forked from OSchip/llvm-project
Chris Lattner
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81525db280
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6ec05f551c
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//===-- LICM.cpp - Loop Invariant Code Motion Pass ------------------------===//
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//
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// This pass is a simple loop invariant code motion pass.
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//
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// Note that this pass does NOT require pre-headers to exist on loops in the
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// CFG, but if there is not distinct preheader for a loop, the hoisted code will
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// be *DUPLICATED* in every basic block, outside of the loop, that preceeds the
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// loop header. Additionally, any use of one of these hoisted expressions
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// cannot be loop invariant itself, because the expression hoisted gets a PHI
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// node that is loop variant.
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//
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// For these reasons, and many more, it makes sense to run a pass before this
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// that ensures that there are preheaders on all loops. That said, we don't
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// REQUIRE it. :)
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/iOperators.h"
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#include "llvm/iPHINode.h"
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#include "llvm/Support/InstVisitor.h"
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#include "llvm/Support/CFG.h"
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#include "Support/STLExtras.h"
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#include "Support/StatisticReporter.h"
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#include <algorithm>
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static Statistic<> NumHoistedNPH("licm\t\t- Number of insts hoisted to multiple"
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" loop preds (bad, no loop pre-header)");
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static Statistic<> NumHoistedPH("licm\t\t- Number of insts hoisted to a loop "
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"pre-header");
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namespace {
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struct LICM : public FunctionPass, public InstVisitor<LICM> {
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const char *getPassName() const { return "Loop Invariant Code Motion"; }
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virtual bool runOnFunction(Function *F);
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// This transformation requires natural loop information...
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.preservesCFG();
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AU.addRequired(LoopInfo::ID);
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}
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private:
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// List of predecessor blocks for the current loop - These blocks are where
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// we hoist loop invariants to for the current loop.
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//
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std::vector<BasicBlock*> LoopPreds, LoopBackEdges;
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Loop *CurLoop; // The current loop we are working on...
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bool Changed; // Set to true when we change anything.
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// visitLoop - Hoist expressions out of the specified loop...
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void visitLoop(Loop *L);
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// notInCurrentLoop - Little predicate that returns true if the specified
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// basic block is in a subloop of the current one, not the current one
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// itself.
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//
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bool notInCurrentLoop(BasicBlock *BB) {
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for (unsigned i = 0, e = CurLoop->getSubLoops().size(); i != e; ++i)
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if (CurLoop->getSubLoops()[i]->contains(BB))
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return true; // A subloop actually contains this block!
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return false;
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}
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// hoist - When an instruction is found to only use loop invariant operands
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// that is safe to hoist, this instruction is called to do the dirty work.
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//
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void hoist(Instruction *I);
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// isLoopInvariant - Return true if the specified value is loop invariant
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inline bool isLoopInvariant(Value *V) {
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if (Instruction *I = dyn_cast<Instruction>(V))
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return !CurLoop->contains(I->getParent());
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return true; // All non-instructions are loop invariant
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}
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// visitBasicBlock - Run LICM on a particular block.
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void visitBasicBlock(BasicBlock *BB);
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// Instruction visitation handlers... these basically control whether or not
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// the specified instruction types are hoisted.
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//
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friend class InstVisitor<LICM>;
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void visitUnaryOperator(Instruction *I) {
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if (isLoopInvariant(I->getOperand(0))) hoist(I);
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}
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void visitBinaryOperator(Instruction *I) {
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if (isLoopInvariant(I->getOperand(0)) &&isLoopInvariant(I->getOperand(1)))
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hoist(I);
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}
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void visitCastInst(CastInst *I) { visitUnaryOperator((Instruction*)I); }
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void visitShiftInst(ShiftInst *I) { visitBinaryOperator((Instruction*)I); }
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void visitGetElementPtrInst(GetElementPtrInst *GEPI) {
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Instruction *I = (Instruction*)GEPI;
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for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
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if (!isLoopInvariant(I->getOperand(i))) return;
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hoist(I);
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}
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};
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}
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Pass *createLICMPass() { return new LICM(); }
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bool LICM::runOnFunction(Function *F) {
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// get our loop information...
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const std::vector<Loop*> &TopLevelLoops =
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getAnalysis<LoopInfo>().getTopLevelLoops();
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// Traverse loops in postorder, hoisting expressions out of the deepest loops
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// first.
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//
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Changed = false;
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std::for_each(TopLevelLoops.begin(), TopLevelLoops.end(),
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bind_obj(this, &LICM::visitLoop));
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return Changed;
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}
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void LICM::visitLoop(Loop *L) {
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// Recurse through all subloops before we process this loop...
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std::for_each(L->getSubLoops().begin(), L->getSubLoops().end(),
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bind_obj(this, &LICM::visitLoop));
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CurLoop = L;
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// Calculate the set of predecessors for this loop. The predecessors for this
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// loop are equal to the predecessors for the header node of the loop that are
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// not themselves in the loop.
