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