mirror of
https://github.com/c64scene-ar/llvm-6502.git
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@8469 91177308-0d34-0410-b5e6-96231b3b80d8
457 lines
18 KiB
C++
457 lines
18 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. An interesting aspect
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// of this pass is that it uses alias analysis for two purposes:
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//
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// 1. Moving loop invariant loads out of loops. If we can determine that a
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// load inside of a loop never aliases anything stored to, we can hoist it
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// like any other instruction.
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// 2. Scalar Promotion of Memory - If there is a store instruction inside of
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// the loop, we try to move the store to happen AFTER the loop instead of
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// inside of the loop. This can only happen if a few conditions are true:
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// A. The pointer stored through is loop invariant
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// B. There are no stores or loads in the loop which _may_ alias the
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// pointer. There are no calls in the loop which mod/ref the pointer.
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// If these conditions are true, we can promote the loads and stores in the
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// loop of the pointer to use a temporary alloca'd variable. We then use
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// the mem2reg functionality to construct the appropriate SSA form for the
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// variable.
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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/PromoteMemToReg.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/AliasSetTracker.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/Instructions.h"
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#include "llvm/DerivedTypes.h"
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#include "llvm/Target/TargetData.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/CommandLine.h"
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#include "Support/Debug.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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namespace {
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cl::opt<bool> DisablePromotion("disable-licm-promotion", cl::Hidden,
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cl::desc("Disable memory promotion in LICM pass"));
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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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Statistic<> NumPromoted("licm", "Number of memory locations promoted to registers");
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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<DominanceFrontier>(); // For scalar promotion (mem2reg)
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AU.addRequired<AliasAnalysis>();
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}
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private:
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LoopInfo *LI; // Current LoopInfo
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AliasAnalysis *AA; // Current AliasAnalysis information
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bool Changed; // Set to true when we change anything.
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BasicBlock *Preheader; // The preheader block of the current loop...
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Loop *CurLoop; // The current loop we are working on...
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AliasSetTracker *CurAST; // AliasSet information for the current loop...
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DominatorTree *DT; // Dominator Tree for the current Loop...
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/// visitLoop - Hoist expressions out of the specified loop...
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///
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void visitLoop(Loop *L, AliasSetTracker &AST);
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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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/// SafeToHoist - Only hoist an instruction if it is not a trapping instruction
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/// or if it is a trapping instruction and is guaranteed to execute
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///
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bool SafeToHoist(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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// Check to see if any of the basic blocks in CurLoop invalidate *V.
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return CurAST->getAliasSetForPointer(V, 0).isMod();
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}
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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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/// PromoteValuesInLoop - Look at the stores in the loop and promote as many
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/// to scalars as we can.
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///
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void PromoteValuesInLoop();
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/// findPromotableValuesInLoop - Check the current loop for stores to
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/// definite pointers, which are not loaded and stored through may aliases.
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/// If these are found, create an alloca for the value, add it to the
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/// PromotedValues list, and keep track of the mapping from value to
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/// alloca...
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///
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void findPromotableValuesInLoop(
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std::vector<std::pair<AllocaInst*, Value*> > &PromotedValues,
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std::map<Value*, AllocaInst*> &Val2AlMap);
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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)) && SafeToHoist(I))
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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)) && SafeToHoist(CI)) 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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if(SafeToHoist(GEPI))
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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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Changed = false;
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// Get our Loop and Alias Analysis information...
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LI = &getAnalysis<LoopInfo>();
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AA = &getAnalysis<AliasAnalysis>();
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DT = &getAnalysis<DominatorTree>();
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// Hoist expressions out of all of the top-level loops.
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const std::vector<Loop*> &TopLevelLoops = LI->getTopLevelLoops();
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for (std::vector<Loop*>::const_iterator I = TopLevelLoops.begin(),
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E = TopLevelLoops.end(); I != E; ++I) {
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AliasSetTracker AST(*AA);
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LICM::visitLoop(*I, AST);
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}
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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, AliasSetTracker &AST) {
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// Recurse through all subloops before we process this loop...
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for (std::vector<Loop*>::const_iterator I = L->getSubLoops().begin(),
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E = L->getSubLoops().end(); I != E; ++I) {
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AliasSetTracker SubAST(*AA);
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LICM::visitLoop(*I, SubAST);
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// Incorporate information about the subloops into this loop...
