forked from OSchip/llvm-project
360 lines
14 KiB
C++
360 lines
14 KiB
C++
//===-- LoopUnswitch.cpp - Hoist loop-invariant conditionals in loop ------===//
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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 the LLVM research group and is distributed under
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// the 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 that contain branches on loop-invariant conditions
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// to have multiple loops. For example, it turns the left into the right code:
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//
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// for (...) if (lic)
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// A for (...)
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// if (lic) A; B; C
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// B else
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// C for (...)
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// A; C
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//
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// This can increase the size of the code exponentially (doubling it every time
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// a loop is unswitched) so we only unswitch if the resultant code will be
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// smaller than a threshold.
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//
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// This pass expects LICM to be run before it to hoist invariant conditions out
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// of the loop, to make the unswitching opportunity obvious.
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "loop-unswitch"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Constants.h"
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#include "llvm/Function.h"
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#include "llvm/Instructions.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/Transforms/Utils/Cloning.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/ADT/Statistic.h"
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#include <algorithm>
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#include <iostream>
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using namespace llvm;
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namespace {
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Statistic<> NumUnswitched("loop-unswitch", "Number of loops unswitched");
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class LoopUnswitch : public FunctionPass {
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LoopInfo *LI; // Loop information
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DominatorSet *DS;
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public:
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virtual bool runOnFunction(Function &F);
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bool visitLoop(Loop *L);
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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.addRequiredID(LoopSimplifyID);
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//AU.addRequired<DominatorSet>();
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AU.addRequired<LoopInfo>();
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AU.addPreserved<LoopInfo>();
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}
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private:
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void VersionLoop(Value *LIC, Loop *L);
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BasicBlock *SplitBlock(BasicBlock *BB, bool SplitAtTop);
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void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, bool Val);
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};
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RegisterOpt<LoopUnswitch> X("loop-unswitch", "Unswitch loops");
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}
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FunctionPass *llvm::createLoopUnswitchPass() { return new LoopUnswitch(); }
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bool LoopUnswitch::runOnFunction(Function &F) {
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bool Changed = false;
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LI = &getAnalysis<LoopInfo>();
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DS = 0; //&getAnalysis<DominatorSet>();
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// Transform all the top-level loops. Copy the loop list so that the child
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// can update the loop tree if it needs to delete the loop.
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std::vector<Loop*> SubLoops(LI->begin(), LI->end());
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for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
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Changed |= visitLoop(SubLoops[i]);
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return Changed;
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}
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bool LoopUnswitch::visitLoop(Loop *L) {
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bool Changed = false;
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// Recurse through all subloops before we process this loop. Copy the loop
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// list so that the child can update the loop tree if it needs to delete the
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// loop.
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std::vector<Loop*> SubLoops(L->begin(), L->end());
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for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
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Changed |= visitLoop(SubLoops[i]);
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// Loop over all of the basic blocks in the loop. If we find an interior
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// block that is branching on a loop-invariant condition, we can unswitch this
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// loop.
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for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
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I != E; ++I) {
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TerminatorInst *TI = (*I)->getTerminator();
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if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
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if (!isa<Constant>(SI) && L->isLoopInvariant(SI->getCondition()))
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DEBUG(std::cerr << "Can't unswitching 'switch' loop %"
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<< L->getHeader()->getName() << ", cost = "
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<< L->getBlocks().size() << "\n" << **I);
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} else if (BranchInst *BI = dyn_cast<BranchInst>(TI))
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if (BI->isConditional() && !isa<Constant>(BI->getCondition()) &&
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L->isLoopInvariant(BI->getCondition())) {
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// Check to see if it would be profitable to unswitch this loop.
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if (L->getBlocks().size() > 10) {
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DEBUG(std::cerr << "NOT unswitching loop %"
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<< L->getHeader()->getName() << ", cost too high: "
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<< L->getBlocks().size() << "\n");
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} else {
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// FIXME: check for profitability.
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//std::cerr << "BEFORE:\n"; LI->dump();
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VersionLoop(BI->getCondition(), L);
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//std::cerr << "AFTER:\n"; LI->dump();
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return true;
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}
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}
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}
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return Changed;
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}
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/// SplitBlock - Split the specified basic block into two pieces. If SplitAtTop
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/// is false, this splits the block so the second half only has an unconditional
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/// branch. If SplitAtTop is true, it makes it so the first half of the block
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/// only has an unconditional branch in it.
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///
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/// This method updates the LoopInfo for this function to correctly reflect the
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/// CFG changes made.
