds())
addToWorklist(P);
return false;
}
/// Return true if there are any users of V outside the specified block.
static bool isUsedOutsideOfBlock(SILValue V, SILBasicBlock *BB) {
for (auto UI : V.getUses())
if (UI->getUser()->getParent() != BB)
return true;
return false;
}
/// couldSimplifyUsers - Check to see if any simplifications are possible if
/// "Val" is substituted for BBArg. If so, return true, if nothing obvious
/// is possible, return false.
static bool couldSimplifyUsers(SILArgument *BBArg, SILValue Val) {
assert(!isa<IntegerLiteralInst>(Val) && !isa<FloatLiteralInst>(Val) &&
"Obvious constants shouldn't reach here");
// If the value being substituted is an enum, check to see if there are any
// switches on it.
auto *EI = dyn_cast<EnumInst>(Val);
if (!EI)
return false;
for (auto UI : BBArg->getUses()) {
auto *User = UI->getUser();
if (isa<SwitchEnumInst>(User) || isa<SelectEnumInst>(User))
return true;
// Also allow enum of enum, which usually can be combined to a single
// instruction. This helps to simplify the creation of an enum from an
// integer raw value.
if (isa<EnumInst>(User))
return true;
}
return false;
}
namespace {
class ThreadingCloner : public SILClonerWithScopes<ThreadingCloner> {
friend class SILVisitor<ThreadingCloner>;
friend class SILCloner<ThreadingCloner>;
SILBasicBlock *FromBB, *DestBB;
public:
// A map of old to new available values.
SmallVector<std::pair<ValueBase *, SILValue>, 16> AvailVals;
ThreadingCloner(BranchInst *BI)
: SILClonerWithScopes(*BI->getFunction()),
FromBB(BI->getDestBB()), DestBB(BI->getParent()) {
// Populate the value map so that uses of the BBArgs in the DestBB are
// replaced with the branch's values.
for (unsigned i = 0, e = BI->getArgs().size(); i != e; ++i) {
ValueMap[FromBB->getBBArg(i)] = BI->getArg(i);
AvailVals.push_back(std::make_pair(FromBB->getBBArg(i), BI->getArg(i)));
}
}
void process(SILInstruction *I) { visit(I); }
SILBasicBlock *remapBasicBlock(SILBasicBlock *BB) { return BB; }
SILValue remapValue(SILValue Value) {
// If this is a use of an instruction in another block, then just use it.
if (auto SI = dyn_cast<SILInstruction>(Value)) {
if (SI->getParent() != FromBB) return Value;
} else if (auto BBArg = dyn_cast<SILArgument>(Value)) {
if (BBArg->getParent() != FromBB) return Value;
} else {
assert(isa<SILUndef>(Value) && "Unexpected Value kind");
return Value;
}
return SILCloner<ThreadingCloner>::remapValue(Value);
}
void postProcess(SILInstruction *Orig, SILInstruction *Cloned) {
DestBB->getInstList().push_back(Cloned);
SILClonerWithScopes<ThreadingCloner>::postProcess(Orig, Cloned);
AvailVals.push_back(std::make_pair(Orig, SILValue(Cloned, 0)));
}
};
} // end anonymous namespace
/// We can not duplicate blocks with AllocStack instructions (they need to be
/// FIFO). Other instructions can be duplicated.
static bool canDuplicateBlock(SILBasicBlock *BB) {
for (auto &I : *BB) {
if (!I.isTriviallyDuplicatable())
return false;
}
return true;
}
/// Check whether we can 'thread' through the switch_enum instruction by
/// duplicating the switch_enum block into SrcBB.
static bool isThreadableSwitchEnumInst(SwitchEnumInst *SEI,
SILBasicBlock *SrcBB, EnumInst *&E0,
EnumInst *&E1) {
auto SEIBB = SEI->getParent();
auto PIt = SEIBB->pred_begin();
auto PEnd = SEIBB->pred_end();
// Recognize a switch_enum preceeded by two direct branch blocks that carry
// the switch_enum operand's value as EnumInsts.
if(std::distance(PIt, PEnd) != 2)
return false;
auto Arg = dyn_cast<SILArgument>(SEI->getOperand());
if (!Arg)
return false;
if (Arg->getParent() != SEIBB)
return false;
// We must not duplicate alloc_stack, dealloc_stack.
if (!canDuplicateBlock(SEIBB))
return false;
auto Idx = Arg->getIndex();
auto IncomingBr0 = dyn_cast<BranchInst>(((*PIt))->getTerminator());
++PIt;
auto IncomingBr1 = dyn_cast<BranchInst>((*PIt)->getTerminator());
// Make sure that we don't have an incoming critical edge.
if (!IncomingBr0 || !IncomingBr1)
return false;
// We cannonicalize to IncomingBr0 to be from the basic block we clone
// into.
if (IncomingBr1->getParent() == SrcBB)
std::swap(IncomingBr0, IncomingBr1);
assert(IncomingBr0->getArgs().size() == SEIBB->getNumBBArg());
assert(IncomingBr1->getArgs().size() == SEIBB->getNumBBArg());
// Make sure that both predecessors arguments are an EnumInst so that we can
// forward the branch.
E0 = dyn_cast<EnumInst>(IncomingBr0->getArg(Idx));
E1 = dyn_cast<EnumInst>(IncomingBr1->getArg(Idx));
if (!E0 || !E1)
return false;
if (E0->getParent() != IncomingBr0->getParent() ||
E1->getParent() != IncomingBr1->getParent())
return false;
// We also need to check for the absence of payload uses. we are not handling
// them.
auto SwitchDestBB0 = SEI->getCaseDestination(E0->getElement());
auto SwitchDestBB1 = SEI->getCaseDestination(E1->getElement());
return SwitchDestBB0->getNumBBArg() == 0 && SwitchDestBB1->getNumBBArg() == 0;
}
void SimplifyCFG::findLoopHeaders() {
/// Find back edges in the CFG. This performs a dfs search and identifies
/// back edges as edges going to an ancestor in the dfs search. If a basic
/// block is the target of such a back edge we will identify it as a header.
LoopHeaders.clear();
SmallPtrSet<SILBasicBlock *, 16> Visited;
SmallPtrSet<SILBasicBlock *, 16> InDFSStack;
SmallVector<std::pair<SILBasicBlock *, SILBasicBlock::succ_iterator>, 16>
DFSStack;
auto EntryBB = &Fn.front();
DFSStack.push_back(std::make_pair(EntryBB, EntryBB->succ_begin()));
Visited.insert(EntryBB);
InDFSStack.insert(EntryBB);
while (!DFSStack.empty()) {
auto &D = DFSStack.back();
// No successors.
if (D.second == D.first->succ_end()) {
// Retreat the dfs search.
DFSStack.pop_back();
InDFSStack.erase(D.first);
} else {
// Visit the next successor.