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AST.add(SubAST);
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}
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CurLoop = L;
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CurAST = &AST;
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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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// Loop over the body of this loop, looking for calls, invokes, and stores.
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// Because subloops have already been incorporated into AST, we skip blocks in
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// subloops.
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//
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const std::vector<BasicBlock*> &LoopBBs = L->getBlocks();
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for (std::vector<BasicBlock*>::const_iterator I = LoopBBs.begin(),
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E = LoopBBs.end(); I != E; ++I)
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if (LI->getLoopFor(*I) == L) // Ignore blocks in subloops...
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AST.add(**I); // Incorporate the specified basic block
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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(DT->getNode(L->getHeader()));
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// Now that all loop invariants have been removed from the loop, promote any
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// memory references to scalars that we can...
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if (!DisablePromotion)
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PromoteValuesInLoop();
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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->getBlock())) 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->getBlock()))
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visit(*N->getBlock());
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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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/// SafeToHoist - Only hoist an instruction if it is not a trapping instruction
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/// or if it is a trapping instruction and is guaranteed to execute
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///
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bool LICM::SafeToHoist(Instruction &Inst) {
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//If it is a trapping instruction, then check if its guaranteed to execute.
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if(Inst.isTrapping()) {
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//Get the instruction's basic block.
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BasicBlock *InstBB = Inst.getParent();
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//Get the Dominator Tree Node for the instruction's basic block/
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DominatorTree::Node *InstDTNode = DT->getNode(InstBB);
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//Get the exit blocks for the current loop.
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const std::vector<BasicBlock* > &ExitBlocks = CurLoop->getExitBlocks();
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//For each exit block, get the DT node and walk up the DT until
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//the instruction's basic block is found or we exit the loop.
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for(unsigned i=0; i < ExitBlocks.size(); ++i) {
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DominatorTree::Node *IDom = DT->getNode(ExitBlocks[i]);
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while(IDom != InstDTNode) {
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//Get next Immediate Dominator.
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IDom = IDom->getIDom();
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//See if we exited the loop.
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if(!CurLoop->contains(IDom->getBlock()))
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return false;
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}
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}
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}
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return true;
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}
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void LICM::visitLoadInst(LoadInst &LI) {
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if (isLoopInvariant(LI.getOperand(0)) && !LI.isVolatile() &&
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!pointerInvalidatedByLoop(LI.getOperand(0)) && SafeToHoist(LI)) {
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hoist(LI);
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++NumHoistedLoads;
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}
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}
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/// PromoteValuesInLoop - Try to promote memory values to scalars by sinking
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/// stores out of the loop and moving loads to before the loop. We do this by
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/// looping over the stores in the loop, looking for stores to Must pointers
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/// which are loop invariant. We promote these memory locations to use allocas
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/// instead. These allocas can easily be raised to register values by the
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/// PromoteMem2Reg functionality.
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///
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void LICM::PromoteValuesInLoop() {
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// PromotedValues - List of values that are promoted out of the loop. Each
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// value has an alloca instruction for it, and a canonical version of the
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// pointer.
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std::vector<std::pair<AllocaInst*, Value*> > PromotedValues;
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std::map<Value*, AllocaInst*> ValueToAllocaMap; // Map of ptr to alloca
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findPromotableValuesInLoop(PromotedValues, ValueToAllocaMap);
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if (ValueToAllocaMap.empty()) return; // If there are values to promote...
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Changed = true;
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NumPromoted += PromotedValues.size();
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// Emit a copy from the value into the alloca'd value in the loop preheader
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TerminatorInst *LoopPredInst = Preheader->getTerminator();
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for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) {
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// Load from the memory we are promoting...
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LoadInst *LI = new LoadInst(PromotedValues[i].second,
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PromotedValues[i].second->getName()+".promoted",
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LoopPredInst);
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// Store into the temporary alloca...
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new StoreInst(LI, PromotedValues[i].first, LoopPredInst);
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}
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// Scan the basic blocks in the loop, replacing uses of our pointers with
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// uses of the allocas in question. If we find a branch that exits the
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// loop, make sure to put reload code into all of the successors of the
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// loop.
