//===-- LICM.cpp - Loop Invariant Code Motion Pass ------------------------===//
//
-// This pass is a simple loop invariant code motion pass.
+// The LLVM Compiler Infrastructure
//
-// Note that this pass does NOT require pre-headers to exist on loops in the
-// CFG, but if there is not distinct preheader for a loop, the hoisted code will
-// be *DUPLICATED* in every basic block, outside of the loop, that preceeds the
-// loop header. Additionally, any use of one of these hoisted expressions
-// cannot be loop invariant itself, because the expression hoisted gets a PHI
-// node that is loop variant.
+// This file was developed by the LLVM research group and is distributed under
+// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
-// For these reasons, and many more, it makes sense to run a pass before this
-// that ensures that there are preheaders on all loops. That said, we don't
-// REQUIRE it. :)
+//===----------------------------------------------------------------------===//
+//
+// 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 mem2reg functionality to construct the appropriate SSA form for the
+// variable.
//
//===----------------------------------------------------------------------===//
+#define DEBUG_TYPE "licm"
#include "llvm/Transforms/Scalar.h"
-#include "llvm/Transforms/Utils/Local.h"
+#include "llvm/Constants.h"
+#include "llvm/DerivedTypes.h"
+#include "llvm/Instructions.h"
+#include "llvm/Target/TargetData.h"
#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/AliasAnalysis.h"
-#include "llvm/iOperators.h"
-#include "llvm/iPHINode.h"
-#include "llvm/iMemory.h"
-#include "llvm/Support/InstVisitor.h"
+#include "llvm/Analysis/AliasSetTracker.h"
+#include "llvm/Analysis/Dominators.h"
+#include "llvm/Transforms/Utils/PromoteMemToReg.h"
#include "llvm/Support/CFG.h"
-#include "Support/STLExtras.h"
-#include "Support/StatisticReporter.h"
+#include "llvm/Support/Compiler.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/ADT/Statistic.h"
#include <algorithm>
-using std::string;
+using namespace llvm;
-static Statistic<> NumHoistedNPH("licm\t\t- Number of insts hoisted to multiple"
- " loop preds (bad, no loop pre-header)");
-static Statistic<> NumHoistedPH("licm\t\t- Number of insts hoisted to a loop "
- "pre-header");
-static Statistic<> NumHoistedLoads("licm\t\t- Number of load insts hoisted");
+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");
namespace {
- struct LICM : public FunctionPass, public InstVisitor<LICM> {
- virtual bool runOnFunction(Function &F);
+ cl::opt<bool>
+ DisablePromotion("disable-licm-promotion", cl::Hidden,
+ cl::desc("Disable memory promotion in LICM pass"));
- // This transformation requires natural loop information...
+ struct VISIBILITY_HIDDEN LICM : public LoopPass {
+ virtual bool runOnLoop(Loop *L, LPPassManager &LPM);
+
+ /// This transformation requires natural loop information & requires that
+ /// loop preheaders be inserted into the CFG...
+ ///
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
- AU.preservesCFG();
+ AU.setPreservesCFG();
+ AU.addRequiredID(LoopSimplifyID);
AU.addRequired<LoopInfo>();
+ AU.addRequired<ETForest>();
+ AU.addRequired<DominanceFrontier>(); // For scalar promotion (mem2reg)
AU.addRequired<AliasAnalysis>();
}
- private:
- // List of predecessor blocks for the current loop - These blocks are where
- // we hoist loop invariants to for the current loop.
- //
- std::vector<BasicBlock*> LoopPreds, LoopBackEdges;
-
- Loop *CurLoop; // The current loop we are working on...
- bool Changed; // Set to true when we change anything.
- AliasAnalysis *AA; // Currently AliasAnalysis information
-
- // visitLoop - Hoist expressions out of the specified loop...
- void visitLoop(Loop *L);
+ bool doFinalization() {
+ LoopToAliasMap.clear();
+ return false;
+ }
- // notInCurrentLoop - Little predicate that returns true if the specified
- // basic block is in a subloop of the current one, not the current one
- // itself.
- //
- bool notInCurrentLoop(BasicBlock *BB) {
- for (unsigned i = 0, e = CurLoop->getSubLoops().size(); i != e; ++i)
- if (CurLoop->getSubLoops()[i]->contains(BB))
+ private:
+ // Various analyses that we use...
