//===-- LICM.cpp - Loop Invariant Code Motion Pass ------------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass performs loop invariant code motion, attempting to remove as much
// code from the body of a loop as possible. It does this by either hoisting
// code into the preheader block, or by sinking code to the exit blocks if it is
// safe. This pass also promotes must-aliased memory locations in the loop to
// live in registers, thus hoisting and sinking "invariant" loads and stores.
//
// This pass uses alias analysis for two purposes:
//
// 1. Moving loop invariant loads and calls out of loops. If we can determine
// that a load or call inside of a loop never aliases anything stored to,
// we can hoist it or sink it like any other instruction.
// 2. Scalar Promotion of Memory - If there is a store instruction inside of
// the loop, we try to move the store to happen AFTER the loop instead of
// inside of the loop. This can only happen if a few conditions are true:
// A. The pointer stored through is loop invariant
// B. There are no stores or loads in the loop which _may_ alias the
// pointer. There are no calls in the loop which mod/ref the pointer.
// If these conditions are true, we can promote the loads and stores in the
// loop of the pointer to use a temporary alloca'd variable. We then use
// the SSAUpdater to construct the appropriate SSA form for the value.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AliasSetTracker.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/PredIteratorCache.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetLibraryInfo.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
#include <algorithm>
using namespace llvm;
#define DEBUG_TYPE "licm"
STATISTIC(NumSunk , "Number of instructions sunk out of loop");
STATISTIC(NumHoisted , "Number of instructions hoisted out of loop");
STATISTIC(NumMovedLoads, "Number of load insts hoisted or sunk");
STATISTIC(NumMovedCalls, "Number of call insts hoisted or sunk");
STATISTIC(NumPromoted , "Number of memory locations promoted to registers");
static cl::opt<bool>
DisablePromotion("disable-licm-promotion", cl::Hidden,
cl::desc("Disable memory promotion in LICM pass"));
namespace {
struct LICM : public LoopPass {
static char ID; // Pass identification, replacement for typeid
LICM() : LoopPass(ID) {
initializeLICMPass(*PassRegistry::getPassRegistry());
}
bool runOnLoop(Loop *L, LPPassManager &LPM) override;
/// This transformation requires natural loop information & requires that
/// loop preheaders be inserted into the CFG...
///
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
AU.addRequired<DominatorTreeWrapperPass>();
AU.addRequired<LoopInfo>();
AU.addRequiredID(LoopSimplifyID);
AU.addPreservedID(LoopSimplifyID);
AU.addRequiredID(LCSSAID);
AU.addPreservedID(LCSSAID);
AU.addRequired<AliasAnalysis>();
AU.addPreserved<AliasAnalysis>();
AU.addPreserved<ScalarEvolution>();
AU.addRequired<TargetLibraryInfo>();
}
using llvm::Pass::doFinalization;
bool doFinalization() override {
assert(LoopToAliasSetMap.empty() && "Didn't free loop alias sets");
return false;
}
private:
AliasAnalysis *AA; // Current AliasAnalysis information
LoopInfo *LI; // Current LoopInfo
DominatorTree *DT; // Dominator Tree for the current Loop.
const DataLayout *DL; // DataLayout for constant folding.
TargetLibraryInfo *TLI; // TargetLibraryInfo for constant folding.
// State that is updated as we process loops.
bool Changed; // Set to true when we change anything.
BasicBlock *Preheader; // The preheader block of the current loop...
Loop *CurLoop; // The current loop we are working on...
AliasSetTracker *CurAST; // AliasSet information for the current loop...
bool MayThrow; // The current loop contains an instruction which
// may throw, thus preventing code motion of
// instructions with side effects.
DenseMap<Loop*, AliasSetTracker*> LoopToAliasSetMap;
/// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
void cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To,
Loop *L) override;
/// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
/// set.
void deleteAnalysisValue(Value *V, Loop *L) override;
/// SinkRegion - Walk the specified region of the CFG (defined by all blocks
/// dominated by the specified block, and that are in the current loop) in
/// reverse depth first order w.r.t the DominatorTree. This allows us to
/// visit uses before definitions, allowing us to sink a loop body in one
/// pass without iteration.