SILBasicBlock *NextSucc = *(D.second);
if (Visited.insert(NextSucc)) {
InDFSStack.insert(NextSucc);
DFSStack.push_back(std::make_pair(NextSucc, NextSucc->succ_begin()));
} else if (InDFSStack.count(NextSucc)) {
// We have already visited this node and it is in our dfs search. This
// is a back-edge.
LoopHeaders.insert(NextSucc);
}
++D.second;
}
}
}
/// Helper function to perform SSA updates in case of jump threading.
static void updateSSAAfterCloning(ThreadingCloner &Cloner,
SILBasicBlock *SrcBB,
SILBasicBlock *DestBB) {
// We are updating SSA form. This means we need to be able to insert phi
// nodes. To make sure we can do this split all critical edges from
// instructions that don't support block arguments.
splitAllCriticalEdges(*DestBB->getParent(), true, nullptr, nullptr);
SILSSAUpdater SSAUp;
for (auto AvailValPair : Cloner.AvailVals) {
ValueBase *Inst = AvailValPair.first;
if (Inst->use_empty())
continue;
for (unsigned i = 0, e = Inst->getNumTypes(); i != e; ++i) {
// Get the result index for the cloned instruction. This is going to be
// the result index stored in the available value for arguments (we look
// through the phi node) and the same index as the original value
// otherwise.
unsigned ResIdx = i;
if (isa<SILArgument>(Inst))
ResIdx = AvailValPair.second.getResultNumber();
SILValue Res(Inst, i);
SILValue NewRes(AvailValPair.second.getDef(), ResIdx);
SmallVector<UseWrapper, 16> UseList;
// Collect the uses of the value.
for (auto Use : Res.getUses())
UseList.push_back(UseWrapper(Use));
SSAUp.Initialize(Res.getType());
SSAUp.AddAvailableValue(DestBB, Res);
SSAUp.AddAvailableValue(SrcBB, NewRes);
if (UseList.empty())
continue;
// Update all the uses.
for (auto U : UseList) {
Operand *Use = U;
SILInstruction *User = Use->getUser();
assert(User && "Missing user");
// Ignore uses in the same basic block.
if (User->getParent() == DestBB)
continue;
SSAUp.RewriteUse(*Use);
}
}
}
}
/// tryJumpThreading - Check to see if it looks profitable to duplicate the
/// destination of an unconditional jump into the bottom of this block.
bool SimplifyCFG::tryJumpThreading(BranchInst *BI) {
auto *DestBB = BI->getDestBB();
auto *SrcBB = BI->getParent();
// If the destination block ends with a return, we don't want to duplicate it.
// We want to maintain the canonical form of a single return where possible.
if (isa<ReturnInst>(DestBB->getTerminator()))
return false;
bool isThreadableCondBr = isa<CondBranchInst>(DestBB->getTerminator()) &&
canDuplicateBlock(DestBB);
// We can jump thread switch enum instructions. But we need to 'thread' it by
// hand - i.e. we need to replace the switch enum by branches - if we don't do
// so the ssaupdater will fail because we can't form 'phi's with anything
// other than branches and conditional branches because only they support
// arguments :(.
EnumInst *EnumInst0 = nullptr;
EnumInst *EnumInst1 = nullptr;
SwitchEnumInst *SEI = dyn_cast<SwitchEnumInst>(DestBB->getTerminator());
bool isThreadableEnumInst =
SEI && isThreadableSwitchEnumInst(SEI, SrcBB, EnumInst0, EnumInst1);
// This code is intentionally simple, and cannot thread if the BBArgs of the
// destination are used outside the DestBB.
bool HasDestBBDefsUsedOutsideBlock = false;
for (auto Arg : DestBB->getBBArgs())
if ((HasDestBBDefsUsedOutsideBlock |= isUsedOutsideOfBlock(Arg, DestBB)))
if (!isThreadableCondBr && !isThreadableEnumInst)
return false;
// We don't have a great cost model at the SIL level, so we don't want to
// blissly duplicate tons of code with a goal of improved performance (we'll
// leave that to LLVM). However, doing limited code duplication can lead to
// major second order simplifications. Here we only do it if there are
// "constant" arguments to the branch or if we know how to fold something
// given the duplication.
bool WantToThread = false;
for (auto V : BI->getArgs()) {
if (isa<IntegerLiteralInst>(V) || isa<FloatLiteralInst>(V)) {
WantToThread = true;
break;
}
}
if (!WantToThread) {
for (unsigned i = 0, e = BI->getArgs().size(); i != e; ++i)
if (couldSimplifyUsers(DestBB->getBBArg(i), BI->getArg(i))) {
WantToThread = true;
break;
}
}
// If we don't have anything that we can simplify, don't do it.
if (!WantToThread) return false;
// If it looks potentially interesting, decide whether we *can* do the
// operation and whether the block is small enough to be worth duplicating.
unsigned Cost = 0;
for (auto &Inst : DestBB->getInstList()) {
// This is a really trivial cost model, which is only intended as a starting
// point.
if (instructionInlineCost(Inst) != InlineCost::Free)
if (++Cost == 4) return false;
// If there is an instruction in the block that has used outside the block,
// duplicating it would require constructing SSA, which we're not prepared
// to do.
if ((HasDestBBDefsUsedOutsideBlock |=
isUsedOutsideOfBlock(&Inst, DestBB))) {
if (!isThreadableCondBr && !isThreadableEnumInst)
return false;
// We can't build SSA for method values that lower to objc methods.
if (auto *MI = dyn_cast<MethodInst>(&Inst))
if (MI->getMember().isForeign)
return false;
}
}
// Don't jump thread through a potential header - this can produce irreducible
// control flow.
if (!isThreadableEnumInst && LoopHeaders.count(DestBB))
return false;
// Okay, it looks like we want to do this and we can. Duplicate the
// destination block into this one, rewriting uses of the BBArgs to use the
// branch arguments as we go.
ThreadingCloner Cloner(BI);
for (auto &I : *DestBB)
Cloner.process(&I);
// Once all the instructions are copied, we can nuke BI itself. We also add
// this block back to the worklist now that the terminator (likely) can be
// simplified.
addToWorklist(BI->getParent());
BI->eraseFromParent();
// Thread the switch enum instruction.
if (isThreadableEnumInst && HasDestBBDefsUsedOutsideBlock) {
assert(EnumInst0 && EnumInst1 && "Need to have two enum instructions");
// We know that the switch enum is fed by enum instructions along all
// incoming edges.
auto SwitchDestBB0 = SEI->getCaseDestination(EnumInst0->getElement());
auto SwitchDestBB1 = SEI->getCaseDestination(EnumInst1->getElement());
assert(EnumInst0->getParent() == SrcBB);
auto ClonedSEI = SrcBB->getTerminator();
auto &InstList0 = EnumInst0->getParent()->getInstList();
InstList0.insert(InstList0.end(),
BranchInst::create(SEI->getLoc(), SwitchDestBB0,
*SEI->getParent()->getParent()));
auto &InstList1 = SEI->getParent()->getInstList();
InstList1.insert(InstList1.end(),
BranchInst::create(SEI->getLoc(), SwitchDestBB1,
*SEI->getParent()->getParent()));
ClonedSEI->eraseFromParent();
SEI->eraseFromParent();
}
if (HasDestBBDefsUsedOutsideBlock) {
updateSSAAfterCloning(Cloner, SrcBB, DestBB);
}
// We may be able to simplify DestBB now that it has one fewer predecessor.