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BasicBlock *LoopUnswitch::SplitBlock(BasicBlock *BB, bool SplitAtTop) {
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BasicBlock::iterator SplitPoint;
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if (!SplitAtTop)
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SplitPoint = BB->getTerminator();
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else {
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SplitPoint = BB->begin();
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while (isa<PHINode>(SplitPoint)) ++SplitPoint;
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}
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BasicBlock *New = BB->splitBasicBlock(SplitPoint, BB->getName()+".tail");
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// New now lives in whichever loop that BB used to.
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if (Loop *L = LI->getLoopFor(BB))
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L->addBasicBlockToLoop(New, *LI);
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return SplitAtTop ? BB : New;
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}
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// RemapInstruction - Convert the instruction operands from referencing the
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// current values into those specified by ValueMap.
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//
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static inline void RemapInstruction(Instruction *I,
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std::map<const Value *, Value*> &ValueMap) {
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for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
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Value *Op = I->getOperand(op);
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std::map<const Value *, Value*>::iterator It = ValueMap.find(Op);
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if (It != ValueMap.end()) Op = It->second;
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I->setOperand(op, Op);
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}
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}
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/// CloneLoop - Recursively clone the specified loop and all of its children,
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/// mapping the blocks with the specified map.
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static Loop *CloneLoop(Loop *L, Loop *PL, std::map<const Value*, Value*> &VM,
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LoopInfo *LI) {
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Loop *New = new Loop();
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if (PL)
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PL->addChildLoop(New);
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else
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LI->addTopLevelLoop(New);
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// Add all of the blocks in L to the new loop.
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for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
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I != E; ++I)
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if (LI->getLoopFor(*I) == L)
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New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), *LI);
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// Add all of the subloops to the new loop.
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for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
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CloneLoop(*I, New, VM, LI);
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return New;
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}
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/// InsertPHINodesForUsesOutsideLoop - If this instruction is used outside of
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/// the specified loop, insert a PHI node in the appropriate exit block to merge
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/// the values in the two different loop versions.
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///
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/// Most values are not used outside of the loop they are defined in, so be
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/// efficient for this case.
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///
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static AllocaInst *
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InsertPHINodesForUsesOutsideLoop(Instruction *OI, Instruction *NI,
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DominatorSet &DS, Loop *OL, Loop *NL,
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std::vector<BasicBlock*> &OldExitBlocks,
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std::map<const Value*, Value*> &ValueMap) {
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assert(OI->getType() == NI->getType() && OI->getOpcode() == NI->getOpcode() &&
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"Hrm, should be mapping between identical instructions!");
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for (Value::use_iterator UI = OI->use_begin(), E = OI->use_end(); UI != E;
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++UI)
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if (!OL->contains(cast<Instruction>(*UI)->getParent()) &&
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!NL->contains(cast<Instruction>(*UI)->getParent()))
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goto UsedOutsideOfLoop;
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return 0;
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UsedOutsideOfLoop:
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// Okay, this instruction is used outside of the current loop. Insert a PHI
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// nodes for the instruction merging the values together.
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// FIXME: For now we just spill the object to the stack, assuming that a
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// subsequent mem2reg pass will clean up after us. This should be improved in
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// two ways:
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// 1. If there is only one exit block, trivially insert the PHI nodes
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// 2. Once we update domfrontier, we should do the promotion after everything
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// is stable again.
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AllocaInst *Result = DemoteRegToStack(*OI);
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// Store to the stack location right after the new instruction.
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BasicBlock::iterator InsertPoint = NI;
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if (InvokeInst *II = dyn_cast<InvokeInst>(NI))
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InsertPoint = II->getNormalDest()->begin();
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else
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++InsertPoint;
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while (isa<PHINode>(InsertPoint)) ++InsertPoint;
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new StoreInst(NI, Result, InsertPoint);
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return Result;
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}
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/// VersionLoop - We determined that the loop is profitable to unswitch and
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/// contains a branch on a loop invariant condition. Split it into loop
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/// versions and test the condition outside of either loop.
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void LoopUnswitch::VersionLoop(Value *LIC, Loop *L) {
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Function *F = L->getHeader()->getParent();
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DEBUG(std::cerr << "loop-unswitch: Unswitching loop %"
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<< L->getHeader()->getName() << " [" << L->getBlocks().size()
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<< " blocks] in Function " << F->getName()
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<< " on cond:" << *LIC << "\n");
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std::vector<BasicBlock*> LoopBlocks;
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// First step, split the preheader and exit blocks, and add these blocks to
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// the LoopBlocks list.
