forked from OSchip/llvm-project
479 lines
19 KiB
C++
479 lines
19 KiB
C++
//===- ADCE.cpp - Code to perform aggressive dead code elimination --------===//
|
|
//
|
|
// 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 file implements "aggressive" dead code elimination. ADCE is DCe where
|
|
// values are assumed to be dead until proven otherwise. This is similar to
|
|
// SCCP, except applied to the liveness of values.
|
|
//
|
|
//===----------------------------------------------------------------------===//
|
|
|
|
#include "llvm/Transforms/Scalar.h"
|
|
#include "llvm/Transforms/Utils/Local.h"
|
|
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
|
|
#include "llvm/Type.h"
|
|
#include "llvm/Analysis/PostDominators.h"
|
|
#include "llvm/iTerminators.h"
|
|
#include "llvm/iPHINode.h"
|
|
#include "llvm/Constant.h"
|
|
#include "llvm/Support/CFG.h"
|
|
#include "Support/Debug.h"
|
|
#include "Support/DepthFirstIterator.h"
|
|
#include "Support/Statistic.h"
|
|
#include "Support/STLExtras.h"
|
|
#include <algorithm>
|
|
|
|
namespace llvm {
|
|
|
|
namespace {
|
|
Statistic<> NumBlockRemoved("adce", "Number of basic blocks removed");
|
|
Statistic<> NumInstRemoved ("adce", "Number of instructions removed");
|
|
|
|
//===----------------------------------------------------------------------===//
|
|
// ADCE Class
|
|
//
|
|
// This class does all of the work of Aggressive Dead Code Elimination.
|
|
// It's public interface consists of a constructor and a doADCE() method.
|
|
//
|
|
class ADCE : public FunctionPass {
|
|
Function *Func; // The function that we are working on
|
|
std::vector<Instruction*> WorkList; // Instructions that just became live
|
|
std::set<Instruction*> LiveSet; // The set of live instructions
|
|
|
|
//===--------------------------------------------------------------------===//
|
|
// The public interface for this class
|
|
//
|
|
public:
|
|
// Execute the Aggressive Dead Code Elimination Algorithm
|
|
//
|
|
virtual bool runOnFunction(Function &F) {
|
|
Func = &F;
|
|
bool Changed = doADCE();
|
|
assert(WorkList.empty());
|
|
LiveSet.clear();
|
|
return Changed;
|
|
}
|
|
// getAnalysisUsage - We require post dominance frontiers (aka Control
|
|
// Dependence Graph)
|
|
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
|
|
AU.addRequired<PostDominatorTree>();
|
|
AU.addRequired<PostDominanceFrontier>();
|
|
}
|
|
|
|
|
|
//===--------------------------------------------------------------------===//
|
|
// The implementation of this class
|
|
//
|
|
private:
|
|
// doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
|
|
// true if the function was modified.
|
|
//
|
|
bool doADCE();
|
|
|
|
void markBlockAlive(BasicBlock *BB);
|
|
|
|
|
|
// dropReferencesOfDeadInstructionsInLiveBlock - Loop over all of the
|
|
// instructions in the specified basic block, dropping references on
|
|
// instructions that are dead according to LiveSet.
|
|
bool dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB);
|
|
|
|
TerminatorInst *convertToUnconditionalBranch(TerminatorInst *TI);
|
|
|
|
inline void markInstructionLive(Instruction *I) {
|
|
if (LiveSet.count(I)) return;
|
|
DEBUG(std::cerr << "Insn Live: " << I);
|
|
LiveSet.insert(I);
|
|
WorkList.push_back(I);
|
|
}
|
|
|
|
inline void markTerminatorLive(const BasicBlock *BB) {
|
|
DEBUG(std::cerr << "Terminator Live: " << BB->getTerminator());
|
|
markInstructionLive(const_cast<TerminatorInst*>(BB->getTerminator()));
|
|
}
|
|
};
|
|
|
|
RegisterOpt<ADCE> X("adce", "Aggressive Dead Code Elimination");
|
|
} // End of anonymous namespace
|
|
|
|
Pass *createAggressiveDCEPass() { return new ADCE(); }
|
|
|
|
void ADCE::markBlockAlive(BasicBlock *BB) {
|
|
// Mark the basic block as being newly ALIVE... and mark all branches that
|
|
// this block is control dependent on as being alive also...