simplifyAfterDroppingPredecessor(DestBB);
++NumJumpThreads;
return true;
}
/// simplifyBranchOperands - Simplify operands of branches, since it can
/// result in exposing opportunities for CFG simplification.
bool SimplifyCFG::simplifyBranchOperands(OperandValueArrayRef Operands) {
bool Simplified = false;
for (auto O = Operands.begin(), E = Operands.end(); O != E; ++O)
if (auto *I = dyn_cast<SILInstruction>(*O))
if (SILValue Result = simplifyInstruction(I)) {
SILValue(I, 0).replaceAllUsesWith(Result.getDef());
if (isInstructionTriviallyDead(I)) {
I->eraseFromParent();
Simplified = true;
}
}
return Simplified;
}
/// \return True if this basic blocks has a single instruction that is the
/// terminator that jumps to another basic block passing all of the arguments
/// in the original order.
static bool isTrampolineBlock(SILBasicBlock *SBB) {
// Ignore blocks with more than one instruction.
if (SBB->getTerminator() != SBB->begin())
return false;
BranchInst *BI = dyn_cast<BranchInst>(SBB->getTerminator());
if (!BI)
return false;
// Disallow infinite loops.
if (BI->getDestBB() == SBB)
return false;
auto BrArgs = BI->getArgs();
if (BrArgs.size() != SBB->getNumBBArg())
return false;
// Check that the arguments are the same and in the right order.
for (int i = 0, e = SBB->getNumBBArg(); i < e; ++i)
if (BrArgs[i] != SBB->getBBArg(i))
return false;
return true;
}
/// simplifyBranchBlock - Simplify a basic block that ends with an unconditional
/// branch.
bool SimplifyCFG::simplifyBranchBlock(BranchInst *BI) {
// First simplify instructions generating branch operands since that
// can expose CFG simplifications.
bool Simplified = simplifyBranchOperands(BI->getArgs());
auto *BB = BI->getParent(), *DestBB = BI->getDestBB();
// If this block branches to a block with a single predecessor, then
// merge the DestBB into this BB.
if (BB != DestBB && DestBB->getSinglePredecessor()) {
// If there are any BB arguments in the destination, replace them with the
// branch operands, since they must dominate the dest block.
for (unsigned i = 0, e = BI->getArgs().size(); i != e; ++i)
SILValue(DestBB->getBBArg(i)).replaceAllUsesWith(BI->getArg(i));
// Zap BI and move all of the instructions from DestBB into this one.
BI->eraseFromParent();
BB->getInstList().splice(BB->end(), DestBB->getInstList(),
DestBB->begin(), DestBB->end());
// Revisit this block now that we've changed it and remove the DestBB.
addToWorklist(BB);
// This can also expose opportunities in the successors of
// the merged block.
for (auto &Succ : BB->getSuccs())
addToWorklist(Succ);
if (LoopHeaders.count(DestBB))
LoopHeaders.insert(BB);
removeFromWorklist(DestBB);
DestBB->eraseFromParent();
++NumBlocksMerged;
return true;
}
// If the destination block is a simple trampoline (jump to another block)
// then jump directly.
if (isTrampolineBlock(DestBB)) {
BranchInst* Br = dyn_cast<BranchInst>(DestBB->getTerminator());
SILBuilderWithScope<1>(BI).createBranch(BI->getLoc(), Br->getDestBB(),
BI->getArgs());
// Eliminating the trampoline can expose opportuntities to improve the
// new block we branch to.
if (LoopHeaders.count(DestBB))
LoopHeaders.insert(BB);
addToWorklist(Br->getDestBB());
BI->eraseFromParent();
removeIfDead(DestBB);
addToWorklist(BB);
return true;
}
// If this unconditional branch has BBArgs, check to see if duplicating the
// destination would allow it to be simplified. This is a simple form of jump
// threading.
if (!BI->getArgs().empty() &&
tryJumpThreading(BI))
return true;
return Simplified;
}
/// simplifyCondBrBlock - Simplify a basic block that ends with a conditional
/// branch.
bool SimplifyCFG::simplifyCondBrBlock(CondBranchInst *BI) {
// First simplify instructions generating branch operands since that
// can expose CFG simplifications.
simplifyBranchOperands(OperandValueArrayRef(BI->getAllOperands()));
auto *ThisBB = BI->getParent();
// If the condition is an integer literal, we can constant fold the branch.
if (auto *IL = dyn_cast<IntegerLiteralInst>(BI->getCondition())) {
bool isFalse = !IL->getValue();
auto LiveArgs = isFalse ? BI->getFalseArgs() : BI->getTrueArgs();
auto *LiveBlock = isFalse ? BI->getFalseBB() : BI->getTrueBB();
auto *DeadBlock = !isFalse ? BI->getFalseBB() : BI->getTrueBB();
auto *ThisBB = BI->getParent();
SILBuilderWithScope<1>(BI).createBranch(BI->getLoc(), LiveBlock, LiveArgs);
BI->eraseFromParent();
if (IL->use_empty()) IL->eraseFromParent();
addToWorklist(ThisBB);
simplifyAfterDroppingPredecessor(DeadBlock);
addToWorklist(LiveBlock);
++NumConstantFolded;
return true;
}
// If the destination block is a simple trampoline (jump to another block)
// then jump directly.
SILBasicBlock *TrueSide = BI->getTrueBB();
SILBasicBlock *FalseSide = BI->getFalseBB();
if (isTrampolineBlock(TrueSide)) {
BranchInst* Br = cast<BranchInst>(TrueSide->getTerminator());
SILBuilderWithScope<1>(BI)
.createCondBranch(BI->getLoc(), BI->getCondition(),
Br->getDestBB(), BI->getTrueArgs(),
BI->getFalseBB(), BI->getFalseArgs());
BI->eraseFromParent();
if (LoopHeaders.count(TrueSide))
LoopHeaders.insert(ThisBB);
removeIfDead(TrueSide);
addToWorklist(ThisBB);
return true;
}
if (isTrampolineBlock(FalseSide)) {
BranchInst* Br = cast<BranchInst>(FalseSide->getTerminator());
SILBuilderWithScope<1>(BI)
.createCondBranch(BI->getLoc(), BI->getCondition(),
BI->getTrueBB(), BI->getTrueArgs(),
Br->getDestBB(), BI->getFalseArgs());
BI->eraseFromParent();
if (LoopHeaders.count(FalseSide))
LoopHeaders.insert(ThisBB);
removeIfDead(FalseSide);
addToWorklist(ThisBB);
return true;
}
// Simplify cond_br where both sides jump to the same blocks with the same
// args.
