Initial checkin of simple&fast SSA based GCSE algorithm

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@2338 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Transforms/Scalar/GCSE.h

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+//===-- GCSE.h - SSA based Global Common Subexpr Elimination -----*- C++ -*--=//
+//
+// This pass is designed to be a very quick global transformation that
+// eliminates global common subexpressions from a function.  It does this by
+// examining the SSA value graph of the function, instead of doing slow
+// bit-vector computations.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_TRANSFORMS_SCALAR_GCSE_H
+#define LLVM_TRANSFORMS_SCALAR_GCSE_H
+
+class Pass;
+Pass *createGCSEPass();
+
+#endif

lib/Transforms/Scalar/GCSE.cpp

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+//===-- GCSE.cpp - SSA based Global Common Subexpr Elimination ------------===//
+//
+// This pass is designed to be a very quick global transformation that
+// eliminates global common subexpressions from a function.  It does this by
+// examining the SSA value graph of the function, instead of doing slow, dense,
+// bit-vector computations.
+//
+// This pass works best if it is proceeded with a simple constant propogation
+// pass and an instruction combination pass because this pass does not do any
+// value numbering (in order to be speedy).
+//
+// This pass does not attempt to CSE load instructions, because it does not use
+// pointer analysis to determine when it is safe.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Scalar/GCSE.h"
+#include "llvm/Pass.h"
+#include "llvm/InstrTypes.h"
+#include "llvm/iMemory.h"
+#include "llvm/Analysis/Dominators.h"
+#include "llvm/Support/InstVisitor.h"
+#include "llvm/Support/InstIterator.h"
+#include <set>
+#include <algorithm>
+using namespace cfg;
+
+namespace {
+  class GCSE : public FunctionPass, public InstVisitor<GCSE, bool> {
+    set<Instruction*> WorkList;
+    DominatorSet        *DomSetInfo;
+    ImmediateDominators *ImmDominator;
+  public:
+    virtual bool runOnFunction(Function *F);
+
+    // Visitation methods, these are invoked depending on the type of
+    // instruction being checked.  They should return true if a common
+    // subexpression was folded.
+    //
+    bool visitUnaryOperator(Instruction *I);
+    bool visitBinaryOperator(Instruction *I);
+    bool visitGetElementPtrInst(GetElementPtrInst *I);
+    bool visitCastInst(CastInst *I){return visitUnaryOperator((Instruction*)I);}
+    bool visitShiftInst(ShiftInst *I) {
+      return visitBinaryOperator((Instruction*)I);
+    }
+    bool visitInstruction(Instruction *) { return false; }
+
+  private:
+    void ReplaceInstWithInst(Instruction *First, BasicBlock::iterator SI);
+    void CommonSubExpressionFound(Instruction *I, Instruction *Other);
+
+    // This transformation requires dominator and immediate dominator info
+    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+      //preservesCFG(AU);
+      AU.addRequired(DominatorSet::ID);
+      AU.addRequired(ImmediateDominators::ID); 
+    }
+  };
+}
+
+// createGCSEPass - The public interface to this file...
+Pass *createGCSEPass() { return new GCSE(); }
+
+
+// GCSE::runOnFunction - This is the main transformation entry point for a
+// function.
+//
+bool GCSE::runOnFunction(Function *F) {
+  bool Changed = false;
+
+  DomSetInfo = &getAnalysis<DominatorSet>();
+  ImmDominator = &getAnalysis<ImmediateDominators>();
+
+  // Step #1: Add all instructions in the function to the worklist for
+  // processing.  All of the instructions are considered to be our
+  // subexpressions to eliminate if possible.
+  //
+  WorkList.insert(inst_begin(F), inst_end(F));
+
+  // Step #2: WorkList processing.  Iterate through all of the instructions,
+  // checking to see if there are any additionally defined subexpressions in the
+  // program.  If so, eliminate them!