|
|
//
|
|
PostDominanceFrontier &CDG = getAnalysis<PostDominanceFrontier>();
|
|
|
|
PostDominanceFrontier::const_iterator It = CDG.find(BB);
|
|
if (It != CDG.end()) {
|
|
// Get the blocks that this node is control dependent on...
|
|
const PostDominanceFrontier::DomSetType &CDB = It->second;
|
|
for_each(CDB.begin(), CDB.end(), // Mark all their terminators as live
|
|
bind_obj(this, &ADCE::markTerminatorLive));
|
|
}
|
|
|
|
// If this basic block is live, and it ends in an unconditional branch, then
|
|
// the branch is alive as well...
|
|
if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
|
|
if (BI->isUnconditional())
|
|
markTerminatorLive(BB);
|
|
}
|
|
|
|
// dropReferencesOfDeadInstructionsInLiveBlock - Loop over all of the
|
|
// instructions in the specified basic block, dropping references on
|
|
// instructions that are dead according to LiveSet.
|
|
bool ADCE::dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB) {
|
|
bool Changed = false;
|
|
for (BasicBlock::iterator I = BB->begin(), E = --BB->end(); I != E; )
|
|
if (!LiveSet.count(I)) { // Is this instruction alive?
|
|
I->dropAllReferences(); // Nope, drop references...
|
|
if (PHINode *PN = dyn_cast<PHINode>(I)) {
|
|
// We don't want to leave PHI nodes in the program that have
|
|
// #arguments != #predecessors, so we remove them now.
|
|
//
|
|
PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
|
|
|
|
// Delete the instruction...
|
|
I = BB->getInstList().erase(I);
|
|
Changed = true;
|
|
} else {
|
|
++I;
|
|
}
|
|
} else {
|
|
++I;
|
|
}
|
|
return Changed;
|
|
}
|
|
|
|
|
|
/// convertToUnconditionalBranch - Transform this conditional terminator
|
|
/// instruction into an unconditional branch because we don't care which of the
|
|
/// successors it goes to. This eliminate a use of the condition as well.
|
|
///
|
|
TerminatorInst *ADCE::convertToUnconditionalBranch(TerminatorInst *TI) {
|
|
BranchInst *NB = new BranchInst(TI->getSuccessor(0), TI);
|
|
BasicBlock *BB = TI->getParent();
|
|
|
|
// Remove entries from PHI nodes to avoid confusing ourself later...
|
|
for (unsigned i = 1, e = TI->getNumSuccessors(); i != e; ++i)
|
|
TI->getSuccessor(i)->removePredecessor(BB);
|
|
|
|
// Delete the old branch itself...
|
|
BB->getInstList().erase(TI);
|
|
return NB;
|
|
}
|
|
|
|
|
|
// doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
|
|
// true if the function was modified.
|
|
//
|
|
bool ADCE::doADCE() {
|
|
bool MadeChanges = false;
|
|
|
|
// Iterate over all of the instructions in the function, eliminating trivially
|
|
// dead instructions, and marking instructions live that are known to be
|
|
// needed. Perform the walk in depth first order so that we avoid marking any
|
|
// instructions live in basic blocks that are unreachable. These blocks will
|
|
// be eliminated later, along with the instructions inside.
|
|
//
|
|
for (df_iterator<Function*> BBI = df_begin(Func), BBE = df_end(Func);
|
|
BBI != BBE; ++BBI) {
|
|
BasicBlock *BB = *BBI;
|
|
for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
|
|
if (II->mayWriteToMemory() || isa<ReturnInst>(II) || isa<UnwindInst>(II)){
|
|
markInstructionLive(II);
|
|
++II; // Increment the inst iterator if the inst wasn't deleted
|
|
} else if (isInstructionTriviallyDead(II)) {
|
|
// Remove the instruction from it's basic block...
|
|
II = BB->getInstList().erase(II);
|
|
++NumInstRemoved;
|
|
MadeChanges = true;
|
|
} else {
|
|
++II; // Increment the inst iterator if the inst wasn't deleted
|
|
}
|
|
}
|
|
}
|
|
|
|
// Check to ensure we have an exit node for this CFG. If we don't, we won't
|
|
// have any post-dominance information, thus we cannot perform our
|
|
// transformations safely.
