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llvm-6502/lib/Transforms/Scalar/LoopUnswitch.cpp
Daniel Dunbar ce63ffb52f More migration to raw_ostream, the water has dried up around the iostream hole. 
- Some clients which used DOUT have moved to DEBUG. We are deprecating the
   "magic" DOUT behavior which avoided calling printing functions when the
   statement was disabled. In addition to being unnecessary magic, it had the
   downside of leaving code in -Asserts builds, and of hiding potentially
   unnecessary computations.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@77019 91177308-0d34-0410-b5e6-96231b3b80d8
2009-07-25 00:23:56 +00:00

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//===-- LoopUnswitch.cpp - Hoist loop-invariant conditionals in loop ------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass transforms loops that contain branches on loop-invariant conditions
// to have multiple loops. For example, it turns the left into the right code:
//
// for (...) if (lic)
// A for (...)
// if (lic) A; B; C
// B else
// C for (...)
// A; C
//
// This can increase the size of the code exponentially (doubling it every time
// a loop is unswitched) so we only unswitch if the resultant code will be
// smaller than a threshold.
//
// This pass expects LICM to be run before it to hoist invariant conditions out
// of the loop, to make the unswitching opportunity obvious.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "loop-unswitch"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
#include "llvm/LLVMContext.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <set>
using namespace llvm;
STATISTIC(NumBranches, "Number of branches unswitched");
STATISTIC(NumSwitches, "Number of switches unswitched");
STATISTIC(NumSelects , "Number of selects unswitched");
STATISTIC(NumTrivial , "Number of unswitches that are trivial");
STATISTIC(NumSimplify, "Number of simplifications of unswitched code");
static cl::opt<unsigned>
Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"),
cl::init(10), cl::Hidden);
namespace {
class VISIBILITY_HIDDEN LoopUnswitch : public LoopPass {
LoopInfo *LI; // Loop information
LPPassManager *LPM;
// LoopProcessWorklist - Used to check if second loop needs processing
// after RewriteLoopBodyWithConditionConstant rewrites first loop.
std::vector<Loop*> LoopProcessWorklist;
SmallPtrSet<Value *,8> UnswitchedVals;
bool OptimizeForSize;
bool redoLoop;
Loop *currentLoop;
DominanceFrontier *DF;
DominatorTree *DT;
BasicBlock *loopHeader;
BasicBlock *loopPreheader;
// LoopBlocks contains all of the basic blocks of the loop, including the
// preheader of the loop, the body of the loop, and the exit blocks of the
// loop, in that order.
std::vector<BasicBlock*> LoopBlocks;
// NewBlocks contained cloned copy of basic blocks from LoopBlocks.
std::vector<BasicBlock*> NewBlocks;
public:
static char ID; // Pass ID, replacement for typeid
explicit LoopUnswitch(bool Os = false) :
LoopPass(&ID), OptimizeForSize(Os), redoLoop(false),
currentLoop(NULL), DF(NULL), DT(NULL), loopHeader(NULL),
loopPreheader(NULL) {}
bool runOnLoop(Loop *L, LPPassManager &LPM);
bool processCurrentLoop();
/// This transformation requires natural loop information & requires that
/// loop preheaders be inserted into the CFG...
///
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequiredID(LoopSimplifyID);
AU.addPreservedID(LoopSimplifyID);
AU.addRequired<LoopInfo>();
AU.addPreserved<LoopInfo>();
AU.addRequiredID(LCSSAID);
AU.addPreservedID(LCSSAID);
AU.addPreserved<DominatorTree>();
AU.addPreserved<DominanceFrontier>();
}
private:
/// RemoveLoopFromWorklist - If the specified loop is on the loop worklist,
/// remove it.
void RemoveLoopFromWorklist(Loop *L) {
std::vector<Loop*>::iterator I = std::find(LoopProcessWorklist.begin(),
LoopProcessWorklist.end(), L);
if (I != LoopProcessWorklist.end())
LoopProcessWorklist.erase(I);
}
void initLoopData() {
loopHeader = currentLoop->getHeader();
loopPreheader = currentLoop->getLoopPreheader();
}
/// Split all of the edges from inside the loop to their exit blocks.
/// Update the appropriate Phi nodes as we do so.
void SplitExitEdges(Loop *L, const SmallVector<BasicBlock *, 8> &ExitBlocks);
bool UnswitchIfProfitable(Value *LoopCond, Constant *Val);
unsigned getLoopUnswitchCost(Value *LIC);
void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val,
BasicBlock *ExitBlock);
void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L);
void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
Constant *Val, bool isEqual);
void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
BasicBlock *TrueDest,
BasicBlock *FalseDest,
Instruction *InsertPt);
void SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L);
void RemoveBlockIfDead(BasicBlock *BB,
std::vector<Instruction*> &Worklist, Loop *l);
void RemoveLoopFromHierarchy(Loop *L);
bool IsTrivialUnswitchCondition(Value *Cond, Constant **Val = 0,
BasicBlock **LoopExit = 0);
};
}
char LoopUnswitch::ID = 0;
static RegisterPass<LoopUnswitch> X("loop-unswitch", "Unswitch loops");
Pass *llvm::createLoopUnswitchPass(bool Os) {
return new LoopUnswitch(Os);
}
/// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is
/// invariant in the loop, or has an invariant piece, return the invariant.
