//===-- LICM.cpp - Loop Invariant Code Motion Pass ------------------------===//
//
-// This pass is a simple loop invariant code motion pass.
+// This pass is a simple loop invariant code motion pass. An interesting aspect
+// of this pass is that it uses alias analysis for two purposes:
+//
+// 1. Moving loop invariant loads out of loops. If we can determine that a
+// load inside of a loop never aliases anything stored to, we can hoist 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.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
+#include "llvm/Transforms/Utils/PromoteMemToReg.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/AliasSetTracker.h"
#include "llvm/Analysis/Dominators.h"
-#include "llvm/iOperators.h"
-#include "llvm/iMemory.h"
+#include "llvm/Instructions.h"
+#include "llvm/DerivedTypes.h"
+#include "llvm/Target/TargetData.h"
#include "llvm/Support/InstVisitor.h"
-#include "Support/STLExtras.h"
-#include "Support/StatisticReporter.h"
+#include "llvm/Support/CFG.h"
+#include "Support/Statistic.h"
+#include "Support/CommandLine.h"
#include "llvm/Assembly/Writer.h"
#include <algorithm>
-using std::string;
namespace {
- Statistic<>NumHoisted("licm\t\t- Number of instructions hoisted out of loop");
- Statistic<> NumHoistedLoads("licm\t\t- Number of load insts hoisted");
+ cl::opt<bool> DisablePromotion("disable-licm-promotion", cl::Hidden,
+ cl::desc("Disable memory promotion in LICM pass"));
+
+ Statistic<> NumHoisted("licm", "Number of instructions hoisted out of loop");
+ Statistic<> NumHoistedLoads("licm", "Number of load insts hoisted");
+ Statistic<> NumPromoted("licm", "Number of memory locations promoted to registers");
struct LICM : public FunctionPass, public InstVisitor<LICM> {
virtual bool runOnFunction(Function &F);
/// loop preheaders be inserted into the CFG...
///
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
- AU.preservesCFG();
+ AU.setPreservesCFG();
AU.addRequiredID(LoopPreheadersID);
AU.addRequired<LoopInfo>();
AU.addRequired<DominatorTree>();
+ AU.addRequired<DominanceFrontier>(); // For scalar promotion (mem2reg)
AU.addRequired<AliasAnalysis>();
}
private:
- Loop *CurLoop; // The current loop we are working on...
- BasicBlock *Preheader; // The preheader block of the current loop...
- bool Changed; // Set to true when we change anything.
- AliasAnalysis *AA; // Currently AliasAnalysis information
+ LoopInfo *LI; // Current LoopInfo
+ AliasAnalysis *AA; // Current AliasAnalysis information
+ 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...
/// visitLoop - Hoist expressions out of the specified loop...
///
- void visitLoop(Loop *L);
+ void visitLoop(Loop *L, AliasSetTracker &AST);
/// HoistRegion - Walk the specified region of the CFG (defined by all
/// blocks dominated by the specified block, and that are in the current
/// pointerInvalidatedByLoop - Return true if the body of this loop may
/// store into the memory location pointed to by V.
///
- bool pointerInvalidatedByLoop(Value *V);
+ bool pointerInvalidatedByLoop(Value *V) {
+ // Check to see if any of the basic blocks in CurLoop invalidate *V.
+ return CurAST->getAliasSetForPointer(V, 0).isMod();
+ }
/// isLoopInvariant - Return true if the specified value is loop invariant
///
return true; // All non-instructions are loop invariant
}
+ /// 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
+ /// definate 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);
+
+
/// Instruction visitation handlers... these basically control whether or
/// not the specified instruction types are hoisted.
///
/// function, hoisting expressions out of loops if possible.
///
bool LICM::runOnFunction(Function &) {
- // Get information about the top level loops in the function...
- const std::vector<Loop*> &TopLevelLoops =
- getAnalysis<LoopInfo>().getTopLevelLoops();
+ Changed = false;
- // Get our alias analysis information...
+ // Get our Loop and Alias Analysis information...
