initial checkin

llvm-svn: 1480

lib/Transforms/Scalar/InstructionCombining.cpp

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+//===- InstructionCombining.cpp - Combine multiple instructions -------------=//
+//
+// InstructionCombining - Combine instructions to form fewer, simple
+//   instructions.  This pass does not modify the CFG, and has a tendancy to
+//   make instructions dead, so a subsequent DCE pass is useful.
+//
+// This pass combines things like:
+//    %Y = add int 1, %X
+//    %Z = add int 1, %Y
+// into:
+//    %Z = add int 2, %X
+//
+// This is a simple worklist driven algorithm.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Scalar/InstructionCombining.h"
+#include "../TransformInternals.h"
+#include "llvm/Optimizations/ConstantHandling.h"
+#include "llvm/Method.h"
+#include "llvm/iMemory.h"
+
+using namespace opt;
+
+static Instruction *CombineBinOp(BinaryOperator *I) {
+  bool Changed = false;
+
+  // First thing we do is make sure that this instruction has a constant on the
+  // right hand side if it has any constant arguments.
+  //
+  if (isa<Constant>(I->getOperand(0)) && !isa<Constant>(I->getOperand(1)))
+    if (!I->swapOperands())
+      Changed = true;
+
+  bool LocalChange = true;
+  while (LocalChange) {
+    LocalChange = false;
+    Value *Op1 = I->getOperand(0);
+    if (Constant *Op2 = dyn_cast<Constant>(I->getOperand(1))) {
+      if (I->getOpcode() == Instruction::Add) {
+        if (Instruction *IOp1 = dyn_cast<Instruction>(Op1)) {
+          if (IOp1->getOpcode() == Instruction::Add &&
+              isa<Constant>(IOp1->getOperand(1))) {
+            // Fold:
+            //    %Y = add int %X, 1
+            //    %Z = add int %Y, 1
+            // into:
+            //    %Z = add int %X, 2
+            //   
+            // Constant fold both constants...
+            Constant *Val = *Op2 + *cast<Constant>(IOp1->getOperand(1));
+            
+            if (Val) {
+              I->setOperand(0, IOp1->getOperand(0));
+              I->setOperand(1, Val);
+              LocalChange = true;
+            }
+          }
+          
+        }
+      }
+    }
+    Changed |= LocalChange;
+  }
+
+  if (!Changed) return 0;
+  return I;
+}
+
+// Combine Indices - If the source pointer to this mem access instruction is a
+// getelementptr instruction, combine the indices of the GEP into this
+// instruction
+//
+static Instruction *CombineIndicies(MemAccessInst *MAI) {
+  GetElementPtrInst *Src =
+    dyn_cast<GetElementPtrInst>(MAI->getPointerOperand());
+  if (!Src) return 0;
+
+  vector<Value *> Indices;
+  
+  // Only special case we have to watch out for is pointer arithmetic on the
+  // 0th index of MAI. 
+  unsigned FirstIdx = MAI->getFirstIndexOperandNumber();
+  if (FirstIdx == MAI->getNumOperands() || 
+      (FirstIdx == MAI->getNumOperands()-1 &&
+       MAI->getOperand(FirstIdx) == ConstantUInt::get(Type::UIntTy, 0))) { 
+    // Replace the index list on this MAI with the index on the getelementptr
+    Indices.insert(Indices.end(), Src->idx_begin(), Src->idx_end());
+  } else if (*MAI->idx_begin() == ConstantUInt::get(Type::UIntTy, 0)) { 
+    // Otherwise we can do the fold if the first index of the GEP is a zero
+    Indices.insert(Indices.end(), Src->idx_begin(), Src->idx_end());
+    Indices.insert(Indices.end(), MAI->idx_begin()+1, MAI->idx_end());
+  }
+
+  if (Indices.empty()) return 0;  // Can't do the fold?
+
+  switch (MAI->getOpcode()) {
+  case Instruction::GetElementPtr:
+    return new GetElementPtrInst(Src->getOperand(0), Indices, MAI->getName());
+  case Instruction::Load:
+    return new LoadInst(Src->getOperand(0), Indices, MAI->getName());
+  case Instruction::Store:
+    return new StoreInst(MAI->getOperand(0), Src->getOperand(0),
+                         Indices, MAI->getName());
+  default:
+    assert(0 && "Unknown memaccessinst!");
+    break;
+  }
+  abort();
+  return 0;
+}
+
+bool InstructionCombining::CombineInstruction(Instruction *I) {
+  Instruction *Result = 0;
+  if (BinaryOperator *BOP = dyn_cast<BinaryOperator>(I))
+    Result = CombineBinOp(BOP);
+  else if (MemAccessInst *MAI = dyn_cast<MemAccessInst>(I))
+    Result = CombineIndicies(MAI);
+
+  if (!Result) return false;
+  if (Result == I) return true;
+
+  // If we get to here, we are to replace I with Result.
+  ReplaceInstWithInst(I, Result);
+  return true;
+}
+
+
+bool InstructionCombining::doit(Method *M) {
+  // Start the worklist out with all of the instructions in the method in it.
+  vector<Instruction*> WorkList(M->inst_begin(), M->inst_end());
+
+  while (!WorkList.empty()) {
+    Instruction *I = WorkList.back();  // Get an instruction from the worklist
+    WorkList.pop_back();
+
+    // Now that we have an instruction, try combining it to simplify it...
+    if (CombineInstruction(I)) {
+      // The instruction was simplified, add all users of the instruction to
+      // the work lists because they might get more simplified now...
+      //
+      for (Value::use_iterator UI = I->use_begin(), UE = I->use_end();
+           UI != UE; ++UI)
+        if (Instruction *User = dyn_cast<Instruction>(*UI))
+          WorkList.push_back(User);
+    }
+  }
+
+  return false;
+}