+ AliasAnalysis *AA; // Current AliasAnalysis information
+ LoopInfo *LI; // Current LoopInfo
+ ETForest *ET; // ETForest for the current Loop...
+ DominanceFrontier *DF; // Current Dominance Frontier
+
+ // 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...
+ std::map<Loop *, AliasSetTracker *> LoopToAliasMap;
+
+ /// 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 ETForest. This allows us to
+ /// visit uses before definitions, allowing us to sink a loop body in one
+ /// pass without iteration.
+ ///
+ void SinkRegion(BasicBlock *BB);
+
+ /// 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 ETForest. This allows us to
+ /// visit definitions before uses, allowing us to hoist a loop body in one
+ /// pass without iteration.
+ ///
+ void HoistRegion(BasicBlock *BB);
+
+ /// 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");
+ for (Loop::iterator I = CurLoop->begin(), E = CurLoop->end(); I != E; ++I)
+ if ((*I)->contains(BB))
return true; // A subloop actually contains this block!
- return false;
+ return false;
}
- // 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);
-
- // pointerInvalidatedByLoop - Return true if the body of this loop may store
- // into the memory location pointed to by V.
- //
- bool pointerInvalidatedByLoop(Value *V);
+ /// isExitBlockDominatedByBlockInLoop - This method checks to see if the
+ /// specified exit block of the loop is dominated by the specified block
+ /// that is in the body of the loop. We use these constraints to
+ /// dramatically limit the amount of the dominator tree that needs to be
+ /// searched.
+ bool isExitBlockDominatedByBlockInLoop(BasicBlock *ExitBlock,
+ BasicBlock *BlockInLoop) const {
+ // If the block in the loop is the loop header, it must be dominated!
+ BasicBlock *LoopHeader = CurLoop->getHeader();
+ if (BlockInLoop == LoopHeader)
+ return true;
+
+ BasicBlock *IDom = ExitBlock;
+
+ // Because the exit block is not in the loop, we know we have to get _at
+ // least_ its immediate dominator.
+ do {
+ // Get next Immediate Dominator.
+ IDom = ET->getIDom(IDom);
+
+ // If we have got to the header of the loop, then the instructions block
+ // did not dominate the exit node, so we can't hoist it.
+ if (IDom == LoopHeader)
+ return false;
+
+ } while (IDom != BlockInLoop);
- // isLoopInvariant - Return true if the specified value is loop invariant
- inline bool isLoopInvariant(Value *V) {
- if (Instruction *I = dyn_cast<Instruction>(V))
- return !CurLoop->contains(I->getParent());
- return true; // All non-instructions are loop invariant
+ return true;
}
- // visitBasicBlock - Run LICM on a particular block.
- void visitBasicBlock(BasicBlock *BB);
+ /// 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);
- // Instruction visitation handlers... these basically control whether or not
- // the specified instruction types are hoisted.
- //
- friend class InstVisitor<LICM>;
- void visitBinaryOperator(Instruction &I) {
- if (isLoopInvariant(I.getOperand(0)) && isLoopInvariant(I.getOperand(1)))
- hoist(I);
- }
- void visitCastInst(CastInst &CI) {
- Instruction &I = (Instruction&)CI;
- if (isLoopInvariant(I.getOperand(0))) hoist(I);
- }
- void visitShiftInst(ShiftInst &I) { visitBinaryOperator((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);
- void visitLoadInst(LoadInst &LI) {
- if (isLoopInvariant(LI.getOperand(0)) &&
- !pointerInvalidatedByLoop(LI.getOperand(0)))
- hoist(LI);
+ /// 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);
+
+ /// pointerInvalidatedByLoop - Return true if the body of this loop may
+ /// store into the memory location pointed to by V.
+ ///
+ bool pointerInvalidatedByLoop(Value *V, unsigned Size) {
+ // Check to see if any of the basic blocks in CurLoop invalidate *V.
+ return CurAST->getAliasSetForPointer(V, Size).isMod();
}
- void visitGetElementPtrInst(GetElementPtrInst &GEPI) {
- Instruction &I = (Instruction&)GEPI;
- for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i)
- if (!isLoopInvariant(I.getOperand(i))) return;
- hoist(I);
- }
+ bool canSinkOrHoistInst(Instruction &I);
+ bool isLoopInvariantInst(Instruction &I);
+ bool isNotUsedInLoop(Instruction &I);
+
+ /// PromoteValuesInLoop - Look at the stores in the loop and promote as many
+ /// to scalars as we can.