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//
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BasicBlock *Header = L->getHeader();
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// Calculate the sets of predecessors and backedges of the loop...
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LoopBackEdges.insert(LoopBackEdges.end(),pred_begin(Header),pred_end(Header));
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std::vector<BasicBlock*>::iterator LPI =
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std::partition(LoopBackEdges.begin(), LoopBackEdges.end(),
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bind_obj(CurLoop, &Loop::contains));
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// Move all predecessors to the LoopPreds vector...
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LoopPreds.insert(LoopPreds.end(), LPI, LoopBackEdges.end());
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// Remove predecessors from backedges list...
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LoopBackEdges.erase(LPI, LoopBackEdges.end());
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// The only way that there could be no predecessors to a loop is if the loop
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// is not reachable. Since we don't care about optimizing dead loops,
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// summarily ignore them.
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//
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if (LoopPreds.empty()) return;
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// We want to visit all of the instructions in this loop... that are not parts
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// of our subloops (they have already had their invariants hoisted out of
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// their loop, into this loop, so there is no need to process the BODIES of
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// the subloops).
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//
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std::vector<BasicBlock*> BBs(L->getBlocks().begin(), L->getBlocks().end());
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// Remove blocks that are actually in subloops...
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BBs.erase(std::remove_if(BBs.begin(), BBs.end(),
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bind_obj(this, &LICM::notInCurrentLoop)), BBs.end());
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// Visit all of the basic blocks we have chosen, hoisting out the instructions
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// as neccesary. This leaves dead copies of the instruction in the loop
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// unfortunately...
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//
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for_each(BBs.begin(), BBs.end(), bind_obj(this, &LICM::visitBasicBlock));
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// Clear out loops state information for the next iteration
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CurLoop = 0;
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LoopPreds.clear();
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LoopBackEdges.clear();
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}
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void LICM::visitBasicBlock(BasicBlock *BB) {
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// This cannot use an iterator, because it might get invalidated when PHI
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// nodes are inserted!
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//
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for (unsigned i = 0; i < BB->size(); ) {
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visit(BB->begin()[i]);
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BasicBlock::iterator It = BB->begin()+i;
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if (dceInstruction(BB->getInstList(), It))
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Changed = true;
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else
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++i;
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}
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}
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void LICM::hoist(Instruction *Inst) {
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if (Inst->use_empty()) return; // Don't (re) hoist dead instructions!
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//cerr << "Hoisting " << Inst;
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BasicBlock *Header = CurLoop->getHeader();
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// Old instruction will be removed, so take it's name...
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string InstName = Inst->getName();
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Inst->setName("");
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// The common case is that we have a pre-header. Generate special case code
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// that is faster if that is the case.
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//
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if (LoopPreds.size() == 1) {
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BasicBlock *Pred = LoopPreds[0];
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// Create a new copy of the instruction, for insertion into Pred.
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Instruction *New = Inst->clone();
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New->setName(InstName);
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// Insert the new node in Pred, before the terminator.
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Pred->getInstList().insert(Pred->end()-1, New);
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// Kill the old instruction.
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Inst->replaceAllUsesWith(New);
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++NumHoistedPH;
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} else {
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// No loop pre-header, insert a PHI node into header to capture all of the
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// incoming versions of the value.
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//
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PHINode *LoopVal = new PHINode(Inst->getType(), InstName+".phi");
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// Insert the new PHI node into the loop header...
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Header->getInstList().push_front(LoopVal);
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// Insert cloned versions of the instruction into all of the loop preds.
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for (unsigned i = 0, e = LoopPreds.size(); i != e; ++i) {
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BasicBlock *Pred = LoopPreds[i];
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// Create a new copy of the instruction, for insertion into Pred.
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Instruction *New = Inst->clone();
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New->setName(InstName);
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// Insert the new node in Pred, before the terminator.
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Pred->getInstList().insert(Pred->end()-1, New);
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// Add the incoming value to the PHI node.
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LoopVal->addIncoming(New, Pred);
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}
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// Add incoming values to the PHI node for all backedges in the loop...
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for (unsigned i = 0, e = LoopBackEdges.size(); i != e; ++i)
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LoopVal->addIncoming(LoopVal, LoopBackEdges[i]);
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// Replace all uses of the old version of the instruction in the loop with
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// the new version that is out of the loop. We know that this is ok,
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// because the new definition is in the loop header, which dominates the
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// entire loop body. The old definition was defined _inside_ of the loop,
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// so the scope cannot extend outside of the loop, so we're ok.
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//
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Inst->replaceAllUsesWith(LoopVal);
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++NumHoistedNPH;
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}
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Changed = true;
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}
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