llvm-6502/lib/Transforms/Utils/Local.cpp
Chris Lattner 564c0a2330 enhance RecursivelyDeleteTriviallyDeadInstructions to make
PHIs dead if they are single-value.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@60194 91177308-0d34-0410-b5e6-96231b3b80d8
2008-11-27 23:18:11 +00:00

248 lines
8.9 KiB
C++

//===-- Local.cpp - Functions to perform local transformations ------------===//
//
// The LLVM Compiler Infrastructure
//
// This file 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/DerivedTypes.h"
#include "llvm/Instructions.h"
#include "llvm/Intrinsics.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Support/GetElementPtrTypeIterator.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
using namespace llvm;
//===----------------------------------------------------------------------===//
// Local constant propagation.
//
// 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 (ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition())) {
// Are we branching on constant?
// YES. Change to unconditional branch...
BasicBlock *Destination = Cond->getZExtValue() ? Dest1 : Dest2;
BasicBlock *OldDest = Cond->getZExtValue() ? 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..
BranchInst::Create(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 ICmpInst(ICmpInst::ICMP_EQ, SI->getCondition(),
SI->getSuccessorValue(1), "cond", SI);
// Insert the new branch...
BranchInst::Create(SI->getSuccessor(1), SI->getSuccessor(0), Cond, SI);
// Delete the old switch...
SI->eraseFromParent();
return true;
}
}
return false;
}
//===----------------------------------------------------------------------===//
// Local dead code elimination...
//
/// isInstructionTriviallyDead - Return true if the result produced by the
/// instruction is not used, and the instruction has no side effects.
///
bool llvm::isInstructionTriviallyDead(Instruction *I) {
if (!I->use_empty() || isa<TerminatorInst>(I)) return false;
if (!I->mayWriteToMemory())
return true;
// Special case intrinsics that "may write to memory" but can be deleted when
// dead.
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I))
// Safe to delete llvm.stacksave if dead.
if (II->getIntrinsicID() == Intrinsic::stacksave)
return true;
return false;
}
/// RecursivelyDeleteTriviallyDeadInstructions - If the specified value is a
/// trivially dead instruction, delete it. If that makes any of its operands
/// trivially dead, delete them too, recursively.
///
/// If DeadInst is specified, the vector is filled with the instructions that
/// are actually deleted.
void llvm::RecursivelyDeleteTriviallyDeadInstructions(Value *V,
SmallVectorImpl<Instruction*> *DeadInst) {
Instruction *I = dyn_cast<Instruction>(V);
if (!I || !I->use_empty()) return;
SmallPtrSet<Instruction*, 16> Insts;
Insts.insert(I);
while (!Insts.empty()) {
I = *Insts.begin();
Insts.erase(I);
// If this is a PHI node, we may be able to make it dead if we know all the
// input values are the same.
if (PHINode *PN = dyn_cast<PHINode>(I)) {
if (Value *PNV = PN->hasConstantValue())
PN->replaceAllUsesWith(PNV);
}
// Okay, if the instruction is dead, delete it.
if (!isInstructionTriviallyDead(I))
continue;
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
if (Instruction *U = dyn_cast<Instruction>(I->getOperand(i)))
Insts.insert(U);
I->eraseFromParent();
if (DeadInst)
DeadInst->push_back(I);
}
}
//===----------------------------------------------------------------------===//
// Control Flow Graph Restructuring...
//
/// MergeBasicBlockIntoOnlyPred - DestBB is a block with one predecessor and its
/// predecessor is known to have one successor (DestBB!). Eliminate the edge
/// between them, moving the instructions in the predecessor into DestBB and
/// deleting the predecessor block.
///
void llvm::MergeBasicBlockIntoOnlyPred(BasicBlock *DestBB) {
// If BB has single-entry PHI nodes, fold them.
while (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) {
Value *NewVal = PN->getIncomingValue(0);
// Replace self referencing PHI with undef, it must be dead.
if (NewVal == PN) NewVal = UndefValue::get(PN->getType());
PN->replaceAllUsesWith(NewVal);
PN->eraseFromParent();
}
BasicBlock *PredBB = DestBB->getSinglePredecessor();
assert(PredBB && "Block doesn't have a single predecessor!");
// Splice all the instructions from PredBB to DestBB.
PredBB->getTerminator()->eraseFromParent();
DestBB->getInstList().splice(DestBB->begin(), PredBB->getInstList());
// Anything that branched to PredBB now branches to DestBB.
PredBB->replaceAllUsesWith(DestBB);
// Nuke BB.
PredBB->eraseFromParent();
}