/// Otherwise, return null.
static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) {
// Constants should be folded, not unswitched on!
if (isa<Constant>(Cond)) return 0;
// TODO: Handle: br (VARIANT|INVARIANT).
// Hoist simple values out.
if (L->makeLoopInvariant(Cond, Changed))
return Cond;
if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond))
if (BO->getOpcode() == Instruction::And ||
BO->getOpcode() == Instruction::Or) {
// If either the left or right side is invariant, we can unswitch on this,
// which will cause the branch to go away in one loop and the condition to
// simplify in the other one.
if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed))
return LHS;
if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed))
return RHS;
}
return 0;
}
bool LoopUnswitch::runOnLoop(Loop *L, LPPassManager &LPM_Ref) {
LI = &getAnalysis<LoopInfo>();
LPM = &LPM_Ref;
DF = getAnalysisIfAvailable<DominanceFrontier>();
DT = getAnalysisIfAvailable<DominatorTree>();
currentLoop = L;
Function *F = currentLoop->getHeader()->getParent();
bool Changed = false;
do {
assert(currentLoop->isLCSSAForm());
redoLoop = false;
Changed |= processCurrentLoop();
} while(redoLoop);
if (Changed) {
// FIXME: Reconstruct dom info, because it is not preserved properly.
if (DT)
DT->runOnFunction(*F);
if (DF)
DF->runOnFunction(*F);
}
return Changed;
}
/// processCurrentLoop - Do actual work and unswitch loop if possible
/// and profitable.
bool LoopUnswitch::processCurrentLoop() {
bool Changed = false;
LLVMContext &Context = currentLoop->getHeader()->getContext();
// Loop over all of the basic blocks in the loop. If we find an interior
// block that is branching on a loop-invariant condition, we can unswitch this
// loop.
for (Loop::block_iterator I = currentLoop->block_begin(),
E = currentLoop->block_end();
I != E; ++I) {
TerminatorInst *TI = (*I)->getTerminator();
if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
// If this isn't branching on an invariant condition, we can't unswitch
// it.
if (BI->isConditional()) {
// See if this, or some part of it, is loop invariant. If so, we can
// unswitch on it if we desire.
Value *LoopCond = FindLIVLoopCondition(BI->getCondition(),
currentLoop, Changed);
if (LoopCond && UnswitchIfProfitable(LoopCond,
Context.getTrue())) {
++NumBranches;
return true;
}
}
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
Value *LoopCond = FindLIVLoopCondition(SI->getCondition(),
currentLoop, Changed);
if (LoopCond && SI->getNumCases() > 1) {
// Find a value to unswitch on:
// FIXME: this should chose the most expensive case!
Constant *UnswitchVal = SI->getCaseValue(1);
// Do not process same value again and again.
if (!UnswitchedVals.insert(UnswitchVal))
continue;
if (UnswitchIfProfitable(LoopCond, UnswitchVal)) {
++NumSwitches;
return true;
}
}
}
// Scan the instructions to check for unswitchable values.
for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end();
BBI != E; ++BBI)
if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) {
Value *LoopCond = FindLIVLoopCondition(SI->getCondition(),
currentLoop, Changed);
if (LoopCond && UnswitchIfProfitable(LoopCond,
Context.getTrue())) {
++NumSelects;
return true;
}
}
}
return Changed;
}
/// isTrivialLoopExitBlock - Check to see if all paths from BB either:
/// 1. Exit the loop with no side effects.
/// 2. Branch to the latch block with no side-effects.
///
/// If these conditions are true, we return true and set ExitBB to the block we
/// exit through.
///
static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB,
BasicBlock *&ExitBB,
std::set<BasicBlock*> &Visited) {
if (!Visited.insert(BB).second) {
// Already visited and Ok, end of recursion.
return true;
} else if (!L->contains(BB)) {
// Otherwise, this is a loop exit, this is fine so long as this is the
// first exit.
if (ExitBB != 0) return false;
ExitBB = BB;
return true;
}
// Otherwise, this is an unvisited intra-loop node. Check all successors.
for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) {
// Check to see if the successor is a trivial loop exit.
if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited))
return false;
}
// Okay, everything after this looks good, check to make sure that this block
// doesn't include any side effects.
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
if (I->mayHaveSideEffects())
return false;
return true;
}
/// isTrivialLoopExitBlock - Return true if the specified block unconditionally
/// leads to an exit from the specified loop, and has no side-effects in the
/// process. If so, return the block that is exited to, otherwise return null.
static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) {
std::set<BasicBlock*> Visited;
Visited.insert(L->getHeader()); // Branches to header are ok.