+  //
+  while (!WorkList.empty()) {
+    Instruction *I = *WorkList.begin();  // Get an instruction from the worklist
+    WorkList.erase(WorkList.begin());
+
+    // Visit the instruction, dispatching to the correct visit function based on
+    // the instruction type.  This does the checking.
+    //
+    Changed |= visit(I);
+  }
+  
+  // When the worklist is empty, return whether or not we changed anything...
+  return Changed;
+}
+
+
+// ReplaceInstWithInst - Destroy the instruction pointed to by SI, making all
+// uses of the instruction use First now instead.
+//
+void GCSE::ReplaceInstWithInst(Instruction *First, BasicBlock::iterator SI) {
+  Instruction *Second = *SI;
+
+  // Add the first instruction back to the worklist
+  WorkList.insert(First);
+
+  // Add all uses of the second instruction to the worklist
+  for (Value::use_iterator UI = Second->use_begin(), UE = Second->use_end();
+       UI != UE; ++UI)
+    WorkList.insert(cast<Instruction>(*UI));
+    
+  // Make all users of 'Second' now use 'First'
+  Second->replaceAllUsesWith(First);
+
+  // Erase the second instruction from the program
+  delete Second->getParent()->getInstList().remove(SI);
+}
+
+// CommonSubExpressionFound - The two instruction I & Other have been found to
+// be common subexpressions.  This function is responsible for eliminating one
+// of them, and for fixing the worklist to be correct.
+//
+void GCSE::CommonSubExpressionFound(Instruction *I, Instruction *Other) {
+  // I has already been removed from the worklist, Other needs to be.
+  assert(WorkList.count(I) == 0 && WorkList.count(Other) &&
+         "I in worklist or Other not!");
+  WorkList.erase(Other);
+
+  // Handle the easy case, where both instructions are in the same basic block
+  BasicBlock *BB1 = I->getParent(), *BB2 = Other->getParent();
+  if (BB1 == BB2) {
+    // Eliminate the second occuring instruction.  Add all uses of the second
+    // instruction to the worklist.
+    //
+    // Scan the basic block looking for the "first" instruction
+    BasicBlock::iterator BI = BB1->begin();
+    while (*BI != I && *BI != Other) {
+      ++BI;
+      assert(BI != BB1->end() && "Instructions not found in parent BB!");
+    }
+
+    // Keep track of which instructions occurred first & second
+    Instruction *First = *BI;
+    Instruction *Second = I != First ? I : Other; // Get iterator to second inst
+    BI = find(BI, BB1->end(), Second);
+    assert(BI != BB1->end() && "Second instruction not found in parent block!");
+
+    // Destroy Second, using First instead.
+    ReplaceInstWithInst(First, BI);    
+
+    // Otherwise, the two instructions are in different basic blocks.  If one
+    // dominates the other instruction, we can simply use it
+    //
+  } else if (DomSetInfo->dominates(BB1, BB2)) {    // I dom Other?
+    BasicBlock::iterator BI = find(BB2->begin(), BB2->end(), Other);
+    assert(BI != BB2->end() && "Other not in parent basic block!");
+    ReplaceInstWithInst(I, BI);    
+  } else if (DomSetInfo->dominates(BB2, BB1)) {    // Other dom I?
+    BasicBlock::iterator BI = find(BB1->begin(), BB1->end(), I);
+    assert(BI != BB1->end() && "I not in parent basic block!");
+    ReplaceInstWithInst(Other, BI);
+  } else {
+    // Handle the most general case now.  In this case, neither I dom Other nor
+    // Other dom I.  Because we are in SSA form, we are guaranteed that the
+    // operands of the two instructions both dominate the uses, so we _know_
+    // that there must exist a block that dominates both instructions (if the
+    // operands of the instructions are globals or constants, worst case we
+    // would get the entry node of the function).  Search for this block now.
+    //
+
+    // Search up the immediate dominator chain of BB1 for the shared dominator
+    BasicBlock *SharedDom = (*ImmDominator)[BB1];
+    while (!DomSetInfo->dominates(SharedDom, BB2))
+      SharedDom = (*ImmDominator)[SharedDom];
+
+    // At this point, shared dom must dominate BOTH BB1 and BB2...