+ ///
+ void PromoteValuesInLoop();
+
+ /// FindPromotableValuesInLoop - Check the current loop for stores to
+ /// definite pointers, which are not loaded and stored through may aliases.
+ /// If these are found, create an alloca for the value, add it to the
+ /// PromotedValues list, and keep track of the mapping from value to
+ /// alloca...
+ ///
+ void FindPromotableValuesInLoop(
+ std::vector<std::pair<AllocaInst*, Value*> > &PromotedValues,
+ std::map<Value*, AllocaInst*> &Val2AlMap);
};
- RegisterOpt<LICM> X("licm", "Loop Invariant Code Motion");
+ RegisterPass<LICM> X("licm", "Loop Invariant Code Motion");
}
-Pass *createLICMPass() { return new LICM(); }
-
-bool LICM::runOnFunction(Function &) {
- // get our loop information...
- const std::vector<Loop*> &TopLevelLoops =
- getAnalysis<LoopInfo>().getTopLevelLoops();
-
- // Get our alias analysis information...
- AA = &getAnalysis<AliasAnalysis>();
+LoopPass *llvm::createLICMPass() { return new LICM(); }
- // Traverse loops in postorder, hoisting expressions out of the deepest loops
- // first.
- //
+/// Hoist expressions out of the specified loop...
+///
+bool LICM::runOnLoop(Loop *L, LPPassManager &LPM) {
Changed = false;
- std::for_each(TopLevelLoops.begin(), TopLevelLoops.end(),
- bind_obj(this, &LICM::visitLoop));
- return Changed;
-}
-void LICM::visitLoop(Loop *L) {
- // Recurse through all subloops before we process this loop...
- std::for_each(L->getSubLoops().begin(), L->getSubLoops().end(),
- bind_obj(this, &LICM::visitLoop));
+ // Get our Loop and Alias Analysis information...
+ LI = &getAnalysis<LoopInfo>();
+ AA = &getAnalysis<AliasAnalysis>();
+ DF = &getAnalysis<DominanceFrontier>();
+ ET = &getAnalysis<ETForest>();
+
+ CurAST = new AliasSetTracker(*AA);
+ // Collect Alias info frmo subloops
+ for (Loop::iterator LoopItr = L->begin(), LoopItrE = L->end();
+ LoopItr != LoopItrE; ++LoopItr) {
+ Loop *InnerL = *LoopItr;
+ AliasSetTracker *InnerAST = LoopToAliasMap[InnerL];
+ assert (InnerAST && "Where is my AST?");
+
+ // What if InnerLoop was modified by other passes ?
+ CurAST->add(*InnerAST);
+ }
+
CurLoop = L;
- // Calculate the set of predecessors for this loop. The predecessors for this
- // loop are equal to the predecessors for the header node of the loop that are
- // not themselves in the loop.
- //
- BasicBlock *Header = L->getHeader();
+ // Get the preheader block to move instructions into...
+ Preheader = L->getLoopPreheader();
+ assert(Preheader&&"Preheader insertion pass guarantees we have a preheader!");
- // Calculate the sets of predecessors and backedges of the loop...
- LoopBackEdges.insert(LoopBackEdges.end(),pred_begin(Header),pred_end(Header));
-
- std::vector<BasicBlock*>::iterator LPI =
- std::partition(LoopBackEdges.begin(), LoopBackEdges.end(),
- bind_obj(CurLoop, &Loop::contains));
-
- // Move all predecessors to the LoopPreds vector...
- LoopPreds.insert(LoopPreds.end(), LPI, LoopBackEdges.end());
-
- // Remove predecessors from backedges list...
- LoopBackEdges.erase(LPI, LoopBackEdges.end());
-
-
- // The only way that there could be no predecessors to a loop is if the loop
- // is not reachable. Since we don't care about optimizing dead loops,
- // summarily ignore them.
+ // 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.
//
- if (LoopPreds.empty()) return;
-
+ for (std::vector<BasicBlock*>::const_iterator I = L->getBlocks().begin(),
+ E = L->getBlocks().end(); I != E; ++I)
+ if (LI->getLoopFor(*I) == L) // Ignore blocks in subloops...