|
|
//
|
|
PostDominatorTree &DT = getAnalysis<PostDominatorTree>();
|
|
if (DT[&Func->getEntryBlock()] == 0) {
|
|
WorkList.clear();
|
|
return MadeChanges;
|
|
}
|
|
|
|
// Scan the function marking blocks without post-dominance information as
|
|
// live. Blocks without post-dominance information occur when there is an
|
|
// infinite loop in the program. Because the infinite loop could contain a
|
|
// function which unwinds, exits or has side-effects, we don't want to delete
|
|
// the infinite loop or those blocks leading up to it.
|
|
for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
|
|
if (DT[I] == 0)
|
|
for (pred_iterator PI = pred_begin(I), E = pred_end(I); PI != E; ++PI)
|
|
markInstructionLive((*PI)->getTerminator());
|
|
|
|
|
|
|
|
DEBUG(std::cerr << "Processing work list\n");
|
|
|
|
// AliveBlocks - Set of basic blocks that we know have instructions that are
|
|
// alive in them...
|
|
//
|
|
std::set<BasicBlock*> AliveBlocks;
|
|
|
|
// Process the work list of instructions that just became live... if they
|
|
// became live, then that means that all of their operands are necessary as
|
|
// well... make them live as well.
|
|
//
|
|
while (!WorkList.empty()) {
|
|
Instruction *I = WorkList.back(); // Get an instruction that became live...
|
|
WorkList.pop_back();
|
|
|
|
BasicBlock *BB = I->getParent();
|
|
if (!AliveBlocks.count(BB)) { // Basic block not alive yet...
|
|
AliveBlocks.insert(BB); // Block is now ALIVE!
|
|
markBlockAlive(BB); // Make it so now!
|
|
}
|
|
|
|
// PHI nodes are a special case, because the incoming values are actually
|
|
// defined in the predecessor nodes of this block, meaning that the PHI
|
|
// makes the predecessors alive.
|
|
//
|
|
if (PHINode *PN = dyn_cast<PHINode>(I))
|
|
for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE; ++PI)
|
|
if (!AliveBlocks.count(*PI)) {
|
|
AliveBlocks.insert(BB); // Block is now ALIVE!
|
|
markBlockAlive(*PI);
|
|
}
|
|
|
|
// Loop over all of the operands of the live instruction, making sure that
|
|
// they are known to be alive as well...
|
|
//
|
|
for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op)
|
|
if (Instruction *Operand = dyn_cast<Instruction>(I->getOperand(op)))
|
|
markInstructionLive(Operand);
|
|
}
|
|
|
|
DEBUG(
|
|
std::cerr << "Current Function: X = Live\n";
|
|
for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I){
|
|
std::cerr << I->getName() << ":\t"
|
|
<< (AliveBlocks.count(I) ? "LIVE\n" : "DEAD\n");
|
|
for (BasicBlock::iterator BI = I->begin(), BE = I->end(); BI != BE; ++BI){
|
|
if (LiveSet.count(BI)) std::cerr << "X ";
|
|
std::cerr << *BI;
|
|
}
|
|
});
|
|
|
|
// Find the first postdominator of the entry node that is alive. Make it the
|
|
// new entry node...
|
|
//
|
|
if (AliveBlocks.size() == Func->size()) { // No dead blocks?
|
|
for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I) {
|
|
// Loop over all of the instructions in the function, telling dead
|
|
// instructions to drop their references. This is so that the next sweep
|
|
// over the program can safely delete dead instructions without other dead
|
|
// instructions still referring to them.
|
|
//
|
|
dropReferencesOfDeadInstructionsInLiveBlock(I);
|
|
|
|
// Check to make sure the terminator instruction is live. If it isn't,
|
|
// this means that the condition that it branches on (we know it is not an
|
|
// unconditional branch), is not needed to make the decision of where to
|
|
// go to, because all outgoing edges go to the same place. We must remove
|
|
// the use of the condition (because it's probably dead), so we convert
|
|
// the terminator to a conditional branch.