///
void SinkRegion(DomTreeNode *N);
/// HoistRegion - Walk the specified region of the CFG (defined by all
/// blocks dominated by the specified block, and that are in the current
/// loop) in depth first order w.r.t the DominatorTree. This allows us to
/// visit definitions before uses, allowing us to hoist a loop body in one
/// pass without iteration.
///
void HoistRegion(DomTreeNode *N);
/// inSubLoop - Little predicate that returns true if the specified basic
/// block is in a subloop of the current one, not the current one itself.
///
bool inSubLoop(BasicBlock *BB) {
assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
return LI->getLoopFor(BB) != CurLoop;
}
/// sink - When an instruction is found to only be used outside of the loop,
/// this function moves it to the exit blocks and patches up SSA form as
/// needed.
///
void sink(Instruction &I);
/// hoist - When an instruction is found to only use loop invariant operands
/// that is safe to hoist, this instruction is called to do the dirty work.
///
void hoist(Instruction &I);
/// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it
/// is not a trapping instruction or if it is a trapping instruction and is
/// guaranteed to execute.
///
bool isSafeToExecuteUnconditionally(Instruction &I);
/// isGuaranteedToExecute - Check that the instruction is guaranteed to
/// execute.
///
bool isGuaranteedToExecute(Instruction &I);
/// pointerInvalidatedByLoop - Return true if the body of this loop may
/// store into the memory location pointed to by V.
///
bool pointerInvalidatedByLoop(Value *V, uint64_t Size,
const MDNode *TBAAInfo) {
// Check to see if any of the basic blocks in CurLoop invalidate *V.
return CurAST->getAliasSetForPointer(V, Size, TBAAInfo).isMod();
}
bool canSinkOrHoistInst(Instruction &I);
bool isNotUsedInLoop(Instruction &I);
void PromoteAliasSet(AliasSet &AS,
SmallVectorImpl<BasicBlock*> &ExitBlocks,
SmallVectorImpl<Instruction*> &InsertPts,
PredIteratorCache &PIC);
/// \brief Create a copy of the instruction in the exit block and patch up
/// SSA.
/// PN is a user of I in ExitBlock that can be used to get the number and
/// list of predecessors fast.
Instruction *CloneInstructionInExitBlock(Instruction &I,
BasicBlock &ExitBlock,
PHINode &PN);
};
}
char LICM::ID = 0;
INITIALIZE_PASS_BEGIN(LICM, "licm", "Loop Invariant Code Motion", false, false)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LoopInfo)
INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
INITIALIZE_PASS_DEPENDENCY(LCSSA)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
INITIALIZE_PASS_END(LICM, "licm", "Loop Invariant Code Motion", false, false)
Pass *llvm::createLICMPass() { return new LICM(); }
/// Hoist expressions out of the specified loop. Note, alias info for inner
/// loop is not preserved so it is not a good idea to run LICM multiple
/// times on one loop.
///
bool LICM::runOnLoop(Loop *L, LPPassManager &LPM) {
if (skipOptnoneFunction(L))
return false;
Changed = false;
// Get our Loop and Alias Analysis information...
LI = &getAnalysis<LoopInfo>();
AA = &getAnalysis<AliasAnalysis>();
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
DL = DLP ? &DLP->getDataLayout() : nullptr;
TLI = &getAnalysis<TargetLibraryInfo>();
assert(L->isLCSSAForm(*DT) && "Loop is not in LCSSA form.");
CurAST = new AliasSetTracker(*AA);
// Collect Alias info from subloops.
for (Loop::iterator LoopItr = L->begin(), LoopItrE = L->end();
LoopItr != LoopItrE; ++LoopItr) {
Loop *InnerL = *LoopItr;
AliasSetTracker *InnerAST = LoopToAliasSetMap[InnerL];
assert(InnerAST && "Where is my AST?");
// What if InnerLoop was modified by other passes ?
CurAST->add(*InnerAST);
// Once we've incorporated the inner loop's AST into ours, we don't need the
// subloop's anymore.
delete InnerAST;
LoopToAliasSetMap.erase(InnerL);
}
CurLoop = L;
// Get the preheader block to move instructions into...
Preheader = L->getLoopPreheader();
// Loop over the body of this loop, looking for calls, invokes, and stores.