+ CurAST->add(**I); // Incorporate the specified basic block
+
// 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).
//
- std::vector<BasicBlock*> BBs(L->getBlocks().begin(), L->getBlocks().end());
-
- // Remove blocks that are actually in subloops...
- BBs.erase(std::remove_if(BBs.begin(), BBs.end(),
- bind_obj(this, &LICM::notInCurrentLoop)), BBs.end());
-
- // Visit all of the basic blocks we have chosen, hoisting out the instructions
- // as neccesary. This leaves dead copies of the instruction in the loop
- // unfortunately...
+ // 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.
//
- for_each(BBs.begin(), BBs.end(), bind_obj(this, &LICM::visitBasicBlock));
+ SinkRegion(L->getHeader());
+ HoistRegion(L->getHeader());
+
+ // Now that all loop invariants have been removed from the loop, promote any
+ // memory references to scalars that we can...
+ if (!DisablePromotion)
+ PromoteValuesInLoop();
// Clear out loops state information for the next iteration
CurLoop = 0;
- LoopPreds.clear();
- LoopBackEdges.clear();
+ Preheader = 0;
+
+ LoopToAliasMap[L] = CurAST;
+ return Changed;
}
-void LICM::visitBasicBlock(BasicBlock *BB) {
- for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
- visit(*I);
-
- if (dceInstruction(I))
- Changed = true;
- else
- ++I;
+/// 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 ETForest. This allows us to visit
+/// uses before definitions, allowing us to sink a loop body in one pass without
+/// iteration.
+///
+void LICM::SinkRegion(BasicBlock *BB) {
+ assert(BB != 0 && "Null sink block?");
+
+ // 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...
+ std::vector<BasicBlock*> Children;
+ ET->getChildren(BB, Children);
+ 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;
+
+ // 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);
+ }
}
}
-void LICM::hoist(Instruction &Inst) {
- if (Inst.use_empty()) return; // Don't (re) hoist dead instructions!
- //cerr << "Hoisting " << Inst;
+/// 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 ETForest. This allows us to visit definitions
+/// before uses, allowing us to hoist a loop body in one pass without iteration.
+///
+void LICM::HoistRegion(BasicBlock *BB) {
+ assert(BB != 0 && "Null hoist block?");
+
+ // 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 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 (isLoopInvariantInst(I) && canSinkOrHoistInst(I) &&
+ isSafeToExecuteUnconditionally(I))
+ hoist(I);
+ }
- BasicBlock *Header = CurLoop->getHeader();
+ std::vector<BasicBlock*> Children;
+ ET->getChildren(BB, Children);
+ for (unsigned i = 0, e = Children.size(); i != e; ++i)
+ HoistRegion(Children[i]);
+}
- // Old instruction will be removed, so take it's name...
- string InstName = Inst.getName();
- Inst.setName("");
+/// 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->isVolatile())
+ return false; // Don't hoist volatile loads!
+
+ // Don't hoist loads which have may-aliased stores in loop.
+ unsigned Size = 0;
+ if (LI->getType()->isSized())
+ Size = AA->getTargetData().getTypeSize(LI->getType());
+ return !pointerInvalidatedByLoop(LI->getOperand(0), Size);
+ } else if (CallInst *CI = dyn_cast<CallInst>(&I)) {
+ // Handle obvious cases efficiently.
+ if (Function *Callee = CI->getCalledFunction()) {
+ AliasAnalysis::ModRefBehavior Behavior =AA->getModRefBehavior(Callee, CI);
+ if (Behavior == AliasAnalysis::DoesNotAccessMemory)
+ return true;
+ else if (Behavior == AliasAnalysis::OnlyReadsMemory) {
+ // 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;
+ }
+ }
- if (isa<LoadInst>(Inst))
- ++NumHoistedLoads;
+ // FIXME: This should use mod/ref information to see if we can hoist or sink
+ // the call.
- // The common case is that we have a pre-header. Generate special case code
- // that is faster if that is the case.
- //
- if (LoopPreds.size() == 1) {
- BasicBlock *Pred = LoopPreds[0];
+ return false;
+ }
- // Create a new copy of the instruction, for insertion into Pred.
- Instruction *New = Inst.clone();
- New->setName(InstName);
+ // Otherwise these instructions are hoistable/sinkable
+ return isa<BinaryOperator>(I) || isa<CastInst>(I) ||
+ isa<SelectInst>(I) || isa<GetElementPtrInst>(I) || isa<CmpInst>(I);
+}
- // Insert the new node in Pred, before the terminator.
