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cases. Ideally, this issue will go away in the future as LCSSA gets smarter about which Phi nodes it inserts. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@29076 91177308-0d34-0410-b5e6-96231b3b80d8
298 lines
11 KiB
C++
298 lines
11 KiB
C++
//===-- LCSSA.cpp - Convert loops into loop-closed SSA form ---------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file was developed by Owen Anderson and is distributed under the
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// University of Illinois Open Source License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This pass transforms loops by placing phi nodes at the end of the loops for
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// all values that are live across the loop boundary. For example, it turns
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// the left into the right code:
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//
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// for (...) for (...)
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// if (c) if(c)
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// X1 = ... X1 = ...
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// else else
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// X2 = ... X2 = ...
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// X3 = phi(X1, X2) X3 = phi(X1, X2)
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// ... = X3 + 4 X4 = phi(X3)
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// ... = X4 + 4
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//
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// This is still valid LLVM; the extra phi nodes are purely redundant, and will
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// be trivially eliminated by InstCombine. The major benefit of this
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// transformation is that it makes many other loop optimizations, such as
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// LoopUnswitching, simpler.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Constants.h"
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#include "llvm/Pass.h"
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#include "llvm/Function.h"
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#include "llvm/Instructions.h"
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#include "llvm/ADT/SetVector.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/Support/CFG.h"
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#include <algorithm>
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#include <map>
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using namespace llvm;
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namespace {
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static Statistic<> NumLCSSA("lcssa",
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"Number of live out of a loop variables");
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class LCSSA : public FunctionPass {
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public:
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LoopInfo *LI; // Loop information
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DominatorTree *DT; // Dominator Tree for the current Function...
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DominanceFrontier *DF; // Current Dominance Frontier
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std::vector<BasicBlock*> LoopBlocks;
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virtual bool runOnFunction(Function &F);
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bool visitSubloop(Loop* L);
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void processInstruction(Instruction* Instr,
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const std::vector<BasicBlock*>& exitBlocks);
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/// This transformation requires natural loop information & requires that
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/// loop preheaders be inserted into the CFG. It maintains both of these,
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/// as well as the CFG. It also requires dominator information.
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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(LoopSimplifyID);
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AU.addPreservedID(LoopSimplifyID);
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AU.addRequired<LoopInfo>();
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AU.addRequired<DominatorTree>();
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AU.addRequired<DominanceFrontier>();
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}
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private:
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SetVector<Instruction*> getLoopValuesUsedOutsideLoop(Loop *L);
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Value *getValueDominatingBlock(BasicBlock *BB,
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std::map<BasicBlock*, Value*>& PotDoms) {
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return getValueDominatingDTNode(DT->getNode(BB), PotDoms);
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}
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Value *getValueDominatingDTNode(DominatorTree::Node *Node,
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std::map<BasicBlock*, Value*>& PotDoms);
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/// inLoop - returns true if the given block is within the current loop
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const bool inLoop(BasicBlock* B) {
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return std::binary_search(LoopBlocks.begin(), LoopBlocks.end(), B);
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}
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};
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RegisterOpt<LCSSA> X("lcssa", "Loop-Closed SSA Form Pass");
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}
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FunctionPass *llvm::createLCSSAPass() { return new LCSSA(); }
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const PassInfo *llvm::LCSSAID = X.getPassInfo();
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/// runOnFunction - Process all loops in the function, inner-most out.
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bool LCSSA::runOnFunction(Function &F) {
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bool changed = false;
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LI = &getAnalysis<LoopInfo>();
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DF = &getAnalysis<DominanceFrontier>();
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DT = &getAnalysis<DominatorTree>();
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for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I) {
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changed |= visitSubloop(*I);
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}
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return changed;
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}
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/// visitSubloop - Recursively process all subloops, and then process the given
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/// loop if it has live-out values.
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bool LCSSA::visitSubloop(Loop* L) {
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for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
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visitSubloop(*I);
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// Speed up queries by creating a sorted list of blocks
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LoopBlocks.clear();
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LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
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std::sort(LoopBlocks.begin(), LoopBlocks.end());
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SetVector<Instruction*> AffectedValues = getLoopValuesUsedOutsideLoop(L);
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// If no values are affected, we can save a lot of work, since we know that
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// nothing will be changed.
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if (AffectedValues.empty())
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return false;
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std::vector<BasicBlock*> exitBlocks;
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L->getExitBlocks(exitBlocks);
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// Iterate over all affected values for this loop and insert Phi nodes
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// for them in the appropriate exit blocks
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for (SetVector<Instruction*>::iterator I = AffectedValues.begin(),
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E = AffectedValues.end(); I != E; ++I) {
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processInstruction(*I, exitBlocks);
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}
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assert(L->isLCSSAForm());
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return true;
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}
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/// processInstruction - Given a live-out instruction, insert LCSSA Phi nodes,
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/// eliminate all out-of-loop uses.
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void LCSSA::processInstruction(Instruction* Instr,
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const std::vector<BasicBlock*>& exitBlocks)
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{
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++NumLCSSA; // We are applying the transformation
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std::map<BasicBlock*, Value*> Phis;
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// Add the base instruction to the Phis list. This makes tracking down
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// the dominating values easier when we're filling in Phi nodes. This will
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// be removed later, before we perform use replacement.