BasicBlock *ExitBB = 0;
if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited))
return ExitBB;
return 0;
}
/// IsTrivialUnswitchCondition - Check to see if this unswitch condition is
/// trivial: that is, that the condition controls whether or not the loop does
/// anything at all. If this is a trivial condition, unswitching produces no
/// code duplications (equivalently, it produces a simpler loop and a new empty
/// loop, which gets deleted).
///
/// If this is a trivial condition, return true, otherwise return false. When
/// returning true, this sets Cond and Val to the condition that controls the
/// trivial condition: when Cond dynamically equals Val, the loop is known to
/// exit. Finally, this sets LoopExit to the BB that the loop exits to when
/// Cond == Val.
///
bool LoopUnswitch::IsTrivialUnswitchCondition(Value *Cond, Constant **Val,
BasicBlock **LoopExit) {
BasicBlock *Header = currentLoop->getHeader();
TerminatorInst *HeaderTerm = Header->getTerminator();
LLVMContext &Context = Header->getContext();
BasicBlock *LoopExitBB = 0;
if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) {
// If the header block doesn't end with a conditional branch on Cond, we
// can't handle it.
if (!BI->isConditional() || BI->getCondition() != Cond)
return false;
// Check to see if a successor of the branch is guaranteed to go to the
// latch block or exit through a one exit block without having any
// side-effects. If so, determine the value of Cond that causes it to do
// this.
if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
BI->getSuccessor(0)))) {
if (Val) *Val = Context.getTrue();
} else if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
BI->getSuccessor(1)))) {
if (Val) *Val = Context.getFalse();
}
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) {
// If this isn't a switch on Cond, we can't handle it.
if (SI->getCondition() != Cond) return false;
// Check to see if a successor of the switch is guaranteed to go to the
// latch block or exit through a one exit block without having any
// side-effects. If so, determine the value of Cond that causes it to do
// this. Note that we can't trivially unswitch on the default case.
for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i)
if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
SI->getSuccessor(i)))) {
// Okay, we found a trivial case, remember the value that is trivial.
if (Val) *Val = SI->getCaseValue(i);
break;
}
}
// If we didn't find a single unique LoopExit block, or if the loop exit block
// contains phi nodes, this isn't trivial.
if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin()))
return false; // Can't handle this.
if (LoopExit) *LoopExit = LoopExitBB;
// We already know that nothing uses any scalar values defined inside of this
// loop. As such, we just have to check to see if this loop will execute any
// side-effecting instructions (e.g. stores, calls, volatile loads) in the
// part of the loop that the code *would* execute. We already checked the
// tail, check the header now.
for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I)
if (I->mayHaveSideEffects())
return false;
return true;
}
/// getLoopUnswitchCost - Return the cost (code size growth) that will happen if
/// we choose to unswitch current loop on the specified value.
///
unsigned LoopUnswitch::getLoopUnswitchCost(Value *LIC) {
// If the condition is trivial, always unswitch. There is no code growth for
// this case.
if (IsTrivialUnswitchCondition(LIC))
return 0;
// FIXME: This is really overly conservative. However, more liberal
// estimations have thus far resulted in excessive unswitching, which is bad
// both in compile time and in code size. This should be replaced once
// someone figures out how a good estimation.
return currentLoop->getBlocks().size();
unsigned Cost = 0;
// FIXME: this is brain dead. It should take into consideration code
// shrinkage.
for (Loop::block_iterator I = currentLoop->block_begin(),
E = currentLoop->block_end();
I != E; ++I) {
BasicBlock *BB = *I;
// Do not include empty blocks in the cost calculation. This happen due to
// loop canonicalization and will be removed.
if (BB->begin() == BasicBlock::iterator(BB->getTerminator()))
continue;
// Count basic blocks.
++Cost;
}
return Cost;
}
/// UnswitchIfProfitable - We have found that we can unswitch currentLoop when
/// LoopCond == Val to simplify the loop. If we decide that this is profitable,
/// unswitch the loop, reprocess the pieces, then return true.
bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val){
initLoopData();
Function *F = loopHeader->getParent();
// Check to see if it would be profitable to unswitch current loop.
unsigned Cost = getLoopUnswitchCost(LoopCond);
// Do not do non-trivial unswitch while optimizing for size.
if (Cost && OptimizeForSize)
return false;
if (Cost && !F->isDeclaration() && F->hasFnAttr(Attribute::OptimizeForSize))
return false;
if (Cost > Threshold) {
// FIXME: this should estimate growth by the amount of code shared by the
// resultant unswitched loops.