- Pred->getInstList().insert(--Pred->end(), New);
+/// 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 (Value::use_iterator UI = I.use_begin(), E = I.use_end(); UI != E; ++UI) {
+ Instruction *User = cast<Instruction>(*UI);
+ if (PHINode *PN = dyn_cast<PHINode>(User)) {
+ // PHI node uses occur in predecessor blocks!
+ for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
+ if (PN->getIncomingValue(i) == &I)
+ if (CurLoop->contains(PN->getIncomingBlock(i)))
+ return false;
+ } else if (CurLoop->contains(User->getParent())) {
+ return false;
+ }
+ }
+ return true;
+}
- // Kill the old instruction...
- Inst.replaceAllUsesWith(New);
- ++NumHoistedPH;
- } else {
- // No loop pre-header, insert a PHI node into header to capture all of the
- // incoming versions of the value.
- //
- PHINode *LoopVal = new PHINode(Inst.getType(), InstName+".phi");
+/// isLoopInvariantInst - Return true if all operands of this instruction are
+/// loop invariant. We also filter out non-hoistable instructions here just for
+/// efficiency.
+///
+bool LICM::isLoopInvariantInst(Instruction &I) {
+ // The instruction is loop invariant if all of its operands are loop-invariant
+ for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i)
+ if (!CurLoop->isLoopInvariant(I.getOperand(i)))
+ return false;
- // Insert the new PHI node into the loop header...
- Header->getInstList().push_front(LoopVal);
+ // If we got this far, the instruction is loop invariant!
+ return true;
+}
+
+/// 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) {
+ DOUT << "LICM sinking instruction: " << I;
- // Insert cloned versions of the instruction into all of the loop preds.
- for (unsigned i = 0, e = LoopPreds.size(); i != e; ++i) {
- BasicBlock *Pred = LoopPreds[i];
-
- // Create a new copy of the instruction, for insertion into Pred.
- Instruction *New = Inst.clone();
- New->setName(InstName);
+ std::vector<BasicBlock*> ExitBlocks;
+ CurLoop->getExitBlocks(ExitBlocks);
- // Insert the new node in Pred, before the terminator.
- Pred->getInstList().insert(--Pred->end(), New);
+ if (isa<LoadInst>(I)) ++NumMovedLoads;
+ else if (isa<CallInst>(I)) ++NumMovedCalls;
+ ++NumSunk;
+ Changed = true;
- // Add the incoming value to the PHI node.
- LoopVal->addIncoming(New, Pred);
+ // The case where there is only a single exit node of this loop is common
+ // enough that we handle it as a special (more efficient) case. It is more
+ // efficient to handle because there are no PHI nodes that need to be placed.
+ if (ExitBlocks.size() == 1) {
+ if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[0], I.getParent())) {
+ // Instruction is not used, just delete it.
+ CurAST->deleteValue(&I);
+ if (!I.use_empty()) // If I has users in unreachable blocks, eliminate.
+ I.replaceAllUsesWith(UndefValue::get(I.getType()));
+ I.eraseFromParent();
+ } else {
+ // Move the instruction to the start of the exit block, after any PHI
+ // nodes in it.
+ I.removeFromParent();
+
+ BasicBlock::iterator InsertPt = ExitBlocks[0]->begin();
+ while (isa<PHINode>(InsertPt)) ++InsertPt;
+ ExitBlocks[0]->getInstList().insert(InsertPt, &I);
+ }
+ } else if (ExitBlocks.size() == 0) {
+ // The instruction is actually dead if there ARE NO exit blocks.
+ CurAST->deleteValue(&I);
+ if (!I.use_empty()) // If I has users in unreachable blocks, eliminate.
+ I.replaceAllUsesWith(UndefValue::get(I.getType()));
+ I.eraseFromParent();
+ } else {
+ // Otherwise, if we have multiple exits, use the PromoteMem2Reg function to
+ // do all of the hard work of inserting PHI nodes as necessary. We convert
+ // the value into a stack object to get it to do this.
+
+ // Firstly, we create a stack object to hold the value...