// Because subloops have already been incorporated into AST, we skip blocks in
// subloops.
//
for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
I != E; ++I) {
BasicBlock *BB = *I;
if (LI->getLoopFor(BB) == L) // Ignore blocks in subloops.
CurAST->add(*BB); // Incorporate the specified basic block
}
MayThrow = false;
// TODO: We've already searched for instructions which may throw in subloops.
// We may want to reuse this information.
for (Loop::block_iterator BB = L->block_begin(), BBE = L->block_end();
(BB != BBE) && !MayThrow ; ++BB)
for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end();
(I != E) && !MayThrow; ++I)
MayThrow |= I->mayThrow();
// We want to visit all of the instructions in this loop... that are not parts
// of our subloops (they have already had their invariants hoisted out of
// their loop, into this loop, so there is no need to process the BODIES of
// the subloops).
//
// Traverse the body of the loop in depth first order on the dominator tree so
// that we are guaranteed to see definitions before we see uses. This allows
// us to sink instructions in one pass, without iteration. After sinking
// instructions, we perform another pass to hoist them out of the loop.
//
if (L->hasDedicatedExits())
SinkRegion(DT->getNode(L->getHeader()));
if (Preheader)
HoistRegion(DT->getNode(L->getHeader()));
// Now that all loop invariants have been removed from the loop, promote any
// memory references to scalars that we can.
if (!DisablePromotion && (Preheader || L->hasDedicatedExits())) {
SmallVector<BasicBlock *, 8> ExitBlocks;
SmallVector<Instruction *, 8> InsertPts;
PredIteratorCache PIC;
// Loop over all of the alias sets in the tracker object.
for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
I != E; ++I)
PromoteAliasSet(*I, ExitBlocks, InsertPts, PIC);
// Once we have promoted values across the loop body we have to recursively
// reform LCSSA as any nested loop may now have values defined within the
// loop used in the outer loop.
// FIXME: This is really heavy handed. It would be a bit better to use an
// SSAUpdater strategy during promotion that was LCSSA aware and reformed
// it as it went.
if (Changed)
formLCSSARecursively(*L, *DT, getAnalysisIfAvailable<ScalarEvolution>());
}
// Check that neither this loop nor its parent have had LCSSA broken. LICM is
// specifically moving instructions across the loop boundary and so it is
// especially in need of sanity checking here.
assert(L->isLCSSAForm(*DT) && "Loop not left in LCSSA form after LICM!");
assert((!L->getParentLoop() || L->getParentLoop()->isLCSSAForm(*DT)) &&
"Parent loop not left in LCSSA form after LICM!");
// Clear out loops state information for the next iteration
CurLoop = nullptr;
Preheader = nullptr;
// If this loop is nested inside of another one, save the alias information
// for when we process the outer loop.
if (L->getParentLoop())
LoopToAliasSetMap[L] = CurAST;
else
delete CurAST;
return Changed;
}
/// SinkRegion - Walk the specified region of the CFG (defined by all blocks
/// dominated by the specified block, and that are in the current loop) in
/// reverse depth first order w.r.t the DominatorTree. This allows us to visit
/// uses before definitions, allowing us to sink a loop body in one pass without
/// iteration.
///
void LICM::SinkRegion(DomTreeNode *N) {
assert(N != nullptr && "Null dominator tree node?");
BasicBlock *BB = N->getBlock();
// If this subregion is not in the top level loop at all, exit.
if (!CurLoop->contains(BB)) return;
// We are processing blocks in reverse dfo, so process children first.
const std::vector<DomTreeNode*> &Children = N->getChildren();
for (unsigned i = 0, e = Children.size(); i != e; ++i)
SinkRegion(Children[i]);
// Only need to process the contents of this block if it is not part of a
// subloop (which would already have been processed).
if (inSubLoop(BB)) return;
for (BasicBlock::iterator II = BB->end(); II != BB->begin(); ) {
Instruction &I = *--II;
// If the instruction is dead, we would try to sink it because it isn't used
// in the loop, instead, just delete it.
if (isInstructionTriviallyDead(&I, TLI)) {
DEBUG(dbgs() << "LICM deleting dead inst: " << I << '\n');
++II;
CurAST->deleteValue(&I);
I.eraseFromParent();
Changed = true;
continue;
}
// Check to see if we can sink this instruction to the exit blocks
// of the loop. We can do this if the all users of the instruction are
// outside of the loop. In this case, it doesn't even matter if the
// operands of the instruction are loop invariant.