TrueSide = BI->getTrueBB();
FalseSide = BI->getFalseBB();
if (TrueSide == FalseSide) {
auto TrueArgs = BI->getTrueArgs();
auto FalseArgs = BI->getFalseArgs();
assert(TrueArgs.size() == FalseArgs.size() && "Invalid args!");
bool SameArgs = true;
for (int i = 0, e = TrueArgs.size(); i < e; i++)
if (TrueArgs[i] != FalseArgs[i]){
SameArgs = false;
break;
}
if (SameArgs) {
SILBuilderWithScope<1>(BI).createBranch(BI->getLoc(), TrueSide, TrueArgs);
BI->eraseFromParent();
addToWorklist(ThisBB);
addToWorklist(TrueSide);
++NumConstantFolded;
return true;
}
}
// If we have a (cond (select_enum)) on a two element enum, always have the
// first case as our checked tag. If we have the second, create a new
// select_enum with the first case and swap our operands. This simplifies
// later dominance based processing.
if (auto *SEI = dyn_cast<SelectEnumInst>(BI->getCondition())) {
EnumDecl *E = SEI->getEnumOperand().getType().getEnumOrBoundGenericEnum();
auto AllElts = E->getAllElements();
auto Iter = AllElts.begin();
EnumElementDecl *FirstElt = *Iter;
if (SEI->getNumCases() >= 1
&& SEI->getCase(0).first != FirstElt) {
++Iter;
if (Iter != AllElts.end() &&
std::next(Iter) == AllElts.end() &&
*Iter == SEI->getCase(0).first) {
EnumElementDecl *SecondElt = *Iter;
SILValue FirstValue;
// SelectEnum must be exhaustive, so the second case must be handled
// either by a case or the default.
if (SEI->getNumCases() >= 2) {
assert(FirstElt == SEI->getCase(1).first
&& "select_enum missing a case?!");
FirstValue = SEI->getCase(1).second;
} else {
FirstValue = SEI->getDefaultResult();
}
std::pair<EnumElementDecl*, SILValue> SwappedCases[2] = {
{FirstElt, SEI->getCase(0).second},
{SecondElt, FirstValue},
};
auto *NewSEI = SILBuilderWithScope<1>(SEI)
.createSelectEnum(SEI->getLoc(),
SEI->getEnumOperand(),
SEI->getType(),
SILValue(),
SwappedCases);
// We only change the condition to be NewEITI instead of all uses since
// EITI may have other uses besides this one that need to be updated.
BI->setCondition(NewSEI);
BI->swapSuccessors();
addToWorklist(BI->getParent());
addToWorklist(TrueSide);
addToWorklist(FalseSide);
return true;
}
}
}
return false;
}
// Does this basic block consist of only an "unreachable" instruction?
static bool isOnlyUnreachable(SILBasicBlock *BB) {
auto *Term = BB->getTerminator();
if (!isa<UnreachableInst>(Term))
return false;
return (&*BB->begin() == BB->getTerminator());
}
/// simplifySwitchEnumUnreachableBlocks - Attempt to replace a
/// switch_enum_inst where all but one block consists of just an
/// "unreachable" with an unchecked_enum_data and branch.
bool SimplifyCFG::simplifySwitchEnumUnreachableBlocks(SwitchEnumInst *SEI) {
auto Count = SEI->getNumCases();
SILBasicBlock *Dest = nullptr;
EnumElementDecl *Element = nullptr;
if (SEI->hasDefault())
if (!isOnlyUnreachable(SEI->getDefaultBB()))
Dest = SEI->getDefaultBB();
for (unsigned i = 0; i < Count; ++i) {
auto EnumCase = SEI->getCase(i);
if (isOnlyUnreachable(EnumCase.second))
continue;
if (Dest)
return false;
assert(!Element && "Did not expect to have an element without a block!");
Element = EnumCase.first;
Dest = EnumCase.second;
}
if (!Dest) {
addToWorklist(SEI->getParent());
SILBuilderWithScope<1>(SEI).createUnreachable(SEI->getLoc());
SEI->eraseFromParent();
return true;
}
if (!Element || !Element->hasArgumentType() || Dest->bbarg_empty()) {
assert(Dest->bbarg_empty() && "Unexpected argument at destination!");
SILBuilderWithScope<1>(SEI).createBranch(SEI->getLoc(), Dest);
addToWorklist(SEI->getParent());
addToWorklist(Dest);
SEI->eraseFromParent();
return true;
}
auto &Mod = SEI->getModule();
auto OpndTy = SEI->getOperand()->getType(0);
auto Ty = OpndTy.getEnumElementType(Element, Mod);
auto *UED = SILBuilderWithScope<1>(SEI)
.createUncheckedEnumData(SEI->getLoc(), SEI->getOperand(), Element, Ty);
assert(Dest->bbarg_size() == 1 && "Expected only one argument!");
ArrayRef<SILValue> Args = { UED };
SILBuilderWithScope<1>(SEI).createBranch(SEI->getLoc(), Dest, Args);
addToWorklist(SEI->getParent());
addToWorklist(Dest);
SEI->eraseFromParent();
return true;
}
/// simplifySwitchEnumBlock - Simplify a basic block that ends with a
/// switch_enum instruction that gets its operand from a an enum
/// instruction.
bool SimplifyCFG::simplifySwitchEnumBlock(SwitchEnumInst *SEI) {
auto *EI = dyn_cast<EnumInst>(SEI->getOperand());
// If the operand is not from an enum, see if this is a case where
// only one destination of the branch has code that does not end
// with unreachable.
if (!EI)
return simplifySwitchEnumUnreachableBlocks(SEI);
auto *LiveBlock = SEI->getCaseDestination(EI->getElement());
auto *ThisBB = SEI->getParent();
bool DroppedLiveBlock = false;
// Copy the successors into a vector, dropping one entry for the liveblock.
SmallVector<SILBasicBlock*, 4> Dests;
for (auto &S : SEI->getSuccessors()) {
if (S == LiveBlock && !DroppedLiveBlock) {
DroppedLiveBlock = true;
continue;
}
Dests.push_back(S);
}
if (EI->hasOperand() && !LiveBlock->bbarg_empty())
SILBuilderWithScope<1>(SEI).createBranch(SEI->getLoc(), LiveBlock,
EI->getOperand());
else
SILBuilderWithScope<1>(SEI).createBranch(SEI->getLoc(), LiveBlock);
SEI->eraseFromParent();
if (EI->use_empty()) EI->eraseFromParent();
addToWorklist(ThisBB);
for (auto B : Dests)
simplifyAfterDroppingPredecessor(B);
addToWorklist(LiveBlock);
++NumConstantFolded;
return true;
}
/// simplifyUnreachableBlock - Simplify blocks ending with unreachable by
/// removing instructions that are safe to delete backwards until we
/// hit an instruction we cannot delete.
bool SimplifyCFG::simplifyUnreachableBlock(UnreachableInst *UI) {
bool Changed = false;
auto BB = UI->getParent();
auto I = std::next(BB->rbegin());
auto End = BB->rend();
SmallVector<SILInstruction *, 8> DeadInstrs;
// Walk backwards deleting instructions that should be safe to delete
// in a block that ends with unreachable.
