//===-- Local.cpp - Functions to perform local transformations ------------===// // // 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 family of functions perform various local transformations to the // program. // //===----------------------------------------------------------------------===// #include "llvm/Transforms/Utils/Local.h" #include "llvm/Constants.h" #include "llvm/Instructions.h" using namespace llvm; //===----------------------------------------------------------------------===// // Local constant propagation... // /// doConstantPropagation - If an instruction references constants, try to fold /// them together... /// bool llvm::doConstantPropagation(BasicBlock::iterator &II) { if (Constant *C = ConstantFoldInstruction(II)) { // Replaces all of the uses of a variable with uses of the constant. II->replaceAllUsesWith(C); // Remove the instruction from the basic block... II = II->getParent()->getInstList().erase(II); return true; } return false; } /// ConstantFoldInstruction - Attempt to constant fold the specified /// instruction. If successful, the constant result is returned, if not, null /// is returned. Note that this function can only fail when attempting to fold /// instructions like loads and stores, which have no constant expression form. /// Constant *llvm::ConstantFoldInstruction(Instruction *I) { if (PHINode *PN = dyn_cast(I)) { if (PN->getNumIncomingValues() == 0) return Constant::getNullValue(PN->getType()); Constant *Result = dyn_cast(PN->getIncomingValue(0)); if (Result == 0) return 0; // Handle PHI nodes specially here... for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) if (PN->getIncomingValue(i) != Result && PN->getIncomingValue(i) != PN) return 0; // Not all the same incoming constants... // If we reach here, all incoming values are the same constant. return Result; } Constant *Op0 = 0, *Op1 = 0; switch (I->getNumOperands()) { default: case 2: Op1 = dyn_cast(I->getOperand(1)); if (Op1 == 0) return 0; // Not a constant?, can't fold case 1: Op0 = dyn_cast(I->getOperand(0)); if (Op0 == 0) return 0; // Not a constant?, can't fold break; case 0: return 0; } if (isa(I) || isa(I)) return ConstantExpr::get(I->getOpcode(), Op0, Op1); switch (I->getOpcode()) { default: return 0; case Instruction::Cast: return ConstantExpr::getCast(Op0, I->getType()); case Instruction::GetElementPtr: std::vector IdxList; IdxList.reserve(I->getNumOperands()-1); if (Op1) IdxList.push_back(Op1); for (unsigned i = 2, e = I->getNumOperands(); i != e; ++i) if (Constant *C = dyn_cast(I->getOperand(i))) IdxList.push_back(C); else return 0; // Non-constant operand return ConstantExpr::getGetElementPtr(Op0, IdxList); } } // ConstantFoldTerminator - If a terminator instruction is predicated on a // constant value, convert it into an unconditional branch to the constant // destination. // bool llvm::ConstantFoldTerminator(BasicBlock *BB) { TerminatorInst *T = BB->getTerminator(); // Branch - See if we are conditional jumping on constant if (BranchInst *BI = dyn_cast(T)) { if (BI->isUnconditional()) return false; // Can't optimize uncond branch BasicBlock *Dest1 = cast(BI->getOperand(0)); BasicBlock *Dest2 = cast(BI->getOperand(1)); if (ConstantBool *Cond = dyn_cast(BI->getCondition())) { // Are we branching on constant? // YES. Change to unconditional branch... BasicBlock *Destination = Cond->getValue() ? Dest1 : Dest2; BasicBlock *OldDest = Cond->getValue() ? Dest2 : Dest1; //cerr << "Function: " << T->getParent()->getParent() // << "\nRemoving branch from " << T->getParent() // << "\n\nTo: " << OldDest << endl; // Let the basic block know that we are letting go of it. Based on this, // it will adjust it's PHI nodes. assert(BI->getParent() && "Terminator not inserted in block!"); OldDest->removePredecessor(BI->getParent()); // Set the unconditional destination, and change the insn to be an // unconditional branch. BI->setUnconditionalDest(Destination); return true; } else if (Dest2 == Dest1) { // Conditional branch to same location? // This branch matches something like this: // br bool %cond, label %Dest, label %Dest // and changes it into: br label %Dest // Let the basic block know that we are letting go of one copy of it. assert(BI->getParent() && "Terminator not inserted in block!"); Dest1->removePredecessor(BI->getParent()); // Change