//
if (isNotUsedInLoop(I) && canSinkOrHoistInst(I)) {
++II;
sink(I);
}
}
}
/// HoistRegion - Walk the specified region of the CFG (defined by all blocks
/// dominated by the specified block, and that are in the current loop) in depth
/// first order w.r.t the DominatorTree. This allows us to visit definitions
/// before uses, allowing us to hoist a loop body in one pass without iteration.
///
void LICM::HoistRegion(DomTreeNode *N) {
assert(N != nullptr && "Null dominator tree node?");
BasicBlock *BB = N->getBlock();
// If this subregion is not in the top level loop at all, exit.
if (!CurLoop->contains(BB)) return;
// Only need to process the contents of this block if it is not part of a
// subloop (which would already have been processed).
if (!inSubLoop(BB))
for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ) {
Instruction &I = *II++;
// Try constant folding this instruction. If all the operands are
// constants, it is technically hoistable, but it would be better to just
// fold it.
if (Constant *C = ConstantFoldInstruction(&I, DL, TLI)) {
DEBUG(dbgs() << "LICM folding inst: " << I << " --> " << *C << '\n');
CurAST->copyValue(&I, C);
CurAST->deleteValue(&I);
I.replaceAllUsesWith(C);
I.eraseFromParent();
continue;
}
// Try hoisting the instruction out to the preheader. We can only do this
// if all of the operands of the instruction are loop invariant and if it
// is safe to hoist the instruction.
//
if (CurLoop->hasLoopInvariantOperands(&I) && canSinkOrHoistInst(I) &&
isSafeToExecuteUnconditionally(I))
hoist(I);
}
const std::vector<DomTreeNode*> &Children = N->getChildren();
for (unsigned i = 0, e = Children.size(); i != e; ++i)
HoistRegion(Children[i]);
}
/// canSinkOrHoistInst - Return true if the hoister and sinker can handle this
/// instruction.
///
bool LICM::canSinkOrHoistInst(Instruction &I) {
// Loads have extra constraints we have to verify before we can hoist them.
if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
if (!LI->isUnordered())
return false; // Don't hoist volatile/atomic loads!
// Loads from constant memory are always safe to move, even if they end up
// in the same alias set as something that ends up being modified.
if (AA->pointsToConstantMemory(LI->getOperand(0)))
return true;
if (LI->getMetadata("invariant.load"))
return true;
// Don't hoist loads which have may-aliased stores in loop.
uint64_t Size = 0;
if (LI->getType()->isSized())
Size = AA->getTypeStoreSize(LI->getType());
return !pointerInvalidatedByLoop(LI->getOperand(0), Size,
LI->getMetadata(LLVMContext::MD_tbaa));
} else if (CallInst *CI = dyn_cast<CallInst>(&I)) {
// Don't sink or hoist dbg info; it's legal, but not useful.
if (isa<DbgInfoIntrinsic>(I))
return false;
// Handle simple cases by querying alias analysis.
AliasAnalysis::ModRefBehavior Behavior = AA->getModRefBehavior(CI);
if (Behavior == AliasAnalysis::DoesNotAccessMemory)
return true;
if (AliasAnalysis::onlyReadsMemory(Behavior)) {
// If this call only reads from memory and there are no writes to memory
// in the loop, we can hoist or sink the call as appropriate.
bool FoundMod = false;
for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
I != E; ++I) {
AliasSet &AS = *I;
if (!AS.isForwardingAliasSet() && AS.isMod()) {
FoundMod = true;
break;
}
}
if (!FoundMod) return true;
}
// FIXME: This should use mod/ref information to see if we can hoist or
// sink the call.
return false;
}
// Only these instructions are hoistable/sinkable.
if (!isa<BinaryOperator>(I) && !isa<CastInst>(I) && !isa<SelectInst>(I) &&
!isa<GetElementPtrInst>(I) && !isa<CmpInst>(I) &&
!isa<InsertElementInst>(I) && !isa<ExtractElementInst>(I) &&
!isa<ShuffleVectorInst>(I) && !isa<ExtractValueInst>(I) &&
!isa<InsertValueInst>(I))
return false;
return isSafeToExecuteUnconditionally(I);
}
/// \brief Returns true if a PHINode is a trivially replaceable with an
/// Instruction.
