Initial checkin of a simple loop unswitching pass. It still needs work,

but it's a start, and seems to do it's basic job.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@13068 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/LoopUnswitch.cpp

@@ -0,0 +1,357 @@
+//===-- 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);
+}