//
DEBUG(errs() << "NOT unswitching loop %"
<< currentLoop->getHeader()->getName() << ", cost too high: "
<< currentLoop->getBlocks().size() << "\n");
return false;
}
Constant *CondVal;
BasicBlock *ExitBlock;
if (IsTrivialUnswitchCondition(LoopCond, &CondVal, &ExitBlock)) {
UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, ExitBlock);
} else {
UnswitchNontrivialCondition(LoopCond, Val, currentLoop);
}
return true;
}
// RemapInstruction - Convert the instruction operands from referencing the
// current values into those specified by ValueMap.
//
static inline void RemapInstruction(Instruction *I,
DenseMap<const Value *, Value*> &ValueMap) {
for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
Value *Op = I->getOperand(op);
DenseMap<const Value *, Value*>::iterator It = ValueMap.find(Op);
if (It != ValueMap.end()) Op = It->second;
I->setOperand(op, Op);
}
}
/// CloneLoop - Recursively clone the specified loop and all of its children,
/// mapping the blocks with the specified map.
static Loop *CloneLoop(Loop *L, Loop *PL, DenseMap<const Value*, Value*> &VM,
LoopInfo *LI, LPPassManager *LPM) {
Loop *New = new Loop();
LPM->insertLoop(New, PL);
// Add all of the blocks in L to the new loop.
for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
I != E; ++I)
if (LI->getLoopFor(*I) == L)
New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), LI->getBase());
// Add all of the subloops to the new loop.
for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
CloneLoop(*I, New, VM, LI, LPM);
return New;
}
/// EmitPreheaderBranchOnCondition - Emit a conditional branch on two values
/// if LIC == Val, branch to TrueDst, otherwise branch to FalseDest. Insert the
/// code immediately before InsertPt.
void LoopUnswitch::EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
BasicBlock *TrueDest,
BasicBlock *FalseDest,
Instruction *InsertPt) {
// Insert a conditional branch on LIC to the two preheaders. The original
// code is the true version and the new code is the false version.
Value *BranchVal = LIC;
if (!isa<ConstantInt>(Val) || Val->getType() != Type::Int1Ty)
BranchVal = new ICmpInst(InsertPt, ICmpInst::ICMP_EQ, LIC, Val, "tmp");
else if (Val != Val->getContext().getTrue())
// We want to enter the new loop when the condition is true.
std::swap(TrueDest, FalseDest);
// Insert the new branch.
BranchInst::Create(TrueDest, FalseDest, BranchVal, InsertPt);
}
/// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable
/// condition in it (a cond branch from its header block to its latch block,
/// where the path through the loop that doesn't execute its body has no
/// side-effects), unswitch it. This doesn't involve any code duplication, just
/// moving the conditional branch outside of the loop and updating loop info.
void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
Constant *Val,
BasicBlock *ExitBlock) {
DEBUG(errs() << "loop-unswitch: Trivial-Unswitch loop %"
<< loopHeader->getName() << " [" << L->getBlocks().size()
<< " blocks] in Function " << L->getHeader()->getParent()->getName()
<< " on cond: " << *Val << " == " << *Cond << "\n");
// First step, split the preheader, so that we know that there is a safe place
// to insert the conditional branch. We will change loopPreheader to have a
// conditional branch on Cond.
BasicBlock *NewPH = SplitEdge(loopPreheader, loopHeader, this);
// Now that we have a place to insert the conditional branch, create a place
// to branch to: this is the exit block out of the loop that we should
// short-circuit to.
// Split this block now, so that the loop maintains its exit block, and so
// that the jump from the preheader can execute the contents of the exit block
// without actually branching to it (the exit block should be dominated by the
// loop header, not the preheader).
assert(!L->contains(ExitBlock) && "Exit block is in the loop?");
BasicBlock *NewExit = SplitBlock(ExitBlock, ExitBlock->begin(), this);
// Okay, now we have a position to branch from and a position to branch to,
// insert the new conditional branch.
EmitPreheaderBranchOnCondition(Cond, Val, NewExit, NewPH,
loopPreheader->getTerminator());
LPM->deleteSimpleAnalysisValue(loopPreheader->getTerminator(), L);
loopPreheader->getTerminator()->eraseFromParent();
// We need to reprocess this loop, it could be unswitched again.
redoLoop = true;
// Now that we know that the loop is never entered when this condition is a
// particular value, rewrite the loop with this info. We know that this will
// at least eliminate the old branch.
