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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@28619 91177308-0d34-0410-b5e6-96231b3b80d8
263 lines
9.4 KiB
C++
263 lines
9.4 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/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/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 <cassert>
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#include <map>
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#include <vector>
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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 Loop...
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DominanceFrontier *DF; // Current Dominance Frontier
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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*>& LoopBlocks,
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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.addPreserved<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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std::set<Instruction*> getLoopValuesUsedOutsideLoop(Loop *L,
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const std::vector<BasicBlock*>& LoopBlocks);
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Instruction *getValueDominatingBlock(BasicBlock *BB,
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std::map<BasicBlock*, Instruction*> PotDoms);
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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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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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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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std::vector<BasicBlock*> LoopBlocks(L->block_begin(), L->block_end());
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std::sort(LoopBlocks.begin(), LoopBlocks.end());
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std::set<Instruction*> AffectedValues = getLoopValuesUsedOutsideLoop(L,
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LoopBlocks);
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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 (std::set<Instruction*>::iterator I = AffectedValues.begin(),
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E = AffectedValues.end(); I != E; ++I) {
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processInstruction(*I, LoopBlocks, exitBlocks);
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}
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return true; // FIXME: Should be more intelligent in our return value.
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}
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/// processInstruction -
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void LCSSA::processInstruction(Instruction* Instr,
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const std::vector<BasicBlock*>& LoopBlocks,
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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*, Instruction*> Phis;
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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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if (DT->getNode(Instr->getParent())->dominates(DT->getNode(*BBI))) {
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PHINode *phi = new PHINode(Instr->getType(), "lcssa", (*BBI)->begin());
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workList.push_back(phi);
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Phis[*BBI] = phi;
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}
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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 Phis.
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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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// 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 (Phis[*P] == 0) {
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// Still doesn't have operands...
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PHINode *phi = new PHINode(Instr->getType(), "lcssa", (*P)->begin());
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Phis[*P] = phi;
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workList.push_back(phi);
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}
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}
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}
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// Get the predecessor blocks of the current Phi, and use them to hook up
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// the operands of the current Phi to any members of DFPhis that dominate
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// it. This is a nop for the Phis inserted directly in the exit blocks,
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// since they are not dominated by any members of DFPhis.
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for (pred_iterator PI = pred_begin(CurPHI->getParent()),
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E = pred_end(CurPHI->getParent()); PI != E; ++PI)
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CurPHI->addIncoming(getValueDominatingBlock(*PI, Phis),
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*PI);
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}
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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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// Don't need to update uses within the loop body
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if (!std::binary_search(LoopBlocks.begin(), LoopBlocks.end(),
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use->getParent()) &&
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!(std::binary_search(exitBlocks.begin(), exitBlocks.end(),
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use->getParent()) && isa<PHINode>(use)))
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Uses.push_back(use);
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}
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// Deliberately remove the initial instruction from Phis set.
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Phis.erase(Instr->getParent());
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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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Instruction* dominator =
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getValueDominatingBlock(phi->getIncomingBlock(i), Phis);
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if (phi->getIncomingValue(i) == Instr)
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phi->setIncomingValue(i, dominator);
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}
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} else {
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(*II)->replaceUsesOfWith(Instr,
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getValueDominatingBlock((*II)->getParent(),
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Phis));
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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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std::set<Instruction*> LCSSA::getLoopValuesUsedOutsideLoop(Loop *L,
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const std::vector<BasicBlock*>& LoopBlocks) {
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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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std::set<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 (!std::binary_search(LoopBlocks.begin(), LoopBlocks.end(), 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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Instruction *LCSSA::getValueDominatingBlock(BasicBlock *BB,
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std::map<BasicBlock*, Instruction*> PotDoms) {
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DominatorTree::Node* bbNode = DT->getNode(BB);
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while (bbNode != 0) {
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std::map<BasicBlock*, Instruction*>::iterator I =
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PotDoms.find(bbNode->getBlock());
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if (I != PotDoms.end()) {
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return (*I).second;
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}
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bbNode = bbNode->getIDom();
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}
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assert(0 && "No dominating value found.");
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return 0;
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}
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