|
|
//
|
|
TerminatorInst *TI = I->getTerminator();
|
|
if (!LiveSet.count(TI))
|
|
convertToUnconditionalBranch(TI);
|
|
}
|
|
|
|
} else { // If there are some blocks dead...
|
|
// If the entry node is dead, insert a new entry node to eliminate the entry
|
|
// node as a special case.
|
|
//
|
|
if (!AliveBlocks.count(&Func->front())) {
|
|
BasicBlock *NewEntry = new BasicBlock();
|
|
new BranchInst(&Func->front(), NewEntry);
|
|
Func->getBasicBlockList().push_front(NewEntry);
|
|
AliveBlocks.insert(NewEntry); // This block is always alive!
|
|
LiveSet.insert(NewEntry->getTerminator()); // The branch is live
|
|
}
|
|
|
|
// Loop over all of the alive blocks in the function. If any successor
|
|
// blocks are not alive, we adjust the outgoing branches to branch to the
|
|
// first live postdominator of the live block, adjusting any PHI nodes in
|
|
// the block to reflect this.
|
|
//
|
|
for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
|
|
if (AliveBlocks.count(I)) {
|
|
BasicBlock *BB = I;
|
|
TerminatorInst *TI = BB->getTerminator();
|
|
|
|
// If the terminator instruction is alive, but the block it is contained
|
|
// in IS alive, this means that this terminator is a conditional branch
|
|
// on a condition that doesn't matter. Make it an unconditional branch
|
|
// to ONE of the successors. This has the side effect of dropping a use
|
|
// of the conditional value, which may also be dead.
|
|
if (!LiveSet.count(TI))
|
|
TI = convertToUnconditionalBranch(TI);
|
|
|
|
// Loop over all of the successors, looking for ones that are not alive.
|
|
// We cannot save the number of successors in the terminator instruction
|
|
// here because we may remove them if we don't have a postdominator...
|
|
//
|
|
for (unsigned i = 0; i != TI->getNumSuccessors(); ++i)
|
|
if (!AliveBlocks.count(TI->getSuccessor(i))) {
|
|
// Scan up the postdominator tree, looking for the first
|
|
// postdominator that is alive, and the last postdominator that is
|
|
// dead...
|
|
//
|
|
PostDominatorTree::Node *LastNode = DT[TI->getSuccessor(i)];
|
|
|
|
// There is a special case here... if there IS no post-dominator for
|
|
// the block we have no owhere to point our branch to. Instead,
|
|
// convert it to a return. This can only happen if the code
|
|
// branched into an infinite loop. Note that this may not be
|
|
// desirable, because we _are_ altering the behavior of the code.
|
|
// This is a well known drawback of ADCE, so in the future if we
|
|
// choose to revisit the decision, this is where it should be.
|
|
//
|
|
if (LastNode == 0) { // No postdominator!
|
|
// Call RemoveSuccessor to transmogrify the terminator instruction
|
|
// to not contain the outgoing branch, or to create a new
|
|
// terminator if the form fundamentally changes (i.e.,
|
|
// unconditional branch to return). Note that this will change a
|
|
// branch into an infinite loop into a return instruction!
|
|
//
|
|
RemoveSuccessor(TI, i);
|
|
|
|
// RemoveSuccessor may replace TI... make sure we have a fresh
|
|
// pointer... and e variable.
|
|
//
|
|
TI = BB->getTerminator();
|
|
|
|
// Rescan this successor...
|
|
--i;
|
|
} else {
|
|
PostDominatorTree::Node *NextNode = LastNode->getIDom();
|
|
|
|
while (!AliveBlocks.count(NextNode->getBlock())) {
|
|
LastNode = NextNode;
|
|
NextNode = NextNode->getIDom();
|
|
}
|
|
|
|
// Get the basic blocks that we need...
|
|
BasicBlock *LastDead = LastNode->getBlock();
|
|
BasicBlock *NextAlive = NextNode->getBlock();
|
|
|
|
// Make the conditional branch now go to the next alive block...
|
|
TI->getSuccessor(i)->removePredecessor(BB);
|
|
TI->setSuccessor(i, NextAlive);
|
|
|
|
// If there are PHI nodes in NextAlive, we need to add entries to
|
|
// the PHI nodes for the new incoming edge. The incoming values
|
|
// should be identical to the incoming values for LastDead.
