Initial checkin of LICM pass

llvm-svn: 2615

lib/Transforms/Scalar/LICM.cpp

@@ -0,0 +1,262 @@
+//===-- LICM.cpp - Loop Invariant Code Motion Pass ------------------------===//
+//
+// This pass is a simple loop invariant code motion pass.
+//
+// Note that this pass does NOT require pre-headers to exist on loops in the
+// CFG, but if there is not distinct preheader for a loop, the hoisted code will
+// be *DUPLICATED* in every basic block, outside of the loop, that preceeds the
+// loop header.  Additionally, any use of one of these hoisted expressions
+// cannot be loop invariant itself, because the expression hoisted gets a PHI
+// node that is loop variant.
+//
+// For these reasons, and many more, it makes sense to run a pass before this
+// that ensures that there are preheaders on all loops.  That said, we don't
+// REQUIRE it. :)
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Transforms/Utils/Local.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/iOperators.h"
+#include "llvm/iPHINode.h"
+#include "llvm/Support/InstVisitor.h"
+#include "llvm/Support/CFG.h"
+#include "Support/STLExtras.h"
+#include "Support/StatisticReporter.h"
+#include <algorithm>
+
+static Statistic<> NumHoistedNPH("licm\t\t- Number of insts hoisted to multiple"
+                                 " loop preds (bad, no loop pre-header)");
+static Statistic<> NumHoistedPH("licm\t\t- Number of insts hoisted to a loop "
+                                "pre-header");
+
+namespace {
+  struct LICM : public FunctionPass, public InstVisitor<LICM> {
+    const char *getPassName() const { return "Loop Invariant Code Motion"; }
+
+    virtual bool runOnFunction(Function *F);
+
+    // This transformation requires natural loop information...
+    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+      AU.preservesCFG();
+      AU.addRequired(LoopInfo::ID); 
+    }
+
+  private:
+    // List of predecessor blocks for the current loop - These blocks are where
+    // we hoist loop invariants to for the current loop.
+    //
+    std::vector<BasicBlock*> LoopPreds, LoopBackEdges;
+
+    Loop *CurLoop;  // The current loop we are working on...
+    bool Changed;   // Set to true when we change anything.
+
+    // visitLoop - Hoist expressions out of the specified loop...    
+    void visitLoop(Loop *L);
+
+    // notInCurrentLoop - Little predicate that returns true if the specified
+    // basic block is in a subloop of the current one, not the current one
+    // itself.
+    //
+    bool notInCurrentLoop(BasicBlock *BB) {
+      for (unsigned i = 0, e = CurLoop->getSubLoops().size(); i != e; ++i)
+        if (CurLoop->getSubLoops()[i]->contains(BB))
+          return true;  // A subloop actually contains this block!
+      return false;      
+    }
+
+    // hoist - When an instruction is found to only use loop invariant operands
+    // that is safe to hoist, this instruction is called to do the dirty work.
+    //
+    void hoist(Instruction *I);
+
+    // isLoopInvariant - Return true if the specified value is loop invariant
+    inline bool isLoopInvariant(Value *V) {
+      if (Instruction *I = dyn_cast<Instruction>(V))
+        return !CurLoop->contains(I->getParent());
+      return true;  // All non-instructions are loop invariant
+    }
+
+    // visitBasicBlock - Run LICM on a particular block.
+    void visitBasicBlock(BasicBlock *BB);
+
+    // Instruction visitation handlers... these basically control whether or not
+    // the specified instruction types are hoisted.
+    //
+    friend class InstVisitor<LICM>;
+    void visitUnaryOperator(Instruction *I) {
+      if (isLoopInvariant(I->getOperand(0))) hoist(I);
+    }
+    void visitBinaryOperator(Instruction *I) {
+      if (isLoopInvariant(I->getOperand(0)) &&isLoopInvariant(I->getOperand(1)))
+        hoist(I);
+    }
+
+    void visitCastInst(CastInst *I) { visitUnaryOperator((Instruction*)I); }
+    void visitShiftInst(ShiftInst *I) { visitBinaryOperator((Instruction*)I); }
+
+    void visitGetElementPtrInst(GetElementPtrInst *GEPI) {
+      Instruction *I = (Instruction*)GEPI;
+      for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
+        if (!isLoopInvariant(I->getOperand(i))) return;
+      hoist(I);
+    }
+  };
+}
+
+Pass *createLICMPass() { return new LICM(); }
+
+bool LICM::runOnFunction(Function *F) {
+  // get our loop information...
+  const std::vector<Loop*> &TopLevelLoops =
+    getAnalysis<LoopInfo>().getTopLevelLoops();
+
+  // Traverse loops in postorder, hoisting expressions out of the deepest loops
+  // first.
+  //
+  Changed = false;
+  std::for_each(TopLevelLoops.begin(), TopLevelLoops.end(),
+                bind_obj(this, &LICM::visitLoop));
+  return Changed;
+}
+
+void LICM::visitLoop(Loop *L) {
+  // Recurse through all subloops before we process this loop...