while (I != End) {
auto MaybeDead = I++;
switch (MaybeDead->getKind()) {
// These technically have side effects, but not ones that matter
// in a block that we shouldn't really reach...
case ValueKind::StrongRetainInst:
case ValueKind::StrongReleaseInst:
case ValueKind::RetainValueInst:
case ValueKind::ReleaseValueInst:
break;
default:
if (MaybeDead->mayHaveSideEffects()) {
if (Changed)
for (auto Dead : DeadInstrs)
Dead->eraseFromParent();
return Changed;
}
}
for (unsigned i = 0, e = MaybeDead->getNumTypes(); i != e; ++i)
if (!SILValue(&*MaybeDead, i).use_empty()) {
auto Undef = SILUndef::get(MaybeDead->getType(i), BB->getModule());
SILValue(&*MaybeDead, i).replaceAllUsesWith(Undef);
}
DeadInstrs.push_back(&*MaybeDead);
Changed = true;
}
// If this block was changed and it now consists of only the unreachable,
// make sure we process its predecessors.
if (Changed) {
for (auto Dead : DeadInstrs)
Dead->eraseFromParent();
if (isOnlyUnreachable(BB))
for (auto *P : BB->getPreds())
addToWorklist(P);
}
return Changed;
}
/// simplifyCheckedCastBranchBlock - Simplify a basic block that ends with a
/// checked_cast_br instruction.
///
/// Check if this BB is reachable only from two basic blocks which end
/// with an unconditional branch and which are reachable from
/// a single basic block that ends with a checked_cast_br with the
/// same condition. If this is the case, we can perform jump-threading.
bool SimplifyCFG::simplifyCheckedCastBranchBlock(CheckedCastBranchInst *CCBI) {
auto *BB = CCBI->getParent();
SmallVector<SILBasicBlock *, 2> Preds;
// It should be an exact checked_cast_br
if (!CCBI->isExact())
return false;
// There should be exactly 2 predecessors.
for (auto Pred : BB->getPreds()) {
Preds.push_back(Pred);
if (Preds.size() > 2)
return false;
}
if (Preds.size() != 2)
return false;
// They should have a single predecessor.
if (!Preds[0]->getSinglePredecessor())
return false;
// They should have the same single predecessor.
if (Preds[0]->getSinglePredecessor() != Preds[1]->getSinglePredecessor())
return false;
// And this predecessor should end with checked_cast_br.
TermInst *PredTI = Preds[0]->getSinglePredecessor()->getTerminator();
// Previous checked_cast_br instruction, whose result is being used
// for jump-threading.
CheckedCastBranchInst *PCCBI = dyn_cast<CheckedCastBranchInst>(PredTI);
if (!PCCBI)
return false;
// Bail if the condition being checked is not exactly the same.
if (PCCBI->isExact() != CCBI->isExact() ||
PCCBI->getCastType() != CCBI->getCastType() ||
PCCBI->getOperand() != CCBI->getOperand())
return false;
// Bail if predecessors do not end with a branch.
for (auto Pred : Preds) {
TermInst *TI = Pred->getTerminator();
auto *BI = dyn_cast<BranchInst>(TI);
if (!BI)
return false;
}
if (!canDuplicateBlock(BB))
return false;
// At this point we know that we can perform a jump-threading.
// Targets of the checked_cast_br instruction being eliminated.
auto *TargetSuccessBB = CCBI->getSuccessBB();
auto *TargetFailureBB = CCBI->getFailureBB();
// Targets of the previous checked_cast_br instruction, whose
// results is being used for jump-threading.
auto *SuccessBB = (PCCBI->getSuccessBB() == Preds[0]) ? Preds[0] : Preds[1];
auto *FailureBB = (PCCBI->getFailureBB() == Preds[0]) ? Preds[0] : Preds[1];
// First process the failure-branch of the PCCBI.
auto *BI = dyn_cast<BranchInst>(FailureBB->getTerminator());
SILBuilderWithScope<2> Builder(CCBI);
SmallVector<SILValue, 8> FailureBBArgs;
for (auto Arg : FailureBB->getBBArgs())
FailureBBArgs.push_back(Arg);
// Replace checked_cast_br by an unconditional branch to its failure-branch.
auto *InsertedBI = Builder.createBranch(BI->getLoc(), TargetFailureBB,
FailureBBArgs);
// Remove the original checked_cast_br instruction.
CCBI->eraseFromParent();
// Then process the success-branch of the PCCBI.
BI = dyn_cast<BranchInst>(SuccessBB->getTerminator());
ThreadingCloner Cloner(dyn_cast<BranchInst>(SuccessBB->getTerminator()));
// Clone the target block into the success-branch block.
for (auto &I : *BB) {
if (&I != InsertedBI && !dyn_cast<CheckedCastBranchInst>(&I))
Cloner.process(&I);
}
// We add this block back to the worklist now that the terminator (likely)
// can be simplified.
addToWorklist(SuccessBB);
// Add an unconditional jump at the end of the block.
auto &InsnList = SuccessBB->getInstList();
SmallVector<SILValue, 8> SuccessBBArgs;
for (auto Arg : SuccessBB->getBBArgs())
SuccessBBArgs.push_back(Arg);
InsertedBI = BranchInst::create(BI->getLoc(), TargetSuccessBB, SuccessBBArgs,
*BB->getParent());
InsnList.insert(InsnList.end(), InsertedBI);
// Remove the original branch.
BI->eraseFromParent();
// Now update the SSA form.
updateSSAAfterCloning(Cloner, SuccessBB, BB);
return true;
}
void RemoveUnreachable::visit(SILBasicBlock *BB) {
if (!Visited.insert(BB))
return;
for (auto &Succ : BB->getSuccs())
visit(Succ);
}
bool RemoveUnreachable::run() {
bool Changed = false;
// Clear each time we run so that we can run multiple times.
Visited.clear();
// Visit all blocks reachable from the entry block of the function.
visit(Fn.begin());
// Remove the blocks we never reached.
for (auto It = Fn.begin(), End = Fn.end(); It != End; ) {
auto *BB = &*It++;
if (!Visited.count(BB)) {
removeDeadBlock(BB);
Changed = true;
}
}
return Changed;
}
bool SimplifyCFG::simplifyBlocks() {
bool Changed = false;
// Add all of the blocks to the function.
for (auto &BB : Fn)
addToWorklist(&BB);
// Iteratively simplify while there is still work to do.
while (SILBasicBlock *BB = popWorklist()) {
// If the block is dead, remove it.
if (removeIfDead(BB)) {
Changed = true;
continue;
}
// Otherwise, try to simplify the terminator.