+ LI = &getAnalysis<LoopInfo>();
AA = &getAnalysis<AliasAnalysis>();
- // Traverse loops in postorder, hoisting expressions out of the deepest loops
- // first.
- //
- Changed = false;
- std::for_each(TopLevelLoops.begin(), TopLevelLoops.end(),
- bind_obj(this, &LICM::visitLoop));
+ // Hoist expressions out of all of the top-level loops.
+ const std::vector<Loop*> &TopLevelLoops = LI->getTopLevelLoops();
+ for (std::vector<Loop*>::const_iterator I = TopLevelLoops.begin(),
+ E = TopLevelLoops.end(); I != E; ++I) {
+ AliasSetTracker AST(*AA);
+ LICM::visitLoop(*I, AST);
+ }
return Changed;
}
/// visitLoop - Hoist expressions out of the specified loop...
///
-void LICM::visitLoop(Loop *L) {
+void LICM::visitLoop(Loop *L, AliasSetTracker &AST) {
// Recurse through all subloops before we process this loop...
- std::for_each(L->getSubLoops().begin(), L->getSubLoops().end(),
- bind_obj(this, &LICM::visitLoop));
+ for (std::vector<Loop*>::const_iterator I = L->getSubLoops().begin(),
+ E = L->getSubLoops().end(); I != E; ++I) {
+ AliasSetTracker SubAST(*AA);
+ LICM::visitLoop(*I, SubAST);
+
+ // Incorporate information about the subloops into this loop...
+ AST.add(SubAST);
+ }
CurLoop = L;
+ CurAST = &AST;
// Get the preheader block to move instructions into...
Preheader = L->getLoopPreheader();
assert(Preheader&&"Preheader insertion pass guarantees we have a preheader!");
+ // 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.
+ //
+ const std::vector<BasicBlock*> &LoopBBs = L->getBlocks();
+ for (std::vector<BasicBlock*>::const_iterator I = LoopBBs.begin(),
+ E = LoopBBs.end(); I != E; ++I)
+ if (LI->getLoopFor(*I) == L) // Ignore blocks in subloops...
+ AST.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
//
HoistRegion(getAnalysis<DominatorTree>()[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;
Preheader = 0;
}
}
-/// 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))
- return true;
- return false;
+/// 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 cannonical 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();
+
+ // 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) {
+ // Load from the memory we are promoting...
+ LoadInst *LI = new LoadInst(PromotedValues[i].second,
+ PromotedValues[i].second->getName()+".promoted",
+ LoopPredInst);
+ // 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. If we find a branch that exits the
+ // loop, make sure to put reload code into all of the successors of the
+ // loop.
+ //
+ 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...
+ }
+ }
+
+ // Check to see if any successors of this block are outside of the loop.
+ // If so, we need to copy the value from the alloca back into the memory
+ // location...
+ //
+ for (succ_iterator SI = succ_begin(*I), SE = succ_end(*I); SI != SE; ++SI)
+ if (!CurLoop->contains(*SI)) {
+ // Copy all of the allocas into their memory locations...
+ BasicBlock::iterator BI = (*SI)->begin();
+ while (isa<PHINode>(*BI))
+ ++BI; // Skip over all of the phi nodes in the block...
+ Instruction *InsertPos = BI;
+ for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) {
+ // Load from the alloca...
+ LoadInst *LI = new LoadInst(PromotedValues[i].first, "", InsertPos);
+ // 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, getAnalysis<DominanceFrontier>(),
+ AA->getTargetData());
+}
+
+/// findPromotableValuesInLoop - Check the current loop for stores to definate
+/// 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, and if the pointer is loop invariant.
+ if (!AS.isForwardingAliasSet() && AS.isMod() && AS.isMustAlias() &&
+ 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));
+
+ for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I)
+ ValueToAllocaMap.insert(std::make_pair(I->first, AI));
+
+ DEBUG(std::cerr << "LICM: Promoting value: " << *V << "\n");
+ }
+ }
+ }
}
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