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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@36994 91177308-0d34-0410-b5e6-96231b3b80d8
276 lines
10 KiB
C++
276 lines
10 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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#define DEBUG_TYPE "lcssa"
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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 "llvm/Support/Compiler.h"
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#include <algorithm>
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#include <map>
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using namespace llvm;
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STATISTIC(NumLCSSA, "Number of live out of a loop variables");
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namespace {
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struct VISIBILITY_HIDDEN LCSSA : public FunctionPass {
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static char ID; // Pass identification, replacement for typeid
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LCSSA() : FunctionPass((intptr_t)&ID) {}
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// Cached analysis information for the current function.
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LoopInfo *LI;
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DominatorTree *DT;
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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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}
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private:
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void getLoopValuesUsedOutsideLoop(Loop *L,
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SetVector<Instruction*> &AffectedValues);
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Value *GetValueForBlock(DominatorTree::Node *BB, Instruction *OrigInst,
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std::map<DominatorTree::Node*, Value*> &Phis);
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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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char LCSSA::ID = 0;
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RegisterPass<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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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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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;
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getLoopValuesUsedOutsideLoop(L, AffectedValues);
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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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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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++NumLCSSA; // We are applying the transformation
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// Keep track of the blocks that have the value available already.
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std::map<DominatorTree::Node*, Value*> Phis;
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DominatorTree::Node *InstrNode = DT->getNode(Instr->getParent());
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// Insert the LCSSA phi's into the exit blocks (dominated by the value), and
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// add them to the Phi's map.
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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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BasicBlock *BB = *BBI;
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DominatorTree::Node *ExitBBNode = DT->getNode(BB);
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Value *&Phi = Phis[ExitBBNode];
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if (!Phi && InstrNode->dominates(ExitBBNode)) {
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PHINode *PN = new PHINode(Instr->getType(), Instr->getName()+".lcssa",
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BB->begin());
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PN->reserveOperandSpace(std::distance(pred_begin(BB), pred_end(BB)));
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// Remember that this phi makes the value alive in this block.
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Phi = PN;
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// Add inputs from inside the loop for this PHI.
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for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
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PN->addIncoming(Instr, *PI);
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}
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}
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// Record all uses of Instr outside the loop. We need to rewrite these. The
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// LCSSA phis won't be included because they use the value in the loop.
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for (Value::use_iterator UI = Instr->use_begin(), E = Instr->use_end();
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UI != E;) {
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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 the user is in the loop, don't rewrite it!
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if (UserBB == Instr->getParent() || inLoop(UserBB)) {
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++UI;
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continue;
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}
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// Otherwise, patch up uses of the value with the appropriate LCSSA Phi,
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// inserting PHI nodes into join points where needed.
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Value *Val = GetValueForBlock(DT->getNode(UserBB), Instr, Phis);
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// Preincrement the iterator to avoid invalidating it when we change the
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// value.
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Use &U = UI.getUse();
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++UI;
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U.set(Val);
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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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void LCSSA::getLoopValuesUsedOutsideLoop(Loop *L,
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SetVector<Instruction*> &AffectedValues) {
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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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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 (*BB != UserBB && !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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}
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/// GetValueForBlock - Get the value to use within the specified basic block.
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/// available values are in Phis.
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Value *LCSSA::GetValueForBlock(DominatorTree::Node *BB, Instruction *OrigInst,
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std::map<DominatorTree::Node*, Value*> &Phis) {
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// If there is no dominator info for this BB, it is unreachable.
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if (BB == 0)
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return UndefValue::get(OrigInst->getType());
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// If we have already computed this value, return the previously computed val.
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Value *&V = Phis[BB];
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if (V) return V;
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DominatorTree::Node *IDom = BB->getIDom();
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// Otherwise, there are two cases: we either have to insert a PHI node or we
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// don't. We need to insert a PHI node if this block is not dominated by one
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// of the exit nodes from the loop (the loop could have multiple exits, and
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// though the value defined *inside* the loop dominated all its uses, each
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// exit by itself may not dominate all the uses).
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//
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// The simplest way to check for this condition is by checking to see if the
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// idom is in the loop. If so, we *know* that none of the exit blocks
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// dominate this block. Note that we *know* that the block defining the
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// original instruction is in the idom chain, because if it weren't, then the
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// original value didn't dominate this use.
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if (!inLoop(IDom->getBlock())) {
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// Idom is not in the loop, we must still be "below" the exit block and must
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// be fully dominated by the value live in the idom.
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return V = GetValueForBlock(IDom, OrigInst, Phis);
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}
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BasicBlock *BBN = BB->getBlock();
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// Otherwise, the idom is the loop, so we need to insert a PHI node. Do so
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// now, then get values to fill in the incoming values for the PHI.
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PHINode *PN = new PHINode(OrigInst->getType(), OrigInst->getName()+".lcssa",
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BBN->begin());
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PN->reserveOperandSpace(std::distance(pred_begin(BBN), pred_end(BBN)));
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V = PN;
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// Fill in the incoming values for the block.
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for (pred_iterator PI = pred_begin(BBN), E = pred_end(BBN); PI != E; ++PI)
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PN->addIncoming(GetValueForBlock(DT->getNode(*PI), OrigInst, Phis), *PI);
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return PN;
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}
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