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llvm-project/llvm/lib/Transforms/Scalar/PRE.cpp

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//===- PRE.cpp - Partial Redundancy 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 the well-known Partial Redundancy Elimination
// optimization, using an SSA formulation based on e-paths. See this paper for
// more information:
//
// E-path_PRE: partial redundancy elimination made easy
// By: Dhananjay M. Dhamdhere In: ACM SIGPLAN Notices. Vol 37, #8, 2002
// http://doi.acm.org/10.1145/596992.597004
//
// This file actually implements a sparse version of the algorithm, using SSA
// and CFG properties instead of bit-vectors.
//
//===----------------------------------------------------------------------===//
#include "llvm/Pass.h"
#include "llvm/Function.h"
#include "llvm/Type.h"
#include "llvm/iPHINode.h"
#include "llvm/iMemory.h"
#include "llvm/Support/CFG.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/Analysis/ValueNumbering.h"
#include "llvm/Transforms/Scalar.h"
#include "Support/Debug.h"
#include "Support/DepthFirstIterator.h"
#include "Support/PostOrderIterator.h"
#include "Support/Statistic.h"
#include "Support/hash_set"
using namespace llvm;
namespace {
Statistic<> NumExprsEliminated("pre", "Number of expressions constantified");
Statistic<> NumRedundant ("pre", "Number of redundant exprs eliminated");
Statistic<> NumInserted ("pre", "Number of expressions inserted");
struct PRE : public FunctionPass {
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequiredID(BreakCriticalEdgesID); // No critical edges for now!
AU.addRequired<PostDominatorTree>();
AU.addRequired<PostDominanceFrontier>();
AU.addRequired<DominatorSet>();
AU.addRequired<DominatorTree>();
AU.addRequired<DominanceFrontier>();
AU.addRequired<ValueNumbering>();
}
virtual bool runOnFunction(Function &F);
private:
// Block information - Map basic blocks in a function back and forth to
// unsigned integers.
std::vector<BasicBlock*> BlockMapping;
hash_map<BasicBlock*, unsigned> BlockNumbering;
// ProcessedExpressions - Keep track of which expressions have already been
// processed.
hash_set<Instruction*> ProcessedExpressions;
// Provide access to the various analyses used...
DominatorSet *DS;
DominatorTree *DT; PostDominatorTree *PDT;
DominanceFrontier *DF; PostDominanceFrontier *PDF;
ValueNumbering *VN;
// AvailableBlocks - Contain a mapping of blocks with available expression
// values to the expression value itself. This can be used as an efficient
// way to find out if the expression is available in the block, and if so,
// which version to use. This map is only used while processing a single
// expression.
//
typedef hash_map<BasicBlock*, Instruction*> AvailableBlocksTy;
AvailableBlocksTy AvailableBlocks;
bool ProcessBlock(BasicBlock *BB);
// Anticipatibility calculation...
void MarkPostDominatingBlocksAnticipatible(PostDominatorTree::Node *N,
std::vector<char> &AntBlocks,
Instruction *Occurrence);
void CalculateAnticipatiblityForOccurrence(unsigned BlockNo,
std::vector<char> &AntBlocks,
Instruction *Occurrence);
void CalculateAnticipatibleBlocks(const std::map<unsigned, Instruction*> &D,
std::vector<char> &AnticipatibleBlocks);
// PRE for an expression
void MarkOccurrenceAvailableInAllDominatedBlocks(Instruction *Occurrence,
BasicBlock *StartBlock);
void ReplaceDominatedAvailableOccurrencesWith(Instruction *NewOcc,
DominatorTree::Node *N);
bool ProcessExpression(Instruction *I);
};
RegisterOpt<PRE> Z("pre", "Partial Redundancy Elimination");
}
bool PRE::runOnFunction(Function &F) {
VN = &getAnalysis<ValueNumbering>();
DS = &getAnalysis<DominatorSet>();
DT = &getAnalysis<DominatorTree>();
DF = &getAnalysis<DominanceFrontier>();
PDT = &getAnalysis<PostDominatorTree>();
PDF = &getAnalysis<PostDominanceFrontier>();
DEBUG(std::cerr << "\n*** Running PRE on func '" << F.getName() << "'...\n");
// Number the basic blocks based on a reverse post-order traversal of the CFG
// so that all predecessors of a block (ignoring back edges) are visited
// before a block is visited.