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Phis[Instr->getParent()] = Instr;
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// Phi nodes that need to be IDF-processed
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std::vector<PHINode*> workList;
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for (std::vector<BasicBlock*>::const_iterator BBI = exitBlocks.begin(),
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BBE = exitBlocks.end(); BBI != BBE; ++BBI) {
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Value*& phi = Phis[*BBI];
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if (phi == 0 &&
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DT->getNode(Instr->getParent())->dominates(DT->getNode(*BBI))) {
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phi = new PHINode(Instr->getType(), Instr->getName()+".lcssa",
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(*BBI)->begin());
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workList.push_back(cast<PHINode>(phi));
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}
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}
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// Phi nodes that need to have their incoming values filled.
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std::vector<PHINode*> needIncomingValues;
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// Calculate the IDF of these LCSSA Phi nodes, inserting new Phi's where
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// necessary. Keep track of these new Phi's in the "Phis" map.
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while (!workList.empty()) {
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PHINode *CurPHI = workList.back();
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workList.pop_back();
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// Even though we've removed this Phi from the work list, we still need
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// to fill in its incoming values.
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needIncomingValues.push_back(CurPHI);
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// Get the current Phi's DF, and insert Phi nodes. Add these new
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// nodes to our worklist.
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DominanceFrontier::const_iterator it = DF->find(CurPHI->getParent());
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if (it != DF->end()) {
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const DominanceFrontier::DomSetType &S = it->second;
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for (DominanceFrontier::DomSetType::const_iterator P = S.begin(),
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PE = S.end(); P != PE; ++P) {
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if (DT->getNode(Instr->getParent())->dominates(DT->getNode(*P))) {
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Value *&Phi = Phis[*P];
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if (Phi == 0) {
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// Still doesn't have operands...
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Phi = new PHINode(Instr->getType(), Instr->getName()+".lcssa",
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(*P)->begin());
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workList.push_back(cast<PHINode>(Phi));
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}
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}
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}
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}
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}
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// Fill in all Phis we've inserted that need their incoming values filled in.
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for (std::vector<PHINode*>::iterator IVI = needIncomingValues.begin(),
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IVE = needIncomingValues.end(); IVI != IVE; ++IVI)
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for (pred_iterator PI = pred_begin((*IVI)->getParent()),
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E = pred_end((*IVI)->getParent()); PI != E; ++PI)
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(*IVI)->addIncoming(getValueDominatingBlock(*PI, Phis),
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*PI);
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// Find all uses of the affected value, and replace them with the
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// appropriate Phi.
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std::vector<Instruction*> Uses;
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for (Instruction::use_iterator UI = Instr->use_begin(), UE = Instr->use_end();
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UI != UE; ++UI) {
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Instruction* use = cast<Instruction>(*UI);
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BasicBlock* UserBB = use->getParent();
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if (PHINode* p = dyn_cast<PHINode>(use)) {
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unsigned OperandNo = UI.getOperandNo();
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UserBB = p->getIncomingBlock(OperandNo/2);
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}
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// Don't need to update uses within the loop body.
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if (!inLoop(use->getParent()))
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Uses.push_back(use);
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}
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for (std::vector<Instruction*>::iterator II = Uses.begin(), IE = Uses.end();
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II != IE; ++II) {
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if (PHINode* phi = dyn_cast<PHINode>(*II)) {
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for (unsigned int i = 0; i < phi->getNumIncomingValues(); ++i) {
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if (phi->getIncomingValue(i) == Instr) {
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Value* dominator =
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getValueDominatingBlock(phi->getIncomingBlock(i), Phis);
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phi->setIncomingValue(i, dominator);
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}
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}
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} else {
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Value *NewVal = getValueDominatingBlock((*II)->getParent(), Phis);
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(*II)->replaceUsesOfWith(Instr, NewVal);
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}
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}
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}
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/// getLoopValuesUsedOutsideLoop - Return any values defined in the loop that
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/// are used by instructions outside of it.
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SetVector<Instruction*> LCSSA::getLoopValuesUsedOutsideLoop(Loop *L) {
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// FIXME: For large loops, we may be able to avoid a lot of use-scanning
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// by using dominance information. In particular, if a block does not
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// dominate any of the loop exits, then none of the values defined in the
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// block could be used outside the loop.
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SetVector<Instruction*> AffectedValues;
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for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
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BB != E; ++BB) {
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for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ++I)
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for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
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++UI) {
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BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
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if (PHINode* p = dyn_cast<PHINode>(*UI)) {
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unsigned OperandNo = UI.getOperandNo();
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UserBB = p->getIncomingBlock(OperandNo/2);
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}
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if (!inLoop(UserBB)) {
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AffectedValues.insert(I);
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break;
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}
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}
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}
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return AffectedValues;
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}
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/// getValueDominatingBlock - Return the value within the potential dominators
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/// map that dominates the given block.
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Value *LCSSA::getValueDominatingDTNode(DominatorTree::Node *Node,
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std::map<BasicBlock*, Value*>& PotDoms) {
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// FIXME: The following insertion should be in place rather than the if
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// statement. Currently, this is due to the fact that LCSSA isn't smart
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// enough to avoid inserting IDF Phis that don't dominate any uses. In some
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// of those cases, it could ask us to provide a dominating value for a block
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// that has none, so we need to return undef.
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//assert(Node != 0 && "Didn't find dom value?");
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if (Node == 0) return UndefValue::get(PotDoms.begin()->second->getType());
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Value *&CacheSlot = PotDoms[Node->getBlock()];
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if (CacheSlot) return CacheSlot;
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// Otherwise, return the value of the idom and remember this for next time.
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return CacheSlot = getValueDominatingDTNode(Node->getIDom(), PotDoms);
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}
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