TermInst *TI = BB->getTerminator();
switch (TI->getKind()) {
case ValueKind::BranchInst:
Changed |= simplifyBranchBlock(cast<BranchInst>(TI));
break;
case ValueKind::CondBranchInst:
Changed |= simplifyCondBrBlock(cast<CondBranchInst>(TI));
break;
case ValueKind::SwitchValueInst:
// FIXME: Optimize for known switch values.
break;
case ValueKind::SwitchEnumInst:
Changed |= simplifySwitchEnumBlock(cast<SwitchEnumInst>(TI));
break;
case ValueKind::UnreachableInst:
Changed |= simplifyUnreachableBlock(cast<UnreachableInst>(TI));
break;
case ValueKind::CheckedCastBranchInst:
Changed |= simplifyCheckedCastBranchBlock(cast<CheckedCastBranchInst>(TI));
break;
default:
break;
}
// Simplify the block argument list.
Changed |= simplifyArgs(BB);
}
return Changed;
}
/// Canonicalize all switch_enum and switch_enum_addr instructions.
/// If possible, replace the default with the corresponding unique case.
void SimplifyCFG::canonicalizeSwitchEnums() {
for (auto &BB : Fn) {
TermInst *TI = BB.getTerminator();
SwitchEnumInstBase *SWI = dyn_cast<SwitchEnumInstBase>(TI);
if (!SWI)
continue;
if (!SWI->hasDefault())
continue;
EnumElementDecl *elementDecl = SWI->getUniqueCaseForDefault();
if (!elementDecl)
continue;
// Construct a new instruction by copying all the case entries.
SmallVector<std::pair<EnumElementDecl*, SILBasicBlock*>, 4> CaseBBs;
for (int idx = 0, numIdcs = SWI->getNumCases(); idx < numIdcs; idx++) {
CaseBBs.push_back(SWI->getCase(idx));
}
// Add the default-entry of the original instruction as case-entry.
CaseBBs.push_back(std::make_pair(elementDecl, SWI->getDefaultBB()));
if (SWI->getKind() == ValueKind::SwitchEnumInst) {
SILBuilderWithScope<1>(SWI).createSwitchEnum(SWI->getLoc(),
SWI->getOperand(), nullptr, CaseBBs);
} else {
assert(SWI->getKind() == ValueKind::SwitchEnumAddrInst &&
"unknown switch_enum instruction");
SILBuilderWithScope<1>(SWI).createSwitchEnumAddr(SWI->getLoc(),
SWI->getOperand(), nullptr, CaseBBs);
}
SWI->eraseFromParent();
}
}
bool SimplifyCFG::run() {
RemoveUnreachable RU(Fn);
// First remove any block not reachable from the entry.
bool Changed = RU.run();
// Find the set of loop headers. We don't want to jump-thread through headers.
findLoopHeaders();
DT = nullptr;
PDT = nullptr;
if (simplifyBlocks()) {
// Simplifying other blocks might have resulted in unreachable
// loops.
RU.run();
// Force dominator recomputation below.
PM->invalidateAnalysis(&Fn, SILAnalysis::InvalidationKind::CFG);
Changed = true;
}
// Do simplifications that require the dominator tree to be accurate.
DominanceAnalysis *DA = PM->getAnalysis<DominanceAnalysis>();
DT = DA->getDomInfo(&Fn);
PDT = DA->getPostDomInfo(&Fn);
Changed |= dominatorBasedSimplify(DT);
DT = nullptr;
PDT = nullptr;
// Now attempt to simplify the remaining blocks.
if (simplifyBlocks()) {
// Simplifying other blocks might have resulted in unreachable
// loops.
RU.run();
Changed = true;
}
// Split all critical edges from non cond_br terminators.
Changed |= splitAllCriticalEdges(Fn, true, nullptr, nullptr);
canonicalizeSwitchEnums();
return Changed;
}
static void
removeArgumentFromTerminator(SILBasicBlock *BB, SILBasicBlock *Dest, int idx) {
TermInst *Branch = BB->getTerminator();
SILBuilderWithScope<2> Builder(Branch);
if (CondBranchInst *CBI = dyn_cast<CondBranchInst>(Branch)) {
DEBUG(llvm::dbgs() << "*** Fixing CondBranchInst.\n");
SmallVector<SILValue, 8> TrueArgs;
SmallVector<SILValue, 8> FalseArgs;
for (auto A : CBI->getTrueArgs())
TrueArgs.push_back(A);
for (auto A : CBI->getFalseArgs())
FalseArgs.push_back(A);
if (Dest == CBI->getTrueBB())
TrueArgs.erase(TrueArgs.begin() + idx);
if (Dest == CBI->getFalseBB())
FalseArgs.erase(FalseArgs.begin() + idx);
Builder.createCondBranch(CBI->getLoc(), CBI->getCondition(),
CBI->getTrueBB(), TrueArgs,
CBI->getFalseBB(), FalseArgs);
Branch->eraseFromParent();
return;
}
if (BranchInst *BI = dyn_cast<BranchInst>(Branch)) {
DEBUG(llvm::dbgs() << "*** Fixing BranchInst.\n");
SmallVector<SILValue, 8> Args;
for (auto A : BI->getArgs())
Args.push_back(A);
Args.erase(Args.begin() + idx);
Builder.createBranch(BI->getLoc(), BI->getDestBB(), Args);
Branch->eraseFromParent();
return;
}
llvm_unreachable("unsupported terminator");
}
/// Is an argument from this terminator considered mandatory?
static bool hasMandatoryArgument(TermInst *term) {
// It's more maintainable to just white-list the instructions that
// *do* have mandatory arguments.
return (!isa<BranchInst>(term) && !isa<CondBranchInst>(term));
}
// Get the element of Aggregate corresponding to the one extracted by
// Extract.
static SILValue getInsertedValue(SILInstruction *Aggregate,
SILInstruction *Extract) {
if (auto *Struct = dyn_cast<StructInst>(Aggregate)) {
auto *SEI = cast<StructExtractInst>(Extract);
return Struct->getFieldValue(SEI->getField());
}
auto *Tuple = cast<TupleInst>(Aggregate);
auto *TEI = cast<TupleExtractInst>(Extract);
return Tuple->getElementValue(TEI->getFieldNo());
}
/// Check a diamond-form property of graphs generated by swith_enum
/// instructions, who only produce integer values by each of BBs handling its
/// case tags.
/// In such graphs, switch_enum dominates any blocks processing cases and all
/// those blocks processing different cases are post-dominated by a single
/// basic block consuming those values, where all the paths join.
static bool isDiamondForm(SILBasicBlock *BB, SILBasicBlock *PredBB,
SILBasicBlock *PostBB, DominanceInfo *DT,
PostDominanceInfo *PDT) {
if (PredBB && !DT->dominates(PredBB, BB))
return false;
if (PostBB && !PDT->dominates(PostBB, BB))
return false;
return true;
}
/// Check if a basic blocks consists of a single branch instruction.
static bool isSingleBranchBlock(SILBasicBlock *BB) {
TermInst *TI = BB->getTerminator();
if (!isa<BranchInst>(TI))
return false;
return TI == BB->begin();
}
/// Find a parent SwitchEnumInst of a basic block. If SEI is set, then
/// only return non-nullptr if the found SwitchEnumInst is the same one as SEI.