RewriteLoopBodyWithConditionConstant(L, Cond, Val, false);
++NumTrivial;
}
/// SplitExitEdges - Split all of the edges from inside the loop to their exit
/// blocks. Update the appropriate Phi nodes as we do so.
void LoopUnswitch::SplitExitEdges(Loop *L,
const SmallVector<BasicBlock *, 8> &ExitBlocks)
{
for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
BasicBlock *ExitBlock = ExitBlocks[i];
std::vector<BasicBlock*> Preds(pred_begin(ExitBlock), pred_end(ExitBlock));
for (unsigned j = 0, e = Preds.size(); j != e; ++j) {
BasicBlock* NewExitBlock = SplitEdge(Preds[j], ExitBlock, this);
BasicBlock* StartBlock = Preds[j];
BasicBlock* EndBlock;
if (NewExitBlock->getSinglePredecessor() == ExitBlock) {
EndBlock = NewExitBlock;
NewExitBlock = EndBlock->getSinglePredecessor();
} else {
EndBlock = ExitBlock;
}
std::set<PHINode*> InsertedPHIs;
PHINode* OldLCSSA = 0;
for (BasicBlock::iterator I = EndBlock->begin();
(OldLCSSA = dyn_cast<PHINode>(I)); ++I) {
Value* OldValue = OldLCSSA->getIncomingValueForBlock(NewExitBlock);
PHINode* NewLCSSA = PHINode::Create(OldLCSSA->getType(),
OldLCSSA->getName() + ".us-lcssa",
NewExitBlock->getTerminator());
NewLCSSA->addIncoming(OldValue, StartBlock);
OldLCSSA->setIncomingValue(OldLCSSA->getBasicBlockIndex(NewExitBlock),
NewLCSSA);
InsertedPHIs.insert(NewLCSSA);
}
BasicBlock::iterator InsertPt = EndBlock->getFirstNonPHI();
for (BasicBlock::iterator I = NewExitBlock->begin();
(OldLCSSA = dyn_cast<PHINode>(I)) && InsertedPHIs.count(OldLCSSA) == 0;
++I) {
PHINode *NewLCSSA = PHINode::Create(OldLCSSA->getType(),
OldLCSSA->getName() + ".us-lcssa",
InsertPt);
OldLCSSA->replaceAllUsesWith(NewLCSSA);
NewLCSSA->addIncoming(OldLCSSA, NewExitBlock);
}
}
}
}
/// UnswitchNontrivialCondition - We determined that the loop is profitable
/// to unswitch when LIC equal Val. Split it into loop versions and test the
/// condition outside of either loop. Return the loops created as Out1/Out2.
void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val,
Loop *L) {
Function *F = loopHeader->getParent();
DEBUG(errs() << "loop-unswitch: Unswitching loop %"
<< loopHeader->getName() << " [" << L->getBlocks().size()
<< " blocks] in Function " << F->getName()
<< " when '" << *Val << "' == " << *LIC << "\n");
LoopBlocks.clear();
NewBlocks.clear();
// First step, split the preheader and exit blocks, and add these blocks to
// the LoopBlocks list.
BasicBlock *NewPreheader = SplitEdge(loopPreheader, loopHeader, this);
LoopBlocks.push_back(NewPreheader);
// We want the loop to come after the preheader, but before the exit blocks.
LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
SmallVector<BasicBlock*, 8> ExitBlocks;
L->getUniqueExitBlocks(ExitBlocks);
// Split all of the edges from inside the loop to their exit blocks. Update
// the appropriate Phi nodes as we do so.
SplitExitEdges(L, ExitBlocks);
// The exit blocks may have been changed due to edge splitting, recompute.
ExitBlocks.clear();
L->getUniqueExitBlocks(ExitBlocks);
// Add exit blocks to the loop blocks.
LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end());
// Next step, clone all of the basic blocks that make up the loop (including
// the loop preheader and exit blocks), keeping track of the mapping between
// the instructions and blocks.
NewBlocks.reserve(LoopBlocks.size());
DenseMap<const Value*, Value*> ValueMap;
for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
BasicBlock *New = CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F);
NewBlocks.push_back(New);
ValueMap[LoopBlocks[i]] = New; // Keep the BB mapping.
LPM->cloneBasicBlockSimpleAnalysis(LoopBlocks[i], New, L);
}
// Splice the newly inserted blocks into the function right before the
// original preheader.
F->getBasicBlockList().splice(LoopBlocks[0], F->getBasicBlockList(),
NewBlocks[0], F->end());
// Now we create the new Loop object for the versioned loop.
Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI, LPM);
Loop *ParentLoop = L->getParentLoop();
if (ParentLoop) {
// Make sure to add the cloned preheader and exit blocks to the parent loop
// as well.
ParentLoop->addBasicBlockToLoop(NewBlocks[0], LI->getBase());
}
for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
BasicBlock *NewExit = cast<BasicBlock>(ValueMap[ExitBlocks[i]]);
// The new exit block should be in the same loop as the old one.
if (Loop *ExitBBLoop = LI->getLoopFor(ExitBlocks[i]))
ExitBBLoop->addBasicBlockToLoop(NewExit, LI->getBase());
assert(NewExit->getTerminator()->getNumSuccessors() == 1 &&
"Exit block should have been split to have one successor!");
BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0);
// If the successor of the exit block had PHI nodes, add an entry for
// NewExit.