a conditional branch to unconditional. BI->setUnconditionalDest(Dest1); return true; } } else if (SwitchInst *SI = dyn_cast(T)) { // If we are switching on a constant, we can convert the switch into a // single branch instruction! ConstantInt *CI = dyn_cast(SI->getCondition()); BasicBlock *TheOnlyDest = SI->getSuccessor(0); // The default dest BasicBlock *DefaultDest = TheOnlyDest; assert(TheOnlyDest == SI->getDefaultDest() && "Default destination is not successor #0?"); // Figure out which case it goes to... for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) { // Found case matching a constant operand? if (SI->getSuccessorValue(i) == CI) { TheOnlyDest = SI->getSuccessor(i); break; } // Check to see if this branch is going to the same place as the default // dest. If so, eliminate it as an explicit compare. if (SI->getSuccessor(i) == DefaultDest) { // Remove this entry... DefaultDest->removePredecessor(SI->getParent()); SI->removeCase(i); --i; --e; // Don't skip an entry... continue; } // Otherwise, check to see if the switch only branches to one destination. // We do this by reseting "TheOnlyDest" to null when we find two non-equal // destinations. if (SI->getSuccessor(i) != TheOnlyDest) TheOnlyDest = 0; } if (CI && !TheOnlyDest) { // Branching on a constant, but not any of the cases, go to the default // successor. TheOnlyDest = SI->getDefaultDest(); } // If we found a single destination that we can fold the switch into, do so // now. if (TheOnlyDest) { // Insert the new branch.. new BranchInst(TheOnlyDest, SI); BasicBlock *BB = SI->getParent(); // Remove entries from PHI nodes which we no longer branch to... for (unsigned i = 0, e = SI->getNumSuccessors(); i != e; ++i) { // Found case matching a constant operand? BasicBlock *Succ = SI->getSuccessor(i); if (Succ == TheOnlyDest) TheOnlyDest = 0; // Don't modify the first branch to TheOnlyDest else Succ->removePredecessor(BB); } // Delete the old switch... BB->getInstList().erase(SI); return true; } else if (SI->getNumSuccessors() == 2) { // Otherwise, we can fold this switch into a conditional branch // instruction if it has only one non-default destination. Value *Cond = new SetCondInst(Instruction::SetEQ, SI->getCondition(), SI->getSuccessorValue(1), "cond", SI); // Insert the new branch... new BranchInst(SI->getSuccessor(1), SI->getSuccessor(0), Cond, SI); // Delete the old switch... SI->getParent()->getInstList().erase(SI); return true; } } return false; } //===----------------------------------------------------------------------===// // Local dead code elimination... // bool llvm::isInstructionTriviallyDead(Instruction *I) { return I->use_empty() && !I->mayWriteToMemory() && !isa(I); } // dceInstruction - Inspect the instruction at *BBI and figure out if it's // [trivially] dead. If so, remove the instruction and update the iterator // to point to the instruction that immediately succeeded the original // instruction. // bool llvm::dceInstruction(BasicBlock::iterator &BBI) { // Look for un"used" definitions... if (isInstructionTriviallyDead(BBI)) { BBI = BBI->getParent()->getInstList().erase(BBI); // Bye bye return true; } return false; } //===----------------------------------------------------------------------===// // PHI Instruction Simplification // /// hasConstantValue - If the specified PHI node always merges together the same /// value, return the value, otherwise return null. /// Value *llvm::hasConstantValue(PHINode *PN) { // If the PHI node only has one incoming value, eliminate the PHI node... if (PN->getNumIncomingValues() == 1) return PN->getIncomingValue(0); // Otherwise if all of the incoming values are the same for the PHI, replace // the PHI node with the incoming value. // Value *InVal = 0; for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) if (PN->getIncomingValue(i) != PN) // Not the PHI node itself... if (InVal && PN->getIncomingValue(i) != InVal) return 0; // Not the same, bail out. else InVal = PN->getIncomingValue(i); // The only case that could cause InVal to be null is if we have a PHI node // that only has entries for itself. In this case, there is no entry into the // loop, so kill the PHI. // if (InVal == 0) InVal = Constant::getNullValue(PN->getType()); // All of the incoming values are the same, return the value now. return InVal; }