+ AllocaInst *AI = 0;
+
+ if (I.getType() != Type::VoidTy)
+ AI = new AllocaInst(I.getType(), 0, I.getName(),
+ I.getParent()->getParent()->getEntryBlock().begin());
+
+ // Secondly, insert load instructions for each use of the instruction
+ // outside of the loop.
+ while (!I.use_empty()) {
+ Instruction *U = cast<Instruction>(I.use_back());
+
+ // If the user is a PHI Node, we actually have to insert load instructions
+ // in all predecessor blocks, not in the PHI block itself!
+ if (PHINode *UPN = dyn_cast<PHINode>(U)) {
+ // Only insert into each predecessor once, so that we don't have
+ // different incoming values from the same block!
+ std::map<BasicBlock*, Value*> InsertedBlocks;
+ for (unsigned i = 0, e = UPN->getNumIncomingValues(); i != e; ++i)
+ if (UPN->getIncomingValue(i) == &I) {
+ BasicBlock *Pred = UPN->getIncomingBlock(i);
+ Value *&PredVal = InsertedBlocks[Pred];
+ if (!PredVal) {
+ // Insert a new load instruction right before the terminator in
+ // the predecessor block.
+ PredVal = new LoadInst(AI, "", Pred->getTerminator());
+ }
+
+ UPN->setIncomingValue(i, PredVal);
+ }
+
+ } else {
+ LoadInst *L = new LoadInst(AI, "", U);
+ U->replaceUsesOfWith(&I, L);
+ }
}
- // Add incoming values to the PHI node for all backedges in the loop...
- for (unsigned i = 0, e = LoopBackEdges.size(); i != e; ++i)
- LoopVal->addIncoming(LoopVal, LoopBackEdges[i]);
+ // Thirdly, insert a copy of the instruction in each exit block of the loop
+ // that is dominated by the instruction, storing the result into the memory
+ // location. Be careful not to insert the instruction into any particular
+ // basic block more than once.
+ std::set<BasicBlock*> InsertedBlocks;
+ BasicBlock *InstOrigBB = I.getParent();
+
+ for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
+ BasicBlock *ExitBlock = ExitBlocks[i];
+
+ if (isExitBlockDominatedByBlockInLoop(ExitBlock, InstOrigBB)) {
+ // If we haven't already processed this exit block, do so now.
+ if (InsertedBlocks.insert(ExitBlock).second) {
+ // Insert the code after the last PHI node...
+ BasicBlock::iterator InsertPt = ExitBlock->begin();
+ while (isa<PHINode>(InsertPt)) ++InsertPt;
+
+ // If this is the first exit block processed, just move the original
+ // instruction, otherwise clone the original instruction and insert
+ // the copy.
+ Instruction *New;
+ if (InsertedBlocks.size() == 1) {
+ I.removeFromParent();
+ ExitBlock->getInstList().insert(InsertPt, &I);
+ New = &I;
+ } else {
+ New = I.clone();
+ CurAST->copyValue(&I, New);
+ if (!I.getName().empty())
+ New->setName(I.getName()+".le");
+ ExitBlock->getInstList().insert(InsertPt, New);
+ }
+
+ // Now that we have inserted the instruction, store it into the alloca
+ if (AI) new StoreInst(New, AI, InsertPt);
+ }
+ }
+ }
- // Replace all uses of the old version of the instruction in the loop with
- // the new version that is out of the loop. We know that this is ok,
- // because the new definition is in the loop header, which dominates the
- // entire loop body. The old definition was defined _inside_ of the loop,
- // so the scope cannot extend outside of the loop, so we're ok.
- //
- Inst.replaceAllUsesWith(LoopVal);
- ++NumHoistedNPH;
+ // If the instruction doesn't dominate any exit blocks, it must be dead.
+ if (InsertedBlocks.empty()) {
+ CurAST->deleteValue(&I);
+ I.eraseFromParent();
+ }
+
+ // Finally, promote the fine value to SSA form.
+ if (AI) {
+ std::vector<AllocaInst*> Allocas;
+ Allocas.push_back(AI);
+ PromoteMemToReg(Allocas, *ET, *DF, AA->getTargetData(), CurAST);
+ }
}
+}
+
+/// 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) {
+ DOUT << "LICM hoisting to " << Preheader->getName() << ": " << I;
+ // Remove the instruction from its current basic block... but don't delete the
+ // instruction.
+ I.removeFromParent();
+
+ // Insert the new node in Preheader, before the terminator.