PHINode *PN;
for (BasicBlock::iterator I = ExitSucc->begin();
(PN = dyn_cast<PHINode>(I)); ++I) {
Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]);
DenseMap<const Value *, Value*>::iterator It = ValueMap.find(V);
if (It != ValueMap.end()) V = It->second;
PN->addIncoming(V, NewExit);
}
}
// Rewrite the code to refer to itself.
for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
for (BasicBlock::iterator I = NewBlocks[i]->begin(),
E = NewBlocks[i]->end(); I != E; ++I)
RemapInstruction(I, ValueMap);
// Rewrite the original preheader to select between versions of the loop.
BranchInst *OldBR = cast<BranchInst>(loopPreheader->getTerminator());
assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] &&
"Preheader splitting did not work correctly!");
// Emit the new branch that selects between the two versions of this loop.
EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR);
LPM->deleteSimpleAnalysisValue(OldBR, L);
OldBR->eraseFromParent();
LoopProcessWorklist.push_back(NewLoop);
redoLoop = true;
// Now we rewrite the original code to know that the condition is true and the
// new code to know that the condition is false.
RewriteLoopBodyWithConditionConstant(L , LIC, Val, false);
// It's possible that simplifying one loop could cause the other to be
// deleted. If so, don't simplify it.
if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop)
RewriteLoopBodyWithConditionConstant(NewLoop, LIC, Val, true);
}
/// RemoveFromWorklist - Remove all instances of I from the worklist vector
/// specified.
static void RemoveFromWorklist(Instruction *I,
std::vector<Instruction*> &Worklist) {
std::vector<Instruction*>::iterator WI = std::find(Worklist.begin(),
Worklist.end(), I);
while (WI != Worklist.end()) {
unsigned Offset = WI-Worklist.begin();
Worklist.erase(WI);
WI = std::find(Worklist.begin()+Offset, Worklist.end(), I);
}
}
/// ReplaceUsesOfWith - When we find that I really equals V, remove I from the
/// program, replacing all uses with V and update the worklist.
static void ReplaceUsesOfWith(Instruction *I, Value *V,
std::vector<Instruction*> &Worklist,
Loop *L, LPPassManager *LPM) {
DOUT << "Replace with '" << *V << "': " << *I;
// Add uses to the worklist, which may be dead now.
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
Worklist.push_back(Use);
// Add users to the worklist which may be simplified now.
for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
UI != E; ++UI)
Worklist.push_back(cast<Instruction>(*UI));
LPM->deleteSimpleAnalysisValue(I, L);
RemoveFromWorklist(I, Worklist);
I->replaceAllUsesWith(V);
I->eraseFromParent();
++NumSimplify;
}
/// RemoveBlockIfDead - If the specified block is dead, remove it, update loop
/// information, and remove any dead successors it has.
///
void LoopUnswitch::RemoveBlockIfDead(BasicBlock *BB,
std::vector<Instruction*> &Worklist,
Loop *L) {
if (pred_begin(BB) != pred_end(BB)) {
// This block isn't dead, since an edge to BB was just removed, see if there
// are any easy simplifications we can do now.
if (BasicBlock *Pred = BB->getSinglePredecessor()) {
// If it has one pred, fold phi nodes in BB.
while (isa<PHINode>(BB->begin()))
ReplaceUsesOfWith(BB->begin(),
cast<PHINode>(BB->begin())->getIncomingValue(0),
Worklist, L, LPM);
// If this is the header of a loop and the only pred is the latch, we now
// have an unreachable loop.
if (Loop *L = LI->getLoopFor(BB))
if (loopHeader == BB && L->contains(Pred)) {
// Remove the branch from the latch to the header block, this makes
// the header dead, which will make the latch dead (because the header
// dominates the latch).
LPM->deleteSimpleAnalysisValue(Pred->getTerminator(), L);
Pred->getTerminator()->eraseFromParent();
new UnreachableInst(Pred);
// The loop is now broken, remove it from LI.
RemoveLoopFromHierarchy(L);
// Reprocess the header, which now IS dead.
RemoveBlockIfDead(BB, Worklist, L);
return;
}
// If pred ends in a uncond branch, add uncond branch to worklist so that
// the two blocks will get merged.
if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator()))
if (BI->isUnconditional())
Worklist.push_back(BI);
}
return;
}
DOUT << "Nuking dead block: " << *BB;
// Remove the instructions in the basic block from the worklist.
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
RemoveFromWorklist(I, Worklist);
// Anything that uses the instructions in this basic block should have their
// uses replaced with undefs.
if (!I->use_empty())
I->replaceAllUsesWith(I->getContext().getUndef(I->getType()));
}
// If this is the edge to the header block for a loop, remove the loop and
// promote all subloops.
if (Loop *BBLoop = LI->getLoopFor(BB)) {
if (BBLoop->getLoopLatch() == BB)
RemoveLoopFromHierarchy(BBLoop);
}
// Remove the block from the loop info, which removes it from any loops it
// was in.
LI->removeBlock(BB);
// Remove phi node entries in successors for this block.