/// We consider only those predecessor blocks which are post-dominated by
/// PostBB and dominated by SEI. This ensures that we have a diamond-like CFG
/// starting at SEI and ending at PostBB.
static SwitchEnumInst *
getSwitchEnumPred(SILBasicBlock *BB, SwitchEnumInst *SEI, SILBasicBlock *PostBB,
SmallPtrSet<SILBasicBlock *, 8> &Blocks,
SmallPtrSet<SILBasicBlock *, 8> Visited, DominanceInfo *DT,
PostDominanceInfo *PDT) {
if (BB->pred_empty())
return nullptr;
// Any BB can be visited only once.
if (Visited.count(BB))
return nullptr;
// Remember that this BB was seen already.
Visited.insert(BB);
// Only consider blocks which are post-dominated by PostBB and
// dominated by SEI.
if (!isDiamondForm(BB, (SEI) ? SEI->getParent() : nullptr, PostBB, DT, PDT))
return nullptr;
// Each BB on the path should have only a single branch instruction.
// The only exception is a BB which has a BB ending with switch_enum
// as its single predecessor. Such a block may have an integer_literal
// instruction before the branch.
for (auto PredBB : BB->getPreds()) {
SwitchEnumInst *PredSEI;
if (isSingleBranchBlock(PredBB)) {
PredSEI = getSwitchEnumPred(PredBB, SEI, PostBB, Blocks, Visited,
DT, PDT);
} else {
// Check if a predecessor BB terminates with a switch_enum instruction
PredSEI = dyn_cast<SwitchEnumInst>(PredBB->getTerminator());
if (!PredSEI)
return nullptr;
// Check that this block only produces the value, but does not
// have any side effects.
auto First = BB->begin();
if (BB->getTerminator() != First) {
// There are some instructions besides the branch.
// It should be only an integer literal instruction.
// Handle only integer values for now.
if (!dyn_cast<IntegerLiteralInst>(First))
return nullptr;
if (BB->getTerminator() != ++First)
return nullptr;
}
// Remember that this BB is immediately reachable from a switch_enum.
Blocks.insert(BB);
}
if (!PredSEI)
return nullptr;
if (SEI && PredSEI != SEI)
return nullptr;
if (!SEI)
SEI = PredSEI;
}
return SEI;
}
/// Helper function to produce a SILValue from a result value
/// produced by a basic block responsible for handling a
/// specific enum tag.
static SILValue
getSILValueFromCaseResult(SILBuilder &B, SILLocation Loc,
SILType Type, SILValue Val) {
if (auto *IL = dyn_cast<IntegerLiteralInst>(Val)) {
auto Value = IL->getValue();
if (Value.getBitWidth() != 1)
return B.createIntegerLiteral(Loc, Type, Value);
else
// This is a boolean value
return B.createIntegerLiteral(Loc, Type, Value.getBoolValue());
} else {
llvm::errs() << "Non IntegerLiteralInst switch case result\n";
Val.dump();
return Val;
}
}
/// Given an integer argument, see if it is ultimately matching whether
/// a given enum is of a given tag. If so, create a new select_enum instruction
/// This is used to simplify arbitrary simple switch_enum diamonds into
/// select_enums.
bool simplifySwitchEnumToSelectEnum(SILBasicBlock *BB, unsigned ArgNum,
SILArgument *IntArg, DominanceInfo *DT,
PostDominanceInfo *PDT) {
if (!DT || !PDT)
return false;
// Mapping from case values to the results corresponding to this case value.
SmallVector<std::pair<EnumElementDecl *, SILValue>, 8> CaseToValue;
// Mapping from BB responsible for a specific case value to the result it
// produces.
llvm::DenseMap<SILBasicBlock *, SILValue> BBToValue;
// switch_enum instruction to be replaced.
SwitchEnumInst *SEI = nullptr;
bool HasNonSwitchEnumPreds = false;
// Iterate over all immediate predecessors of the target basic block.
// - Check that each one stems directly or indirectly from the same
// switch_enum instruction.
// - Remember for each case tag of the switch_enum instruction which
// integer value it produces.
// - Check that each block handling a given case tag of a switch_enum
// only produces an integer value and does not have any side-effects.
for (auto P : BB->getPreds()) {
// Only handle branch instructions.
auto *TI = P->getTerminator();
if (!isa<BranchInst>(TI))
return false;
// Find the Nth argument passed to BB.
auto Arg = TI->getOperand(ArgNum);
auto *SI = dyn_cast<SILInstruction>(Arg);
if (!SI) {
return false;
} else {
// Handle integer values
auto *IntLit = dyn_cast<IntegerLiteralInst>(SI);
if (!IntLit) {
// TODO: SI may be any instruction that dominates the switch_enum
// instruction?
//if (!DT->dominates(SI->getParent(), P))
return false;
}
}
// Set of blocks that branch to/reach this basic block P and are immediate
// successors of a switch_enum instruction.
SmallPtrSet<SILBasicBlock *, 8> Blocks;
// Set of blocks visited during a search for a parent switch_enum instruction.
SmallPtrSet<SILBasicBlock *, 8> Visited;
// Try to find a parent SwitchEnumInst for the current predecessor of BB.
auto *PredSEI = getSwitchEnumPred(P, SEI, BB, Blocks, Visited, DT, PDT);
// Predecessor is not produced by a switch_enum instruction, bail.
if (!PredSEI) {
HasNonSwitchEnumPreds = true;
continue;
}
// Check if all predecessors stem from the same switch_enum instruction.
if (SEI) {
if (SEI != PredSEI) {
// It comes from a different switch_enum instruction, bail.
HasNonSwitchEnumPreds = true;
continue;
}
} else {
SEI = PredSEI;
}
// Remember the result value used to branch to this instruction.
for (auto B : Blocks)
BBToValue[B] = Arg;
}
if (!SEI)
return false;
// Insert the new enum_select instruction right after enum_switch
SILBuilder B(SEI);
// Form a set of case_tag:result pairs for select_enum
for (unsigned i = 0, e = SEI->getNumCases(); i != e; ++i) {
std::pair<EnumElementDecl *, SILBasicBlock *> Pair = SEI->getCase(i);
// If one of the branches is not covered, bail
if (!BBToValue.count(Pair.second))
return false;
auto CaseValue = BBToValue[Pair.second];
auto CaseSILValue = getSILValueFromCaseResult(B, SEI->getLoc(),
IntArg->getType(),
CaseValue);
CaseToValue.push_back(std::make_pair(Pair.first, CaseSILValue));
}
// Default value for select_enum.
SILValue DefaultSILValue = SILValue();
if (SEI->hasDefault()) {
// Try to define a default case for enum_select based
// on the default case of enum_switch.
// If default branch is not covered, bail
if (!BBToValue.count(SEI->getDefaultBB()))
return false;
auto DefaultValue = BBToValue[SEI->getDefaultBB()];
DefaultSILValue = getSILValueFromCaseResult(B, SEI->getLoc(),
IntArg->getType(),
DefaultValue);
} else {
// Try to see if enum_switch covers all possible cases.