///
/// This is true when all incoming values are that instruction. This pattern
/// occurs most often with LCSSA PHI nodes.
static bool isTriviallyReplacablePHI(PHINode &PN, Instruction &I) {
for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i)
if (PN.getIncomingValue(i) != &I)
return false;
return true;
}
/// isNotUsedInLoop - Return true if the only users of this instruction are
/// outside of the loop. If this is true, we can sink the instruction to the
/// exit blocks of the loop.
///
bool LICM::isNotUsedInLoop(Instruction &I) {
for (User *U : I.users()) {
Instruction *UI = cast<Instruction>(U);
if (PHINode *PN = dyn_cast<PHINode>(UI)) {
// A PHI node where all of the incoming values are this instruction are
// special -- they can just be RAUW'ed with the instruction and thus
// don't require a use in the predecessor. This is a particular important
// special case because it is the pattern found in LCSSA form.
if (isTriviallyReplacablePHI(*PN, I)) {
if (CurLoop->contains(PN))
return false;
else
continue;
}
// Otherwise, PHI node uses occur in predecessor blocks if the incoming
// values. Check for such a use being inside the loop.
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
if (PN->getIncomingValue(i) == &I)
if (CurLoop->contains(PN->getIncomingBlock(i)))
return false;
continue;
}
if (CurLoop->contains(UI))
return false;
}
return true;
}
Instruction *LICM::CloneInstructionInExitBlock(Instruction &I,
BasicBlock &ExitBlock,
PHINode &PN) {
Instruction *New = I.clone();
ExitBlock.getInstList().insert(ExitBlock.getFirstInsertionPt(), New);
if (!I.getName().empty()) New->setName(I.getName() + ".le");
// Build LCSSA PHI nodes for any in-loop operands. Note that this is
// particularly cheap because we can rip off the PHI node that we're
// replacing for the number and blocks of the predecessors.
// OPT: If this shows up in a profile, we can instead finish sinking all
// invariant instructions, and then walk their operands to re-establish
// LCSSA. That will eliminate creating PHI nodes just to nuke them when
// sinking bottom-up.
for (User::op_iterator OI = New->op_begin(), OE = New->op_end(); OI != OE;
++OI)
if (Instruction *OInst = dyn_cast<Instruction>(*OI))
if (Loop *OLoop = LI->getLoopFor(OInst->getParent()))
if (!OLoop->contains(&PN)) {
PHINode *OpPN =
PHINode::Create(OInst->getType(), PN.getNumIncomingValues(),
OInst->getName() + ".lcssa", ExitBlock.begin());
for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i)
OpPN->addIncoming(OInst, PN.getIncomingBlock(i));
*OI = OpPN;
}
return New;
}
/// sink - When an instruction is found to only be used outside of the loop,
/// this function moves it to the exit blocks and patches up SSA form as needed.
/// This method is guaranteed to remove the original instruction from its
/// position, and may either delete it or move it to outside of the loop.
///
void LICM::sink(Instruction &I) {
DEBUG(dbgs() << "LICM sinking instruction: " << I << "\n");
if (isa<LoadInst>(I)) ++NumMovedLoads;
else if (isa<CallInst>(I)) ++NumMovedCalls;
++NumSunk;
Changed = true;
#ifndef NDEBUG
SmallVector<BasicBlock *, 32> ExitBlocks;
CurLoop->getUniqueExitBlocks(ExitBlocks);
SmallPtrSet<BasicBlock *, 32> ExitBlockSet(ExitBlocks.begin(), ExitBlocks.end());
#endif
// Clones of this instruction. Don't create more than one per exit block!
SmallDenseMap<BasicBlock *, Instruction *, 32> SunkCopies;
// If this instruction is only used outside of the loop, then all users are
// PHI nodes in exit blocks due to LCSSA form. Just RAUW them with clones of
// the instruction.
while (!I.use_empty()) {
// The user must be a PHI node.