|
|
//
|
|
for (BasicBlock::iterator II = NextAlive->begin();
|
|
PHINode *PN = dyn_cast<PHINode>(II); ++II)
|
|
if (LiveSet.count(PN)) { // Only modify live phi nodes
|
|
// Get the incoming value for LastDead...
|
|
int OldIdx = PN->getBasicBlockIndex(LastDead);
|
|
assert(OldIdx != -1 &&"LastDead is not a pred of NextAlive!");
|
|
Value *InVal = PN->getIncomingValue(OldIdx);
|
|
|
|
// Add an incoming value for BB now...
|
|
PN->addIncoming(InVal, BB);
|
|
}
|
|
}
|
|
}
|
|
|
|
// Now loop over all of the instructions in the basic block, telling
|
|
// dead instructions to drop their references. This is so that the next
|
|
// sweep over the program can safely delete dead instructions without
|
|
// other dead instructions still referring to them.
|
|
//
|
|
dropReferencesOfDeadInstructionsInLiveBlock(BB);
|
|
}
|
|
}
|
|
|
|
// We make changes if there are any dead blocks in the function...
|
|
if (unsigned NumDeadBlocks = Func->size() - AliveBlocks.size()) {
|
|
MadeChanges = true;
|
|
NumBlockRemoved += NumDeadBlocks;
|
|
}
|
|
|
|
// Loop over all of the basic blocks in the function, removing control flow
|
|
// edges to live blocks (also eliminating any entries in PHI functions in
|
|
// referenced blocks).
|
|
//
|
|
for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
|
|
if (!AliveBlocks.count(BB)) {
|
|
// Remove all outgoing edges from this basic block and convert the
|
|
// terminator into a return instruction.
|
|
std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
|
|
|
|
if (!Succs.empty()) {
|
|
// Loop over all of the successors, removing this block from PHI node
|
|
// entries that might be in the block...
|
|
while (!Succs.empty()) {
|
|
Succs.back()->removePredecessor(BB);
|
|
Succs.pop_back();
|
|
}
|
|
|
|
// Delete the old terminator instruction...
|
|
const Type *TermTy = BB->getTerminator()->getType();
|
|
if (TermTy != Type::VoidTy)
|
|
BB->getTerminator()->replaceAllUsesWith(
|
|
Constant::getNullValue(TermTy));
|
|
BB->getInstList().pop_back();
|
|
const Type *RetTy = Func->getReturnType();
|
|
new ReturnInst(RetTy != Type::VoidTy ?
|
|
Constant::getNullValue(RetTy) : 0, BB);
|
|
}
|
|
}
|
|
|
|
|
|
// Loop over all of the basic blocks in the function, dropping references of
|
|
// the dead basic blocks. We must do this after the previous step to avoid
|
|
// dropping references to PHIs which still have entries...
|
|
//
|
|
for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
|
|
if (!AliveBlocks.count(BB))
|
|
BB->dropAllReferences();
|
|
|
|
// Now loop through all of the blocks and delete the dead ones. We can safely
|
|
// do this now because we know that there are no references to dead blocks
|
|
// (because they have dropped all of their references... we also remove dead
|
|
// instructions from alive blocks.
|
|
//
|
|
for (Function::iterator BI = Func->begin(); BI != Func->end(); )
|
|
if (!AliveBlocks.count(BI)) { // Delete dead blocks...
|
|
BI = Func->getBasicBlockList().erase(BI);
|
|
} else { // Scan alive blocks...
|
|
for (BasicBlock::iterator II = BI->begin(); II != --BI->end(); )
|
|
if (!LiveSet.count(II)) { // Is this instruction alive?
|
|
// Nope... remove the instruction from it's basic block...
|
|
II = BI->getInstList().erase(II);
|
|
++NumInstRemoved;
|
|
MadeChanges = true;
|
|
} else {
|
|
++II;
|
|
}
|
|
|
|
++BI; // Increment iterator...
|
|
}
|
|
|
|
return MadeChanges;
|
|
}
|
|
|
|
} // End llvm namespace
|