+    assert(SharedDom && DomSetInfo->dominates(SharedDom, BB1) &&
+           DomSetInfo->dominates(SharedDom, BB2) && "Dominators broken!");
+
+    // Rip 'I' out of BB1, and move it to the end of SharedDom.
+    BB1->getInstList().remove(I);
+    SharedDom->getInstList().insert(SharedDom->end()-1, I);
+
+    // Eliminate 'Other' now.
+    BasicBlock::iterator BI = find(BB2->begin(), BB2->end(), Other);
+    assert(BI != BB2->end() && "I not in parent basic block!");
+    ReplaceInstWithInst(I, BI);
+  }
+}
+
+//===----------------------------------------------------------------------===//
+//
+// Visitation methods, these are invoked depending on the type of instruction
+// being checked.  They should return true if a common subexpression was folded.
+//
+//===----------------------------------------------------------------------===//
+
+bool GCSE::visitUnaryOperator(Instruction *I) {
+  Value *Op = I->getOperand(0);
+  Function *F = I->getParent()->getParent();
+  
+  for (Value::use_iterator UI = Op->use_begin(), UE = Op->use_end();
+       UI != UE; ++UI)
+    if (Instruction *Other = dyn_cast<Instruction>(*UI))
+      // Check to see if this new binary operator is not I, but same operand...
+      if (Other != I && Other->getOpcode() == I->getOpcode() &&
+          Other->getOperand(0) == Op &&     // Is the operand the same?
+          // Is it embeded in the same function?  (This could be false if LHS
+          // is a constant or global!)
+          Other->getParent()->getParent() == F &&
+
+          // Check that the types are the same, since this code handles casts...
+          Other->getType() == I->getType()) {
+        
+        // These instructions are identical.  Handle the situation.
+        CommonSubExpressionFound(I, Other);
+        return true;   // One instruction eliminated!
+      }
+  
+  return false;
+}
+
+bool GCSE::visitBinaryOperator(Instruction *I) {
+  Value *LHS = I->getOperand(0), *RHS = I->getOperand(1);
+  Function *F = I->getParent()->getParent();
+  
+  for (Value::use_iterator UI = LHS->use_begin(), UE = LHS->use_end();
+       UI != UE; ++UI)
+    if (Instruction *Other = dyn_cast<Instruction>(*UI))
+      // Check to see if this new binary operator is not I, but same operand...
+      if (Other != I && Other->getOpcode() == I->getOpcode() &&
+          // Are the LHS and RHS the same?
+          Other->getOperand(0) == LHS && Other->getOperand(1) == RHS &&
+          // Is it embeded in the same function?  (This could be false if LHS
+          // is a constant or global!)
+          Other->getParent()->getParent() == F) {
+        
+        // These instructions are identical.  Handle the situation.
+        CommonSubExpressionFound(I, Other);
+        return true;   // One instruction eliminated!
+      }
+  
+  return false;
+}
+
+bool GCSE::visitGetElementPtrInst(GetElementPtrInst *I) {
+  Value *Op = I->getOperand(0);
+  Function *F = I->getParent()->getParent();
+  
+  for (Value::use_iterator UI = Op->use_begin(), UE = Op->use_end();
+       UI != UE; ++UI)
+    if (GetElementPtrInst *Other = dyn_cast<GetElementPtrInst>(*UI))
+      // Check to see if this new binary operator is not I, but same operand...
+      if (Other != I && Other->getParent()->getParent() == F &&
+          Other->getType() == I->getType()) {
+
+        // Check to see that all operators past the 0th are the same...
+        unsigned i = 1, e = I->getNumOperands();
+        for (; i != e; ++i)
+          if (I->getOperand(i) != Other->getOperand(i)) break;
+        
+        if (i == e) {
+          // These instructions are identical.  Handle the situation.
+          CommonSubExpressionFound(I, Other);
+          return true;   // One instruction eliminated!
+        }
+      }
+  
+  return false;
+}