PHINode *PN = cast<PHINode>(I.user_back());
BasicBlock *ExitBlock = PN->getParent();
assert(ExitBlockSet.count(ExitBlock) &&
"The LCSSA PHI is not in an exit block!");
Instruction *New;
auto It = SunkCopies.find(ExitBlock);
if (It != SunkCopies.end())
New = It->second;
else
New = SunkCopies[ExitBlock] =
CloneInstructionInExitBlock(I, *ExitBlock, *PN);
PN->replaceAllUsesWith(New);
PN->eraseFromParent();
}
CurAST->deleteValue(&I);
I.eraseFromParent();
}
/// hoist - When an instruction is found to only use loop invariant operands
/// that is safe to hoist, this instruction is called to do the dirty work.
///
void LICM::hoist(Instruction &I) {
DEBUG(dbgs() << "LICM hoisting to " << Preheader->getName() << ": "
<< I << "\n");
// Move the new node to the Preheader, before its terminator.
I.moveBefore(Preheader->getTerminator());
if (isa<LoadInst>(I)) ++NumMovedLoads;
else if (isa<CallInst>(I)) ++NumMovedCalls;
++NumHoisted;
Changed = true;
}
/// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it is
/// not a trapping instruction or if it is a trapping instruction and is
/// guaranteed to execute.
///
bool LICM::isSafeToExecuteUnconditionally(Instruction &Inst) {
// If it is not a trapping instruction, it is always safe to hoist.
if (isSafeToSpeculativelyExecute(&Inst, DL))
return true;
return isGuaranteedToExecute(Inst);
}
bool LICM::isGuaranteedToExecute(Instruction &Inst) {
// Somewhere in this loop there is an instruction which may throw and make us
// exit the loop.
if (MayThrow)
return false;
// Otherwise we have to check to make sure that the instruction dominates all
// of the exit blocks. If it doesn't, then there is a path out of the loop
// which does not execute this instruction, so we can't hoist it.
// If the instruction is in the header block for the loop (which is very
// common), it is always guaranteed to dominate the exit blocks. Since this
// is a common case, and can save some work, check it now.
if (Inst.getParent() == CurLoop->getHeader())
return true;
// Get the exit blocks for the current loop.
SmallVector<BasicBlock*, 8> ExitBlocks;
CurLoop->getExitBlocks(ExitBlocks);
// Verify that the block dominates each of the exit blocks of the loop.
for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
if (!DT->dominates(Inst.getParent(), ExitBlocks[i]))
return false;
// As a degenerate case, if the loop is statically infinite then we haven't
// proven anything since there are no exit blocks.
if (ExitBlocks.empty())
return false;
return true;
}
namespace {
class LoopPromoter : public LoadAndStorePromoter {
Value *SomePtr; // Designated pointer to store to.
SmallPtrSet<Value*, 4> &PointerMustAliases;
SmallVectorImpl<BasicBlock*> &LoopExitBlocks;
SmallVectorImpl<Instruction*> &LoopInsertPts;
PredIteratorCache &PredCache;
AliasSetTracker &AST;
LoopInfo &LI;
DebugLoc DL;
int Alignment;
MDNode *TBAATag;
Value *maybeInsertLCSSAPHI(Value *V, BasicBlock *BB) const {
if (Instruction *I = dyn_cast<Instruction>(V))
if (Loop *L = LI.getLoopFor(I->getParent()))
if (!L->contains(BB)) {
// We need to create an LCSSA PHI node for the incoming value and
// store that.