//
BlockMapping.reserve(F.size());
{
ReversePostOrderTraversal<Function*> RPOT(&F);
DEBUG(std::cerr << "Block order: ");
for (ReversePostOrderTraversal<Function*>::rpo_iterator I = RPOT.begin(),
E = RPOT.end(); I != E; ++I) {
// Keep track of mapping...
BasicBlock *BB = *I;
BlockNumbering.insert(std::make_pair(BB, BlockMapping.size()));
BlockMapping.push_back(BB);
DEBUG(std::cerr << BB->getName() << " ");
}
DEBUG(std::cerr << "\n");
}
// Traverse the current function depth-first in dominator-tree order. This
// ensures that we see all definitions before their uses (except for PHI
// nodes), allowing us to hoist dependent expressions correctly.
bool Changed = false;
for (unsigned i = 0, e = BlockMapping.size(); i != e; ++i)
Changed |= ProcessBlock(BlockMapping[i]);
// Free memory
BlockMapping.clear();
BlockNumbering.clear();
ProcessedExpressions.clear();
return Changed;
}
// ProcessBlock - Process any expressions first seen in this block...
//
bool PRE::ProcessBlock(BasicBlock *BB) {
bool Changed = false;
// PRE expressions first defined in this block...
Instruction *PrevInst = 0;
for (BasicBlock::iterator I = BB->begin(); I != BB->end(); )
if (ProcessExpression(I)) {
// The current instruction may have been deleted, make sure to back up to
// PrevInst instead.
if (PrevInst)
I = PrevInst;
else
I = BB->begin();
Changed = true;
} else {
PrevInst = I++;
}
return Changed;
}
void PRE::MarkPostDominatingBlocksAnticipatible(PostDominatorTree::Node *N,
std::vector<char> &AntBlocks,
Instruction *Occurrence) {
unsigned BlockNo = BlockNumbering[N->getBlock()];
if (AntBlocks[BlockNo]) return; // Already known to be anticipatible??
// Check to see if any of the operands are defined in this block, if so, the
// entry of this block does not anticipate the expression. This computes
// "transparency".
for (unsigned i = 0, e = Occurrence->getNumOperands(); i != e; ++i)
if (Instruction *I = dyn_cast<Instruction>(Occurrence->getOperand(i)))
if (I->getParent() == N->getBlock()) // Operand is defined in this block!
return;
if (isa<LoadInst>(Occurrence))
return; // FIXME: compute transparency for load instructions using AA
// Insert block into AntBlocks list...
AntBlocks[BlockNo] = true;
for (PostDominatorTree::Node::iterator I = N->begin(), E = N->end(); I != E;
++I)
MarkPostDominatingBlocksAnticipatible(*I, AntBlocks, Occurrence);
}
void PRE::CalculateAnticipatiblityForOccurrence(unsigned BlockNo,
std::vector<char> &AntBlocks,
Instruction *Occurrence) {
if (AntBlocks[BlockNo]) return; // Block already anticipatible!
BasicBlock *BB = BlockMapping[BlockNo];
// For each occurrence, mark all post-dominated blocks as anticipatible...
MarkPostDominatingBlocksAnticipatible(PDT->getNode(BB), AntBlocks,
Occurrence);
// Next, mark any blocks in the post-dominance frontier as anticipatible iff
// all successors are anticipatible.
//
PostDominanceFrontier::iterator PDFI = PDF->find(BB);
if (PDFI != DF->end())
for (std::set<BasicBlock*>::iterator DI = PDFI->second.begin();
DI != PDFI->second.end(); ++DI) {
BasicBlock *PDFBlock = *DI;
bool AllSuccessorsAnticipatible = true;
for (succ_iterator SI = succ_begin(PDFBlock), SE = succ_end(PDFBlock);
SI != SE; ++SI)
if (!AntBlocks[BlockNumbering[*SI]]) {
AllSuccessorsAnticipatible = false;
break;
}
if (AllSuccessorsAnticipatible)
CalculateAnticipatiblityForOccurrence(BlockNumbering[PDFBlock],
AntBlocks, Occurrence);
}
}
void PRE::CalculateAnticipatibleBlocks(const std::map<unsigned,
Instruction*> &Defs,
std::vector<char> &AntBlocks) {
// Initialize to zeros...
AntBlocks.resize(BlockMapping.size());
// Loop over all of the expressions...
for (std::map<unsigned, Instruction*>::const_iterator I = Defs.begin(),
E = Defs.end(); I != E; ++I)
CalculateAnticipatiblityForOccurrence(I->first, AntBlocks, I->second);
}
/// MarkOccurrenceAvailableInAllDominatedBlocks - Add entries to AvailableBlocks
/// for all nodes dominated by the occurrence to indicate that it is now the
/// available occurrence to use in any of these blocks.