+ Preheader->getInstList().insert(Preheader->getTerminator(), &I);
+
+ if (isa<LoadInst>(I)) ++NumMovedLoads;
+ else if (isa<CallInst>(I)) ++NumMovedCalls;
+ ++NumHoisted;
Changed = true;
}
-// pointerInvalidatedByLoop - Return true if the body of this loop may store
-// into the memory location pointed to by V.
-//
-bool LICM::pointerInvalidatedByLoop(Value *V) {
- // Check to see if any of the basic blocks in CurLoop invalidate V.
- for (unsigned i = 0, e = CurLoop->getBlocks().size(); i != e; ++i)
- if (AA->canBasicBlockModify(*CurLoop->getBlocks()[i], V))
+/// 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 (!Inst.isTrapping()) return true;
+
+ // 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;
+
+ // It's always safe to load from a global or alloca.
+ if (isa<LoadInst>(Inst))
+ if (isa<AllocationInst>(Inst.getOperand(0)) ||
+ isa<GlobalVariable>(Inst.getOperand(0)))
return true;
- return false;
+
+ // Get the exit blocks for the current loop.
+ std::vector<BasicBlock*> ExitBlocks;
+ CurLoop->getExitBlocks(ExitBlocks);
+
+ // For each exit block, walk up the ET until the
+ // instruction's basic block is found or we exit the loop.
+ for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
+ if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[i], Inst.getParent()))
+ return false;
+
+ return true;
+}
+
+
+/// PromoteValuesInLoop - 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. We promote these memory locations to use allocas
+/// instead. These allocas can easily be raised to register values by the
+/// PromoteMem2Reg functionality.
+///
+void LICM::PromoteValuesInLoop() {
+ // PromotedValues - List of values that are promoted out of the loop. Each
+ // value has an alloca instruction for it, and a canonical version of the
+ // pointer.
+ std::vector<std::pair<AllocaInst*, Value*> > PromotedValues;
+ std::map<Value*, AllocaInst*> ValueToAllocaMap; // Map of ptr to alloca
+
+ FindPromotableValuesInLoop(PromotedValues, ValueToAllocaMap);
+ if (ValueToAllocaMap.empty()) return; // If there are values to promote.
+
+ Changed = true;
+ NumPromoted += PromotedValues.size();
+
+ std::vector<Value*> PointerValueNumbers;
+
+ // Emit a copy from the value into the alloca'd value in the loop preheader
+ TerminatorInst *LoopPredInst = Preheader->getTerminator();
+ for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) {
+ Value *Ptr = PromotedValues[i].second;
+
+ // If we are promoting a pointer value, update alias information for the
+ // inserted load.
+ Value *LoadValue = 0;
+ if (isa<PointerType>(cast<PointerType>(Ptr->getType())->getElementType())) {
+ // Locate a load or store through the pointer, and assign the same value
+ // to LI as we are loading or storing. Since we know that the value is
+ // stored in this loop, this will always succeed.
+ for (Value::use_iterator UI = Ptr->use_begin(), E = Ptr->use_end();
+ UI != E; ++UI)
+ if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
+ LoadValue = LI;
+ break;
+ } else if (StoreInst *SI = dyn_cast<StoreInst>(*UI)) {
+ if (SI->getOperand(1) == Ptr) {
+ LoadValue = SI->getOperand(0);
+ break;
+ }
+ }
+ assert(LoadValue && "No store through the pointer found!");
+ PointerValueNumbers.push_back(LoadValue); // Remember this for later.
+ }
+
+ // Load from the memory we are promoting.
+ LoadInst *LI = new LoadInst(Ptr, Ptr->getName()+".promoted", LoopPredInst);
+
+ if (LoadValue) CurAST->copyValue(LoadValue, LI);
+
+ // Store into the temporary alloca.
+ new StoreInst(LI, PromotedValues[i].first, LoopPredInst);
+ }
+
+ // Scan the basic blocks in the loop, replacing uses of our pointers with
+ // uses of the allocas in question.
+ //
+ const std::vector<BasicBlock*> &LoopBBs = CurLoop->getBlocks();
+ for (std::vector<BasicBlock*>::const_iterator I = LoopBBs.begin(),
+ E = LoopBBs.end(); I != E; ++I) {
+ // Rewrite all loads and stores in the block of the pointer...