PHINode *PN = PHINode::Create(
I->getType(), PredCache.GetNumPreds(BB),
I->getName() + ".lcssa", BB->begin());
for (BasicBlock **PI = PredCache.GetPreds(BB); *PI; ++PI)
PN->addIncoming(I, *PI);
return PN;
}
return V;
}
public:
LoopPromoter(Value *SP, const SmallVectorImpl<Instruction *> &Insts,
SSAUpdater &S, SmallPtrSet<Value *, 4> &PMA,
SmallVectorImpl<BasicBlock *> &LEB,
SmallVectorImpl<Instruction *> &LIP, PredIteratorCache &PIC,
AliasSetTracker &ast, LoopInfo &li, DebugLoc dl, int alignment,
MDNode *TBAATag)
: LoadAndStorePromoter(Insts, S), SomePtr(SP), PointerMustAliases(PMA),
LoopExitBlocks(LEB), LoopInsertPts(LIP), PredCache(PIC), AST(ast),
LI(li), DL(dl), Alignment(alignment), TBAATag(TBAATag) {}
bool isInstInList(Instruction *I,
const SmallVectorImpl<Instruction*> &) const override {
Value *Ptr;
if (LoadInst *LI = dyn_cast<LoadInst>(I))
Ptr = LI->getOperand(0);
else
Ptr = cast<StoreInst>(I)->getPointerOperand();
return PointerMustAliases.count(Ptr);
}
void doExtraRewritesBeforeFinalDeletion() const override {
// Insert stores after in the loop exit blocks. Each exit block gets a
// store of the live-out values that feed them. Since we've already told
// the SSA updater about the defs in the loop and the preheader
// definition, it is all set and we can start using it.
for (unsigned i = 0, e = LoopExitBlocks.size(); i != e; ++i) {
BasicBlock *ExitBlock = LoopExitBlocks[i];
Value *LiveInValue = SSA.GetValueInMiddleOfBlock(ExitBlock);
LiveInValue = maybeInsertLCSSAPHI(LiveInValue, ExitBlock);
Value *Ptr = maybeInsertLCSSAPHI(SomePtr, ExitBlock);
Instruction *InsertPos = LoopInsertPts[i];
StoreInst *NewSI = new StoreInst(LiveInValue, Ptr, InsertPos);
NewSI->setAlignment(Alignment);
NewSI->setDebugLoc(DL);
if (TBAATag) NewSI->setMetadata(LLVMContext::MD_tbaa, TBAATag);
}
}
void replaceLoadWithValue(LoadInst *LI, Value *V) const override {
// Update alias analysis.
AST.copyValue(LI, V);
}
void instructionDeleted(Instruction *I) const override {
AST.deleteValue(I);
}
};
} // end anon namespace
/// PromoteAliasSet - Try to promote memory values to scalars by sinking
/// stores out of the loop and moving loads to before the loop. We do this by
/// looping over the stores in the loop, looking for stores to Must pointers
/// which are loop invariant.
///
void LICM::PromoteAliasSet(AliasSet &AS,
SmallVectorImpl<BasicBlock*> &ExitBlocks,
SmallVectorImpl<Instruction*> &InsertPts,
PredIteratorCache &PIC) {
// We can promote this alias set if it has a store, if it is a "Must" alias
// set, if the pointer is loop invariant, and if we are not eliminating any
// volatile loads or stores.
if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() ||
AS.isVolatile() || !CurLoop->isLoopInvariant(AS.begin()->getValue()))
return;
assert(!AS.empty() &&
"Must alias set should have at least one pointer element in it!");
Value *SomePtr = AS.begin()->getValue();
// It isn't safe to promote a load/store from the loop if the load/store is
// conditional. For example, turning:
//
// for () { if (c) *P += 1; }
//
// into:
//
// tmp = *P; for () { if (c) tmp +=1; } *P = tmp;
//
// is not safe, because *P may only be valid to access if 'c' is true.
//
// It is safe to promote P if all uses are direct load/stores and if at
// least one is guaranteed to be executed.
bool GuaranteedToExecute = false;
SmallVector<Instruction*, 64> LoopUses;
SmallPtrSet<Value*, 4> PointerMustAliases;
// We start with an alignment of one and try to find instructions that allow
// us to prove better alignment.
unsigned Alignment = 1;
MDNode *TBAATag = nullptr;
// Check that all of the pointers in the alias set have the same type. We
// cannot (yet) promote a memory location that is loaded and stored in
// different sizes. While we are at it, collect alignment and TBAA info.
for (AliasSet::iterator ASI = AS.begin(), E = AS.end(); ASI != E; ++ASI) {
Value *ASIV = ASI->getValue();
PointerMustAliases.insert(ASIV);
// Check that all of the pointers in the alias set have the same type. We
// cannot (yet) promote a memory location that is loaded and stored in
// different sizes.
if (SomePtr->getType() != ASIV->getType())
return;
for (User *U : ASIV->users()) {
// Ignore instructions that are outside the loop.