///
void PRE::MarkOccurrenceAvailableInAllDominatedBlocks(Instruction *Occurrence,
BasicBlock *BB) {
// FIXME: There are much more efficient ways to get the blocks dominated
// by a block. Use them.
//
DominatorTree::Node *N = DT->getNode(Occurrence->getParent());
for (df_iterator<DominatorTree::Node*> DI = df_begin(N), E = df_end(N);
DI != E; ++DI)
AvailableBlocks[(*DI)->getBlock()] = Occurrence;
}
/// ReplaceDominatedAvailableOccurrencesWith - This loops over the region
/// dominated by N, replacing any available expressions with NewOcc.
void PRE::ReplaceDominatedAvailableOccurrencesWith(Instruction *NewOcc,
DominatorTree::Node *N) {
BasicBlock *BB = N->getBlock();
Instruction *&ExistingAvailableVal = AvailableBlocks[BB];
// If there isn't a definition already active in this node, make this the new
// active definition...
if (ExistingAvailableVal == 0) {
ExistingAvailableVal = NewOcc;
for (DominatorTree::Node::iterator I = N->begin(), E = N->end(); I != E;++I)
ReplaceDominatedAvailableOccurrencesWith(NewOcc, *I);
} else {
// If there is already an active definition in this block, replace it with
// NewOcc, and force it into all dominated blocks.
DEBUG(std::cerr << " Replacing dominated occ %"
<< ExistingAvailableVal->getName() << " with %" << NewOcc->getName()
<< "\n");
assert(ExistingAvailableVal != NewOcc && "NewOcc already inserted??");
ExistingAvailableVal->replaceAllUsesWith(NewOcc);
++NumRedundant;
assert(ExistingAvailableVal->getParent() == BB &&
"OldOcc not defined in current block?");
BB->getInstList().erase(ExistingAvailableVal);
// Mark NewOCC as the Available expression in all blocks dominated by BB
for (df_iterator<DominatorTree::Node*> DI = df_begin(N), E = df_end(N);
DI != E; ++DI)
AvailableBlocks[(*DI)->getBlock()] = NewOcc;
}
}
/// ProcessExpression - Given an expression (instruction) process the
/// instruction to remove any partial redundancies induced by equivalent
/// computations. Note that we only need to PRE each expression once, so we
/// keep track of whether an expression has been PRE'd already, and don't PRE an
/// expression again. Expressions may be seen multiple times because process
/// the entire equivalence class at once, which may leave expressions later in
/// the control path.
///
bool PRE::ProcessExpression(Instruction *Expr) {
if (Expr->mayWriteToMemory() || Expr->getType() == Type::VoidTy ||
isa<PHINode>(Expr))
return false; // Cannot move expression
if (ProcessedExpressions.count(Expr)) return false; // Already processed.
// Ok, this is the first time we have seen the expression. Build a set of
// equivalent expressions using SSA def/use information. We consider
// expressions to be equivalent if they are the same opcode and have
// equivalent operands. As a special case for SSA, values produced by PHI
// nodes are considered to be equivalent to all of their operands.
//
std::vector<Value*> Values;
VN->getEqualNumberNodes(Expr, Values);
#if 0
// FIXME: This should handle PHI nodes correctly. To do this, we need to
// consider expressions of the following form equivalent to this set of
// expressions:
//
// If an operand is a PHI node, add any occurrences of the expression with the
// PHI operand replaced with the PHI node operands. This is only valid if the
// PHI operand occurrences exist in blocks post-dominated by the incoming edge
// of the PHI node.
#endif
// We have to be careful to handle expression definitions which dominated by
// other expressions. These can be directly eliminated in favor of their
// dominating value. Keep track of which blocks contain definitions (the key)
// and if a block contains a definition, which instruction it is.
//
std::map<unsigned, Instruction*> Definitions;
Definitions.insert(std::make_pair(BlockNumbering[Expr->getParent()], Expr));
bool Changed = false;
// Look at all of the equal values. If any of the values is not an
// instruction, replace all other expressions immediately with it (it must be
// an argument or a constant or something). Otherwise, convert the list of
// values into a list of expression (instruction) definitions ordering
// according to their dominator tree ordering.
