llvm-6502/lib/Transforms/Utils/Local.cpp
2004-03-12 05:53:03 +00:00

275 lines
10 KiB
C++

//===-- 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<PHINode>(I)) {
if (PN->getNumIncomingValues() == 0)
return Constant::getNullValue(PN->getType());
Constant *Result = dyn_cast<Constant>(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<Constant>(I->getOperand(1));
if (Op1 == 0) return 0; // Not a constant?, can't fold
case 1:
Op0 = dyn_cast<Constant>(I->getOperand(0));
if (Op0 == 0) return 0; // Not a constant?, can't fold
break;
case 0: return 0;
}
if (isa<BinaryOperator>(I) || isa<ShiftInst>(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::Select:
if (Constant *Op2 = dyn_cast<Constant>(I->getOperand(2)))
return ConstantExpr::getSelect(Op0, Op1, Op2);
return 0;
case Instruction::GetElementPtr:
std::vector<Constant*> 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<Constant>(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<BranchInst>(T)) {
if (BI->isUnconditional()) return false; // Can't optimize uncond branch
BasicBlock *Dest1 = cast<BasicBlock>(BI->getOperand(0));
BasicBlock *Dest2 = cast<BasicBlock>(BI->getOperand(1));
if (ConstantBool *Cond = dyn_cast<ConstantBool>(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<SwitchInst>(T)) {
// If we are switching on a constant, we can convert the switch into a
// single branch instruction!
ConstantInt *CI = dyn_cast<ConstantInt>(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<TerminatorInst>(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;
}