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setPreservesCFG to be less confusing. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@4255 91177308-0d34-0410-b5e6-96231b3b80d8
224 lines
8.0 KiB
C++
224 lines
8.0 KiB
C++
//===-- 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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//===----------------------------------------------------------------------===//
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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/Analysis/AliasAnalysis.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/iOperators.h"
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#include "llvm/iMemory.h"
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#include "llvm/Support/InstVisitor.h"
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#include "Support/STLExtras.h"
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#include "Support/Statistic.h"
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#include "llvm/Assembly/Writer.h"
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#include <algorithm>
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using std::string;
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namespace {
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Statistic<> NumHoisted("licm", "Number of instructions hoisted out of loop");
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Statistic<> NumHoistedLoads("licm", "Number of load insts hoisted");
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struct LICM : public FunctionPass, public InstVisitor<LICM> {
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virtual bool runOnFunction(Function &F);
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/// This transformation requires natural loop information & requires that
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/// loop preheaders be inserted into the CFG...
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///
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.setPreservesCFG();
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AU.addRequiredID(LoopPreheadersID);
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AU.addRequired<LoopInfo>();
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AU.addRequired<DominatorTree>();
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AU.addRequired<AliasAnalysis>();
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}
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private:
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Loop *CurLoop; // The current loop we are working on...
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BasicBlock *Preheader; // The preheader block of the current loop...
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bool Changed; // Set to true when we change anything.
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AliasAnalysis *AA; // Currently AliasAnalysis information
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/// visitLoop - Hoist expressions out of the specified loop...
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///
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void visitLoop(Loop *L);
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/// HoistRegion - Walk the specified region of the CFG (defined by all
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/// blocks dominated by the specified block, and that are in the current
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/// loop) in depth first order w.r.t the DominatorTree. This allows us to
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/// visit defintions before uses, allowing us to hoist a loop body in one
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/// pass without iteration.
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///
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void HoistRegion(DominatorTree::Node *N);
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/// inSubLoop - Little predicate that returns true if the specified basic
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/// block is in a subloop of the current one, not the current one itself.
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///
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bool inSubLoop(BasicBlock *BB) {
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assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
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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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/// pointerInvalidatedByLoop - Return true if the body of this loop may
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/// store into the memory location pointed to by V.
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///
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bool pointerInvalidatedByLoop(Value *V);
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/// isLoopInvariant - Return true if the specified value is loop invariant
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///
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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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/// Instruction visitation handlers... these basically control whether or
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/// not the specified instruction types are hoisted.
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///
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friend class InstVisitor<LICM>;
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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 &CI) {
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Instruction &I = (Instruction&)CI;
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if (isLoopInvariant(I.getOperand(0))) hoist(I);
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}
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void visitShiftInst(ShiftInst &I) { visitBinaryOperator((Instruction&)I); }
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void visitLoadInst(LoadInst &LI);
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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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RegisterOpt<LICM> X("licm", "Loop Invariant Code Motion");
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}
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Pass *createLICMPass() { return new LICM(); }
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/// runOnFunction - For LICM, this simply traverses the loop structure of the
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/// function, hoisting expressions out of loops if possible.
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///
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bool LICM::runOnFunction(Function &) {
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// Get information about the top level loops in the function...
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const std::vector<Loop*> &TopLevelLoops =
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getAnalysis<LoopInfo>().getTopLevelLoops();
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// Get our alias analysis information...
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AA = &getAnalysis<AliasAnalysis>();
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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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/// visitLoop - Hoist expressions out of the specified loop...
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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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// Get the preheader block to move instructions into...
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Preheader = L->getLoopPreheader();
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assert(Preheader&&"Preheader insertion pass guarantees we have a preheader!");
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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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// Traverse the body of the loop in depth first order on the dominator tree so
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// that we are guaranteed to see definitions before we see uses. This allows
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// us to perform the LICM transformation in one pass, without iteration.
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//
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HoistRegion(getAnalysis<DominatorTree>()[L->getHeader()]);
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// Clear out loops state information for the next iteration
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CurLoop = 0;
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Preheader = 0;
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}
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/// HoistRegion - Walk the specified region of the CFG (defined by all blocks
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/// dominated by the specified block, and that are in the current loop) in depth
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/// first order w.r.t the DominatorTree. This allows us to visit defintions
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/// before uses, allowing us to hoist a loop body in one pass without iteration.
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///
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void LICM::HoistRegion(DominatorTree::Node *N) {
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assert(N != 0 && "Null dominator tree node?");
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// If this subregion is not in the top level loop at all, exit.
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if (!CurLoop->contains(N->getNode())) return;
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// Only need to hoist the contents of this block if it is not part of a
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// subloop (which would already have been hoisted)
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if (!inSubLoop(N->getNode()))
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visit(*N->getNode());
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const std::vector<DominatorTree::Node*> &Children = N->getChildren();
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for (unsigned i = 0, e = Children.size(); i != e; ++i)
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HoistRegion(Children[i]);
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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 LICM::hoist(Instruction &Inst) {
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DEBUG(std::cerr << "LICM hoisting to";
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WriteAsOperand(std::cerr, Preheader, false);
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std::cerr << ": " << Inst);
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// Remove the instruction from its current basic block... but don't delete the
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// instruction.
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Inst.getParent()->getInstList().remove(&Inst);
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// Insert the new node in Preheader, before the terminator.
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Preheader->getInstList().insert(Preheader->getTerminator(), &Inst);
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++NumHoisted;
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Changed = true;
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}
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void LICM::visitLoadInst(LoadInst &LI) {
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if (isLoopInvariant(LI.getOperand(0)) &&
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!pointerInvalidatedByLoop(LI.getOperand(0))) {
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hoist(LI);
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++NumHoistedLoads;
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}
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}
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/// pointerInvalidatedByLoop - Return true if the body of this loop may store
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/// into the memory location pointed to by V.
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///
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bool LICM::pointerInvalidatedByLoop(Value *V) {
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// Check to see if any of the basic blocks in CurLoop invalidate V.
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for (unsigned i = 0, e = CurLoop->getBlocks().size(); i != e; ++i)
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if (AA->canBasicBlockModify(*CurLoop->getBlocks()[i], V))
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return true;
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return false;
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}
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