+ for (BasicBlock::iterator II = (*I)->begin(), E = (*I)->end();
+ II != E; ++II) {
+ if (LoadInst *L = dyn_cast<LoadInst>(II)) {
+ std::map<Value*, AllocaInst*>::iterator
+ I = ValueToAllocaMap.find(L->getOperand(0));
+ if (I != ValueToAllocaMap.end())
+ L->setOperand(0, I->second); // Rewrite load instruction...
+ } else if (StoreInst *S = dyn_cast<StoreInst>(II)) {
+ std::map<Value*, AllocaInst*>::iterator
+ I = ValueToAllocaMap.find(S->getOperand(1));
+ if (I != ValueToAllocaMap.end())
+ S->setOperand(1, I->second); // Rewrite store instruction...
+ }
+ }
+ }
+
+ // Now that the body of the loop uses the allocas instead of the original
+ // memory locations, insert code to copy the alloca value back into the
+ // original memory location on all exits from the loop. Note that we only
+ // want to insert one copy of the code in each exit block, though the loop may
+ // exit to the same block more than once.
+ //
+ std::set<BasicBlock*> ProcessedBlocks;
+
+ std::vector<BasicBlock*> ExitBlocks;
+ CurLoop->getExitBlocks(ExitBlocks);
+ for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
+ if (ProcessedBlocks.insert(ExitBlocks[i]).second) {
+ // Copy all of the allocas into their memory locations.
+ BasicBlock::iterator BI = ExitBlocks[i]->begin();
+ while (isa<PHINode>(*BI))
+ ++BI; // Skip over all of the phi nodes in the block.
+ Instruction *InsertPos = BI;
+ unsigned PVN = 0;
+ for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) {
+ // Load from the alloca.
+ LoadInst *LI = new LoadInst(PromotedValues[i].first, "", InsertPos);
+
+ // If this is a pointer type, update alias info appropriately.
+ if (isa<PointerType>(LI->getType()))
+ CurAST->copyValue(PointerValueNumbers[PVN++], LI);
+
+ // Store into the memory we promoted.
+ new StoreInst(LI, PromotedValues[i].second, InsertPos);
+ }
+ }
+
+ // Now that we have done the deed, use the mem2reg functionality to promote
+ // all of the new allocas we just created into real SSA registers.
+ //
+ std::vector<AllocaInst*> PromotedAllocas;
+ PromotedAllocas.reserve(PromotedValues.size());
+ for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i)
+ PromotedAllocas.push_back(PromotedValues[i].first);
+ PromoteMemToReg(PromotedAllocas, *ET, *DF, AA->getTargetData(), CurAST);
+}
+
+/// FindPromotableValuesInLoop - Check the current loop for stores to definite
+/// pointers, which are not loaded and stored through may aliases. If these are
+/// found, create an alloca for the value, add it to the PromotedValues list,
+/// and keep track of the mapping from value to alloca.
+///
+void LICM::FindPromotableValuesInLoop(
+ std::vector<std::pair<AllocaInst*, Value*> > &PromotedValues,
+ std::map<Value*, AllocaInst*> &ValueToAllocaMap) {
+ Instruction *FnStart = CurLoop->getHeader()->getParent()->begin()->begin();
+
+ // Loop over all of the alias sets in the tracker object.
+ for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
+ I != E; ++I) {
+ AliasSet &AS = *I;
+ // 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()->first)) {
+ assert(AS.begin() != AS.end() &&
+ "Must alias set should have at least one pointer element in it!");
+ Value *V = AS.begin()->first;
+
+ // 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.
+ bool PointerOk = true;
+ for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I)
+ if (V->getType() != I->first->getType()) {
+ PointerOk = false;
+ break;
+ }
+
+ if (PointerOk) {
+ const Type *Ty = cast<PointerType>(V->getType())->getElementType();
+ AllocaInst *AI = new AllocaInst(Ty, 0, V->getName()+".tmp", FnStart);
+ PromotedValues.push_back(std::make_pair(AI, V));
+
+ // Update the AST and alias analysis.
+ CurAST->copyValue(V, AI);
+
+ for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I)
+ ValueToAllocaMap.insert(std::make_pair(I->first, AI));
+
+ DOUT << "LICM: Promoting value: " << *V << "\n";
+ }
+ }
+ }
}
projects / oota-llvm.git / blobdiff