Instruction *UI = dyn_cast<Instruction>(U);
if (!UI || !CurLoop->contains(UI))
continue;
// If there is an non-load/store instruction in the loop, we can't promote
// it.
if (LoadInst *load = dyn_cast<LoadInst>(UI)) {
assert(!load->isVolatile() && "AST broken");
if (!load->isSimple())
return;
} else if (StoreInst *store = dyn_cast<StoreInst>(UI)) {
// Stores *of* the pointer are not interesting, only stores *to* the
// pointer.
if (UI->getOperand(1) != ASIV)
continue;
assert(!store->isVolatile() && "AST broken");
if (!store->isSimple())
return;
// Note that we only check GuaranteedToExecute inside the store case
// so that we do not introduce stores where they did not exist before
// (which would break the LLVM concurrency model).
// If the alignment of this instruction allows us to specify a more
// restrictive (and performant) alignment and if we are sure this
// instruction will be executed, update the alignment.
// Larger is better, with the exception of 0 being the best alignment.
unsigned InstAlignment = store->getAlignment();
if ((InstAlignment > Alignment || InstAlignment == 0) && Alignment != 0)
if (isGuaranteedToExecute(*UI)) {
GuaranteedToExecute = true;
Alignment = InstAlignment;
}
if (!GuaranteedToExecute)
GuaranteedToExecute = isGuaranteedToExecute(*UI);
} else
return; // Not a load or store.
// Merge the TBAA tags.
if (LoopUses.empty()) {
// On the first load/store, just take its TBAA tag.
TBAATag = UI->getMetadata(LLVMContext::MD_tbaa);
} else if (TBAATag) {
TBAATag = MDNode::getMostGenericTBAA(TBAATag,
UI->getMetadata(LLVMContext::MD_tbaa));
}
LoopUses.push_back(UI);
}
}
// If there isn't a guaranteed-to-execute instruction, we can't promote.
if (!GuaranteedToExecute)
return;
// Otherwise, this is safe to promote, lets do it!
DEBUG(dbgs() << "LICM: Promoting value stored to in loop: " <<*SomePtr<<'\n');
Changed = true;
++NumPromoted;
// Grab a debug location for the inserted loads/stores; given that the
// inserted loads/stores have little relation to the original loads/stores,
// this code just arbitrarily picks a location from one, since any debug
// location is better than none.
DebugLoc DL = LoopUses[0]->getDebugLoc();
// Figure out the loop exits and their insertion points, if this is the
// first promotion.
if (ExitBlocks.empty()) {
CurLoop->getUniqueExitBlocks(ExitBlocks);
InsertPts.resize(ExitBlocks.size());
for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
InsertPts[i] = ExitBlocks[i]->getFirstInsertionPt();
}
// We use the SSAUpdater interface to insert phi nodes as required.
SmallVector<PHINode*, 16> NewPHIs;
SSAUpdater SSA(&NewPHIs);
LoopPromoter Promoter(SomePtr, LoopUses, SSA, PointerMustAliases, ExitBlocks,
InsertPts, PIC, *CurAST, *LI, DL, Alignment, TBAATag);
// Set up the preheader to have a definition of the value. It is the live-out
// value from the preheader that uses in the loop will use.
LoadInst *PreheaderLoad =
new LoadInst(SomePtr, SomePtr->getName()+".promoted",
Preheader->getTerminator());
PreheaderLoad->setAlignment(Alignment);
PreheaderLoad->setDebugLoc(DL);
if (TBAATag) PreheaderLoad->setMetadata(LLVMContext::MD_tbaa, TBAATag);
SSA.AddAvailableValue(Preheader, PreheaderLoad);
// Rewrite all the loads in the loop and remember all the definitions from
// stores in the loop.
Promoter.run(LoopUses);
// If the SSAUpdater didn't use the load in the preheader, just zap it now.
if (PreheaderLoad->use_empty())
PreheaderLoad->eraseFromParent();
}
/// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
void LICM::cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L) {
AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);
if (!AST)
return;
AST->copyValue(From, To);
}
/// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
/// set.
void LICM::deleteAnalysisValue(Value *V, Loop *L) {
AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);
if (!AST)
return;
AST->deleteValue(V);
}