TerminatorInst *TI = BB->getTerminator();
SmallVector<BasicBlock*, 4> Succs;
for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) {
Succs.push_back(TI->getSuccessor(i));
TI->getSuccessor(i)->removePredecessor(BB);
}
// Unique the successors, remove anything with multiple uses.
array_pod_sort(Succs.begin(), Succs.end());
Succs.erase(std::unique(Succs.begin(), Succs.end()), Succs.end());
// Remove the basic block, including all of the instructions contained in it.
LPM->deleteSimpleAnalysisValue(BB, L);
BB->eraseFromParent();
// Remove successor blocks here that are not dead, so that we know we only
// have dead blocks in this list. Nondead blocks have a way of becoming dead,
// then getting removed before we revisit them, which is badness.
//
for (unsigned i = 0; i != Succs.size(); ++i)
if (pred_begin(Succs[i]) != pred_end(Succs[i])) {
// One exception is loop headers. If this block was the preheader for a
// loop, then we DO want to visit the loop so the loop gets deleted.
// We know that if the successor is a loop header, that this loop had to
// be the preheader: the case where this was the latch block was handled
// above and headers can only have two predecessors.
if (!LI->isLoopHeader(Succs[i])) {
Succs.erase(Succs.begin()+i);
--i;
}
}
for (unsigned i = 0, e = Succs.size(); i != e; ++i)
RemoveBlockIfDead(Succs[i], Worklist, L);
}
/// RemoveLoopFromHierarchy - We have discovered that the specified loop has
/// become unwrapped, either because the backedge was deleted, or because the
/// edge into the header was removed. If the edge into the header from the
/// latch block was removed, the loop is unwrapped but subloops are still alive,
/// so they just reparent loops. If the loops are actually dead, they will be
/// removed later.
void LoopUnswitch::RemoveLoopFromHierarchy(Loop *L) {
LPM->deleteLoopFromQueue(L);
RemoveLoopFromWorklist(L);
}
// RewriteLoopBodyWithConditionConstant - We know either that the value LIC has
// the value specified by Val in the specified loop, or we know it does NOT have
// that value. Rewrite any uses of LIC or of properties correlated to it.
void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
Constant *Val,
bool IsEqual) {
assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
// FIXME: Support correlated properties, like:
// for (...)
// if (li1 < li2)
// ...
// if (li1 > li2)
// ...
// FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches,
// selects, switches.
std::vector<User*> Users(LIC->use_begin(), LIC->use_end());
std::vector<Instruction*> Worklist;
LLVMContext &Context = Val->getContext();
// If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC
// in the loop with the appropriate one directly.
if (IsEqual || (isa<ConstantInt>(Val) && Val->getType() == Type::Int1Ty)) {
Value *Replacement;
if (IsEqual)
Replacement = Val;
else
Replacement = ConstantInt::get(Type::Int1Ty,
!cast<ConstantInt>(Val)->getZExtValue());
for (unsigned i = 0, e = Users.size(); i != e; ++i)
if (Instruction *U = cast<Instruction>(Users[i])) {
if (!L->contains(U->getParent()))
continue;
U->replaceUsesOfWith(LIC, Replacement);
Worklist.push_back(U);
}
} else {
// Otherwise, we don't know the precise value of LIC, but we do know that it
// is certainly NOT "Val". As such, simplify any uses in the loop that we
// can. This case occurs when we unswitch switch statements.
for (unsigned i = 0, e = Users.size(); i != e; ++i)
if (Instruction *U = cast<Instruction>(Users[i])) {
if (!L->contains(U->getParent()))
continue;
Worklist.push_back(U);
// If we know that LIC is not Val, use this info to simplify code.
if (SwitchInst *SI = dyn_cast<SwitchInst>(U)) {
for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) {
if (SI->getCaseValue(i) == Val) {
// Found a dead case value. Don't remove PHI nodes in the
// successor if they become single-entry, those PHI nodes may
// be in the Users list.
// FIXME: This is a hack. We need to keep the successor around
// and hooked up so as to preserve the loop structure, because
// trying to update it is complicated. So instead we preserve the
// loop structure and put the block on an dead code path.
BasicBlock *SISucc = SI->getSuccessor(i);
BasicBlock* Old = SI->getParent();
BasicBlock* Split = SplitBlock(Old, SI, this);
Instruction* OldTerm = Old->getTerminator();
BranchInst::Create(Split, SISucc,
Context.getTrue(), OldTerm);
LPM->deleteSimpleAnalysisValue(Old->getTerminator(), L);
Old->getTerminator()->eraseFromParent();
PHINode *PN;
for (BasicBlock::iterator II = SISucc->begin();
(PN = dyn_cast<PHINode>(II)); ++II) {
Value *InVal = PN->removeIncomingValue(Split, false);
PN->addIncoming(InVal, Old);
}
SI->removeCase(i);
break;
}
}
}
// TODO: We could do other simplifications, for example, turning
// LIC == Val -> false.