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//
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const std::vector<BasicBlock*> &LoopBBs = CurLoop->getBlocks();
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for (std::vector<BasicBlock*>::const_iterator I = LoopBBs.begin(),
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E = LoopBBs.end(); I != E; ++I) {
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// Rewrite all loads and stores in the block of the pointer...
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for (BasicBlock::iterator II = (*I)->begin(), E = (*I)->end();
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II != E; ++II) {
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if (LoadInst *L = dyn_cast<LoadInst>(II)) {
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std::map<Value*, AllocaInst*>::iterator
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I = ValueToAllocaMap.find(L->getOperand(0));
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if (I != ValueToAllocaMap.end())
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L->setOperand(0, I->second); // Rewrite load instruction...
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} else if (StoreInst *S = dyn_cast<StoreInst>(II)) {
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std::map<Value*, AllocaInst*>::iterator
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I = ValueToAllocaMap.find(S->getOperand(1));
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if (I != ValueToAllocaMap.end())
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S->setOperand(1, I->second); // Rewrite store instruction...
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}
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}
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// Check to see if any successors of this block are outside of the loop.
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// If so, we need to copy the value from the alloca back into the memory
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// location...
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//
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for (succ_iterator SI = succ_begin(*I), SE = succ_end(*I); SI != SE; ++SI)
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if (!CurLoop->contains(*SI)) {
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// Copy all of the allocas into their memory locations...
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BasicBlock::iterator BI = (*SI)->begin();
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while (isa<PHINode>(*BI))
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++BI; // Skip over all of the phi nodes in the block...
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Instruction *InsertPos = BI;
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for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) {
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// Load from the alloca...
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LoadInst *LI = new LoadInst(PromotedValues[i].first, "", InsertPos);
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// Store into the memory we promoted...
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new StoreInst(LI, PromotedValues[i].second, InsertPos);
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}
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}
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}
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// Now that we have done the deed, use the mem2reg functionality to promote
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// all of the new allocas we just created into real SSA registers...
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//
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std::vector<AllocaInst*> PromotedAllocas;
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PromotedAllocas.reserve(PromotedValues.size());
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for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i)
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PromotedAllocas.push_back(PromotedValues[i].first);
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PromoteMemToReg(PromotedAllocas, getAnalysis<DominanceFrontier>(),
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AA->getTargetData());
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}
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/// findPromotableValuesInLoop - Check the current loop for stores to definite
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/// pointers, which are not loaded and stored through may aliases. If these are
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/// found, create an alloca for the value, add it to the PromotedValues list,
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/// and keep track of the mapping from value to alloca...
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///
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void LICM::findPromotableValuesInLoop(
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std::vector<std::pair<AllocaInst*, Value*> > &PromotedValues,
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std::map<Value*, AllocaInst*> &ValueToAllocaMap) {
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Instruction *FnStart = CurLoop->getHeader()->getParent()->begin()->begin();
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// Loop over all of the alias sets in the tracker object...
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for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
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I != E; ++I) {
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AliasSet &AS = *I;
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// We can promote this alias set if it has a store, if it is a "Must" alias
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// set, and if the pointer is loop invariant.
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if (!AS.isForwardingAliasSet() && AS.isMod() && AS.isMustAlias() &&
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isLoopInvariant(AS.begin()->first)) {
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assert(AS.begin() != AS.end() &&
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"Must alias set should have at least one pointer element in it!");
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Value *V = AS.begin()->first;
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// Check that all of the pointers in the alias set have the same type. We
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// cannot (yet) promote a memory location that is loaded and stored in
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// different sizes.
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bool PointerOk = true;
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for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I)
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if (V->getType() != I->first->getType()) {
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PointerOk = false;
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break;
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}
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if (PointerOk) {
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const Type *Ty = cast<PointerType>(V->getType())->getElementType();
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AllocaInst *AI = new AllocaInst(Ty, 0, V->getName()+".tmp", FnStart);
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PromotedValues.push_back(std::make_pair(AI, V));
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for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I)
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ValueToAllocaMap.insert(std::make_pair(I->first, AI));
|
|
|
|
DEBUG(std::cerr << "LICM: Promoting value: " << *V << "\n");
|
|
}
|
|
}
|
|
}
|
|
}
|