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BasicBlock *OrigPreheader = L->getLoopPreheader();
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LoopBlocks.push_back(SplitBlock(OrigPreheader, false));
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// We want the loop to come after the preheader, but before the exit blocks.
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LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
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std::vector<BasicBlock*> ExitBlocks;
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L->getExitBlocks(ExitBlocks);
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std::sort(ExitBlocks.begin(), ExitBlocks.end());
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ExitBlocks.erase(std::unique(ExitBlocks.begin(), ExitBlocks.end()),
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ExitBlocks.end());
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for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
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LoopBlocks.push_back(ExitBlocks[i] = SplitBlock(ExitBlocks[i], true));
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// Next step, clone all of the basic blocks that make up the loop (including
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// the loop preheader and exit blocks), keeping track of the mapping between
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// the instructions and blocks.
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std::vector<BasicBlock*> NewBlocks;
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NewBlocks.reserve(LoopBlocks.size());
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std::map<const Value*, Value*> ValueMap;
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for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
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NewBlocks.push_back(CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F));
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ValueMap[LoopBlocks[i]] = NewBlocks.back(); // Keep the BB mapping.
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}
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// Splice the newly inserted blocks into the function right before the
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// original preheader.
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F->getBasicBlockList().splice(LoopBlocks[0], F->getBasicBlockList(),
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NewBlocks[0], F->end());
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// Now we create the new Loop object for the versioned loop.
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Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI);
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if (Loop *Parent = L->getParentLoop()) {
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// Make sure to add the cloned preheader and exit blocks to the parent loop
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// as well.
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Parent->addBasicBlockToLoop(NewBlocks[0], *LI);
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for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
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Parent->addBasicBlockToLoop(cast<BasicBlock>(ValueMap[ExitBlocks[i]]),
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*LI);
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}
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// Rewrite the code to refer to itself.
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for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
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for (BasicBlock::iterator I = NewBlocks[i]->begin(),
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E = NewBlocks[i]->end(); I != E; ++I)
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RemapInstruction(I, ValueMap);
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// If the instructions are used outside of the loop, insert a PHI node in any
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// exit blocks dominated by the instruction.
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for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
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for (BasicBlock::iterator OI = LoopBlocks[i]->begin(),
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E = LoopBlocks[i]->end(); OI != E; ++OI)
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if (!OI->use_empty()) {
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std::map<const Value*,Value*>::iterator OII = ValueMap.find(OI);
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// The PHINode rewriting stuff can insert stores that are not in the
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// mapping. Don't mess around with them.
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if (OII != ValueMap.end()) {
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Instruction *NI = cast<Instruction>(OII->second);
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InsertPHINodesForUsesOutsideLoop(OI, NI, *DS, L, NewLoop,
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ExitBlocks, ValueMap);
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}
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}
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// Rewrite the original preheader to select between versions of the loop.
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assert(isa<BranchInst>(OrigPreheader->getTerminator()) &&
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cast<BranchInst>(OrigPreheader->getTerminator())->isUnconditional() &&
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OrigPreheader->getTerminator()->getSuccessor(0) == LoopBlocks[0] &&
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"Preheader splitting did not work correctly!");
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// Remove the unconditional branch to LoopBlocks[0].
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OrigPreheader->getInstList().pop_back();
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// Insert a conditional branch on LIC to the two preheaders. The original
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// code is the true version and the new code is the false version.
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new BranchInst(LoopBlocks[0], NewBlocks[0], LIC, OrigPreheader);
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// Now we rewrite the original code to know that the condition is true and the
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// new code to know that the condition is false.
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RewriteLoopBodyWithConditionConstant(L, LIC, true);
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RewriteLoopBodyWithConditionConstant(NewLoop, LIC, false);
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++NumUnswitched;
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// Try to unswitch each of our new loops now!
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visitLoop(L);
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visitLoop(NewLoop);
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}
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// RewriteLoopBodyWithConditionConstant - We know that the boolean value LIC has
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// the value specified by Val in the specified loop. Rewrite any uses of LIC or
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// of properties correlated to it.
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void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
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bool Val) {
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// FIXME: Support correlated properties, like:
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// for (...)
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// if (li1 < li2)
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// ...
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// if (li1 > li2)
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// ...
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ConstantBool *BoolVal = ConstantBool::get(Val);
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std::vector<User*> Users(LIC->use_begin(), LIC->use_end());
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for (unsigned i = 0, e = Users.size(); i != e; ++i)
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if (Instruction *U = dyn_cast<Instruction>(Users[i]))
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if (L->contains(U->getParent()))
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U->replaceUsesOfWith(LIC, BoolVal);
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}
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