+  std::for_each(L->getSubLoops().begin(), L->getSubLoops().end(),
+                bind_obj(this, &LICM::visitLoop));
+  CurLoop = L;
+
+  // Calculate the set of predecessors for this loop.  The predecessors for this
+  // loop are equal to the predecessors for the header node of the loop that are
+  // not themselves in the loop.
+  //
+  BasicBlock *Header = L->getHeader();
+
+  // Calculate the sets of predecessors and backedges of the loop...
+  LoopBackEdges.insert(LoopBackEdges.end(),pred_begin(Header),pred_end(Header));
+
+  std::vector<BasicBlock*>::iterator LPI =
+    std::partition(LoopBackEdges.begin(), LoopBackEdges.end(),
+                   bind_obj(CurLoop, &Loop::contains));
+
+  // Move all predecessors to the LoopPreds vector...
+  LoopPreds.insert(LoopPreds.end(), LPI, LoopBackEdges.end());
+
+  // Remove predecessors from backedges list...
+  LoopBackEdges.erase(LPI, LoopBackEdges.end());
+ 
+
+  // The only way that there could be no predecessors to a loop is if the loop
+  // is not reachable.  Since we don't care about optimizing dead loops,
+  // summarily ignore them.
+  //
+  if (LoopPreds.empty()) return;
+  
+  // We want to visit all of the instructions in this loop... that are not parts
+  // of our subloops (they have already had their invariants hoisted out of
+  // their loop, into this loop, so there is no need to process the BODIES of
+  // the subloops).
+  //
+  std::vector<BasicBlock*> BBs(L->getBlocks().begin(), L->getBlocks().end());
+
+  // Remove blocks that are actually in subloops...
+  BBs.erase(std::remove_if(BBs.begin(), BBs.end(), 
+                           bind_obj(this, &LICM::notInCurrentLoop)), BBs.end());
+
+  // Visit all of the basic blocks we have chosen, hoisting out the instructions
+  // as neccesary.  This leaves dead copies of the instruction in the loop
+  // unfortunately...
+  //
+  for_each(BBs.begin(), BBs.end(), bind_obj(this, &LICM::visitBasicBlock));
+
+  // Clear out loops state information for the next iteration
+  CurLoop = 0;
+  LoopPreds.clear();
+  LoopBackEdges.clear();
+}
+
+void LICM::visitBasicBlock(BasicBlock *BB) {
+  // This cannot use an iterator, because it might get invalidated when PHI
+  // nodes are inserted!
+  //
+  for (unsigned i = 0; i < BB->size(); ) {
+    visit(BB->begin()[i]);
+    
+    BasicBlock::iterator It = BB->begin()+i;
+    if (dceInstruction(BB->getInstList(), It))
+      Changed = true;
+    else
+      ++i;
+  }
+}
+
+
+void LICM::hoist(Instruction *Inst) {
+  if (Inst->use_empty()) return;  // Don't (re) hoist dead instructions!
+  //cerr << "Hoisting " << Inst;
+
+  BasicBlock *Header = CurLoop->getHeader();
+
+  // Old instruction will be removed, so take it's name...
+  string InstName = Inst->getName();
+  Inst->setName("");
+
+  // The common case is that we have a pre-header.  Generate special case code
+  // that is faster if that is the case.
+  //
+  if (LoopPreds.size() == 1) {
+    BasicBlock *Pred = LoopPreds[0];
+
+    // Create a new copy of the instruction, for insertion into Pred.
+    Instruction *New = Inst->clone();
+    New->setName(InstName);
+
+    // Insert the new node in Pred, before the terminator.
+    Pred->getInstList().insert(Pred->end()-1, New);
+
+    // Kill the old instruction.
+    Inst->replaceAllUsesWith(New);
+    ++NumHoistedPH;
+
+  } else {
+    // No loop pre-header, insert a PHI node into header to capture all of the
+    // incoming versions of the value.
+    //
+    PHINode *LoopVal = new PHINode(Inst->getType(), InstName+".phi");
+
+    // Insert the new PHI node into the loop header...
+    Header->getInstList().push_front(LoopVal);
+
+    // Insert cloned versions of the instruction into all of the loop preds.
+    for (unsigned i = 0, e = LoopPreds.size(); i != e; ++i) {
+      BasicBlock *Pred = LoopPreds[i];
+      
+      // Create a new copy of the instruction, for insertion into Pred.
+      Instruction *New = Inst->clone();
+      New->setName(InstName);
+
+      // Insert the new node in Pred, before the terminator.
+      Pred->getInstList().insert(Pred->end()-1, New);
+
+      // Add the incoming value to the PHI node.
+      LoopVal->addIncoming(New, Pred);
+    }
+
+    // Add incoming values to the PHI node for all backedges in the loop...
+    for (unsigned i = 0, e = LoopBackEdges.size(); i != e; ++i)
+      LoopVal->addIncoming(LoopVal, LoopBackEdges[i]);
+
+    // Replace all uses of the old version of the instruction in the loop with
+    // the new version that is out of the loop.  We know that this is ok,
+    // because the new definition is in the loop header, which dominates the
+    // entire loop body.  The old definition was defined _inside_ of the loop,
+    // so the scope cannot extend outside of the loop, so we're ok.
+    //
+    Inst->replaceAllUsesWith(LoopVal);
+    ++NumHoistedNPH;
+  }
+
+  Changed = true;
+}
+