}
}
SimplifyCode(Worklist, L);
}
/// SimplifyCode - Okay, now that we have simplified some instructions in the
/// loop, walk over it and constant prop, dce, and fold control flow where
/// possible. Note that this is effectively a very simple loop-structure-aware
/// optimizer. During processing of this loop, L could very well be deleted, so
/// it must not be used.
///
/// FIXME: When the loop optimizer is more mature, separate this out to a new
/// pass.
///
void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L) {
while (!Worklist.empty()) {
Instruction *I = Worklist.back();
Worklist.pop_back();
// Simple constant folding.
if (Constant *C = ConstantFoldInstruction(I, I->getContext())) {
ReplaceUsesOfWith(I, C, Worklist, L, LPM);
continue;
}
// Simple DCE.
if (isInstructionTriviallyDead(I)) {
DOUT << "Remove dead instruction '" << *I;
// Add uses to the worklist, which may be dead now.
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
Worklist.push_back(Use);
LPM->deleteSimpleAnalysisValue(I, L);
RemoveFromWorklist(I, Worklist);
I->eraseFromParent();
++NumSimplify;
continue;
}
// Special case hacks that appear commonly in unswitched code.
switch (I->getOpcode()) {
case Instruction::Select:
if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(0))) {
ReplaceUsesOfWith(I, I->getOperand(!CB->getZExtValue()+1), Worklist, L,
LPM);
continue;
}
break;
case Instruction::And:
if (isa<ConstantInt>(I->getOperand(0)) &&
I->getOperand(0)->getType() == Type::Int1Ty) // constant -> RHS
cast<BinaryOperator>(I)->swapOperands();
if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(1)))
if (CB->getType() == Type::Int1Ty) {
if (CB->isOne()) // X & 1 -> X
ReplaceUsesOfWith(I, I->getOperand(0), Worklist, L, LPM);
else // X & 0 -> 0
ReplaceUsesOfWith(I, I->getOperand(1), Worklist, L, LPM);
continue;
}
break;
case Instruction::Or:
if (isa<ConstantInt>(I->getOperand(0)) &&
I->getOperand(0)->getType() == Type::Int1Ty) // constant -> RHS
cast<BinaryOperator>(I)->swapOperands();
if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(1)))
if (CB->getType() == Type::Int1Ty) {
if (CB->isOne()) // X | 1 -> 1
ReplaceUsesOfWith(I, I->getOperand(1), Worklist, L, LPM);
else // X | 0 -> X
ReplaceUsesOfWith(I, I->getOperand(0), Worklist, L, LPM);
continue;
}
break;
case Instruction::Br: {
BranchInst *BI = cast<BranchInst>(I);
if (BI->isUnconditional()) {
// If BI's parent is the only pred of the successor, fold the two blocks
// together.
BasicBlock *Pred = BI->getParent();
BasicBlock *Succ = BI->getSuccessor(0);
BasicBlock *SinglePred = Succ->getSinglePredecessor();
if (!SinglePred) continue; // Nothing to do.
assert(SinglePred == Pred && "CFG broken");
DEBUG(errs() << "Merging blocks: " << Pred->getName() << " <- "
<< Succ->getName() << "\n");
// Resolve any single entry PHI nodes in Succ.
while (PHINode *PN = dyn_cast<PHINode>(Succ->begin()))
ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist, L, LPM);
// Move all of the successor contents from Succ to Pred.
Pred->getInstList().splice(BI, Succ->getInstList(), Succ->begin(),
Succ->end());
LPM->deleteSimpleAnalysisValue(BI, L);
BI->eraseFromParent();
RemoveFromWorklist(BI, Worklist);
// If Succ has any successors with PHI nodes, update them to have
// entries coming from Pred instead of Succ.
Succ->replaceAllUsesWith(Pred);
// Remove Succ from the loop tree.
LI->removeBlock(Succ);
LPM->deleteSimpleAnalysisValue(Succ, L);
Succ->eraseFromParent();
++NumSimplify;
} else if (ConstantInt *CB = dyn_cast<ConstantInt>(BI->getCondition())){
// Conditional branch. Turn it into an unconditional branch, then
// remove dead blocks.
break; // FIXME: Enable.
DOUT << "Folded branch: " << *BI;
BasicBlock *DeadSucc = BI->getSuccessor(CB->getZExtValue());
BasicBlock *LiveSucc = BI->getSuccessor(!CB->getZExtValue());
DeadSucc->removePredecessor(BI->getParent(), true);
Worklist.push_back(BranchInst::Create(LiveSucc, BI));
LPM->deleteSimpleAnalysisValue(BI, L);
BI->eraseFromParent();
RemoveFromWorklist(BI, Worklist);
++NumSimplify;
RemoveBlockIfDead(DeadSucc, Worklist, L);
}
break;
}
}
}
}
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