// If it does, then pick one of those cases as a default.
// Count the number of possible case tags for a given enum type
auto *Enum = SEI->getOperand().getType().getEnumOrBoundGenericEnum();
unsigned ElemCount = 0;
for (auto E : Enum->getAllElements()) {
if (E)
ElemCount++;
}
// Check if all possible cases are covered.
if (ElemCount == SEI->getNumCases()) {
// This enum_switch instruction is exhaustive.
// Make the last case a default.
auto Pair = CaseToValue.pop_back_val();
DefaultSILValue = Pair.second;
}
}
// We don't need to have explicit cases for any case tags which produce the
// same result as the default branch.
if (DefaultSILValue != SILValue()) {
auto DefaultValue = DefaultSILValue;
auto *DefaultSI = dyn_cast<IntegerLiteralInst>(DefaultValue);
for (auto I = CaseToValue.begin(); I != CaseToValue.end();) {
auto CaseValue = I->second;
if (CaseValue == DefaultValue) {
I = CaseToValue.erase(I);
continue;
}
if (DefaultSI) {
if (auto CaseSI = dyn_cast<IntegerLiteralInst>(CaseValue)) {
if (DefaultSI->getValue() == CaseSI->getValue()) {
I = CaseToValue.erase(I);
continue;
}
}
}
++I;
}
}
// Create a new select_enum instruction
auto SelectInst = B.createSelectEnum(SEI->getLoc(), SEI->getOperand(),
IntArg->getType(),
DefaultSILValue, CaseToValue);
if (!HasNonSwitchEnumPreds) {
IntArg->replaceAllUsesWith(SelectInst);
} else {
// Don't know which values should be passed if there is more
// than one basic block argument.
if (BB->bbarg_size() > 1)
return false;
// Do not replace the bbarg
SmallVector<SILValue, 4> Args;
Args.push_back(SelectInst);
B.setInsertionPoint(SelectInst->getNextNode());
B.createBranch(SEI->getLoc(), BB, Args);
// Remove switch_enum instruction
SEI->getParent()->getTerminator()->eraseFromParent();
}
return true;
}
// Attempt to simplify the ith argument of BB. We simplify cases
// where there is a single use of the argument that is an extract from
// a struct or tuple and where the predecessors all build the struct
// or tuple and pass it directly.
bool SimplifyCFG::simplifyArgument(SILBasicBlock *BB, unsigned i) {
auto *A = BB->getBBArg(i);
// If we are reading an i1, then check to see if it comes from
// a switch_enum. If so, we may be able to lower this sequence to
// a select_enum.
if (A->getType().is<BuiltinIntegerType>())
return simplifySwitchEnumToSelectEnum(BB, i, A, DT, PDT);
// For now, just focus on cases where there is a single use.
if (!A->hasOneUse())
return false;
auto *Use = *A->use_begin();
auto *User = cast<SILInstruction>(Use->getUser());
if (!dyn_cast<StructExtractInst>(User) &&
!dyn_cast<TupleExtractInst>(User))
return false;
// For now, just handle the case where all predecessors are
// unconditional branches.
for (auto *Pred : BB->getPreds()) {
if (!isa<BranchInst>(Pred->getTerminator()))
return false;
auto *Branch = cast<BranchInst>(Pred->getTerminator());
if (!isa<StructInst>(Branch->getArg(i)) &&
!isa<TupleInst>(Branch->getArg(i)))
return false;
}
// Okay, we'll replace the BB arg with one with the right type, replace
// the uses in this block, and then rewrite the branch operands.
A->replaceAllUsesWith(SILUndef::get(A->getType(), BB->getModule()));
auto *NewArg = BB->replaceBBArg(i, User->getType(0));
User->replaceAllUsesWith(NewArg);
User->eraseFromParent();
// Rewrite the branch operand for each incoming branch.
for (auto *Pred : BB->getPreds()) {
if (auto *Branch = cast<BranchInst>(Pred->getTerminator())) {
auto V = getInsertedValue(cast<SILInstruction>(Branch->getArg(i)),
User);
Branch->setOperand(i, V);
addToWorklist(Pred);
}
}
return true;
}
bool SimplifyCFG::simplifyArgs(SILBasicBlock *BB) {
// Ignore blocks with no arguments.
if (BB->bbarg_empty())
return false;
// Ignore the entry block.
if (BB->pred_empty())
return false;
// Ignore blocks that are successors of terminators with mandatory args.
for (SILBasicBlock *pred : BB->getPreds()) {
if (hasMandatoryArgument(pred->getTerminator()))
return false;
}
bool Changed = false;
for (int i = BB->getNumBBArg() - 1; i >= 0; --i) {
SILArgument *A = BB->getBBArg(i);
// Try to simplify the argument
if (!A->use_empty()) {
if (simplifyArgument(BB, i))
Changed = true;
continue;
}
DEBUG(llvm::dbgs() << "*** Erasing " << i <<"th BB argument.\n");
NumDeadArguments++;
Changed = true;
BB->eraseArgument(i);
// Determine the set of predecessors in case any predecessor has
// two edges to this block (e.g. a conditional branch where both
// sides reach this block).
llvm::SmallPtrSet<SILBasicBlock *, 4> PredBBs;
for (auto *Pred : BB->getPreds())
PredBBs.insert(Pred);
for (auto *Pred : PredBBs)
removeArgumentFromTerminator(Pred, BB, i);
}
return Changed;
}
namespace {
class SimplifyCFGPass : public SILFunctionTransform {
/// The entry point to the transformation.
void run() {
if (SimplifyCFG(*getFunction(), PM).run())
invalidateAnalysis(SILAnalysis::InvalidationKind::CFG);
}
StringRef getName() override { return "Simplify CFG"; }
};
} // end anonymous namespace
SILTransform *swift::createSimplifyCFG() {
return new SimplifyCFGPass();
}
namespace {
class SplitCriticalEdges : public SILFunctionTransform {
bool OnlyNonCondBrEdges;
public:
SplitCriticalEdges(bool SplitOnlyNonCondBrEdges)
: OnlyNonCondBrEdges(SplitOnlyNonCondBrEdges) {}
void run() {
auto &Fn = *getFunction();
// Split all critical egdes from all or non only cond_br terminators.
bool Changed =
splitAllCriticalEdges(Fn, OnlyNonCondBrEdges, nullptr, nullptr);
if (Changed)
invalidateAnalysis(SILAnalysis::InvalidationKind::CFG);
}
StringRef getName() override { return "Split Critical Edges"; }
};
} // End anonymous namespace.
/// Splits all critical edges in a function.
SILTransform *swift::createSplitAllCriticalEdges() {
return new SplitCriticalEdges(false);
}
/// Splits all critical edges from non cond_br terminators in a function.
SILTransform *swift::createSplitNonCondBrCriticalEdges() {
return new SplitCriticalEdges(true);
}
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