//
Value *NonInstValue = 0;
for (unsigned i = 0, e = Values.size(); i != e; ++i)
if (Instruction *I = dyn_cast<Instruction>(Values[i])) {
Instruction *&BlockInst = Definitions[BlockNumbering[I->getParent()]];
if (BlockInst && BlockInst != I) { // Eliminate direct redundancy
if (DS->dominates(I, BlockInst)) { // I dom BlockInst
BlockInst->replaceAllUsesWith(I);
BlockInst->getParent()->getInstList().erase(BlockInst);
} else { // BlockInst dom I
I->replaceAllUsesWith(BlockInst);
I->getParent()->getInstList().erase(I);
I = BlockInst;
}
++NumRedundant;
}
BlockInst = I;
} else {
NonInstValue = Values[i];
}
std::vector<Value*>().swap(Values); // Done with the values list
if (NonInstValue) {
// This is the good, though unlikely, case where we find out that this
// expression is equal to a constant or argument directly. We can replace
// this and all of the other equivalent instructions with the value
// directly.
//
for (std::map<unsigned, Instruction*>::iterator I = Definitions.begin(),
E = Definitions.end(); I != E; ++I) {
Instruction *Inst = I->second;
// Replace the value with the specified non-instruction value.
Inst->replaceAllUsesWith(NonInstValue); // Fixup any uses
Inst->getParent()->getInstList().erase(Inst); // Erase the instruction
}
NumExprsEliminated += Definitions.size();
return true; // Program modified!
}
// There are no expressions equal to this one. Exit early.
assert(!Definitions.empty() && "no equal expressions??");
#if 0
if (Definitions.size() == 1) {
ProcessedExpressions.insert(Definitions.begin()->second);
return Changed;
}
#endif
DEBUG(std::cerr << "\n====--- Expression: " << *Expr);
const Type *ExprType = Expr->getType();
// AnticipatibleBlocks - Blocks where the current expression is anticipatible.
// This is logically std::vector<bool> but using 'char' for performance.
std::vector<char> AnticipatibleBlocks;
// Calculate all of the blocks which the current expression is anticipatible.
CalculateAnticipatibleBlocks(Definitions, AnticipatibleBlocks);
// Print out anticipatible blocks...
DEBUG(std::cerr << "AntBlocks: ";
for (unsigned i = 0, e = AnticipatibleBlocks.size(); i != e; ++i)
if (AnticipatibleBlocks[i])
std::cerr << BlockMapping[i]->getName() <<" ";
std::cerr << "\n";);
// AvailabilityFrontier - Calculates the availability frontier for the current
// expression. The availability frontier contains the blocks on the dominance
// frontier of the current available expressions, iff they anticipate a
// definition of the expression.
hash_set<unsigned> AvailabilityFrontier;
Instruction *NonPHIOccurrence = 0;
while (!Definitions.empty() || !AvailabilityFrontier.empty()) {
if (!Definitions.empty() &&
(AvailabilityFrontier.empty() ||
Definitions.begin()->first < *AvailabilityFrontier.begin())) {
Instruction *Occurrence = Definitions.begin()->second;
BasicBlock *BB = Occurrence->getParent();
Definitions.erase(Definitions.begin());
DEBUG(std::cerr << "PROCESSING Occurrence: " << *Occurrence);
// Check to see if there is already an incoming value for this block...
AvailableBlocksTy::iterator LBI = AvailableBlocks.find(BB);
if (LBI != AvailableBlocks.end()) {
// Yes, there is a dominating definition for this block. Replace this
// occurrence with the incoming value.
if (LBI->second != Occurrence) {
DEBUG(std::cerr << " replacing with: " << *LBI->second);
Occurrence->replaceAllUsesWith(LBI->second);
BB->getInstList().erase(Occurrence); // Delete instruction
++NumRedundant;
}
} else {
ProcessedExpressions.insert(Occurrence);
if (!isa<PHINode>(Occurrence))
NonPHIOccurrence = Occurrence; // Keep an occurrence of this expr
// Okay, there is no incoming value for this block, so this expression
// is a new definition that is good for this block and all blocks
// dominated by it. Add this information to the AvailableBlocks map.
//
MarkOccurrenceAvailableInAllDominatedBlocks(Occurrence, BB);
// Update the dominance frontier for the definitions so far... if a node
// in the dominator frontier now has all of its predecessors available,
// and the block is in an anticipatible region, we can insert a PHI node
// in that block.
DominanceFrontier::iterator DFI = DF->find(BB);
if (DFI != DF->end()) {
for (std::set<BasicBlock*>::iterator DI = DFI->second.begin();
DI != DFI->second.end(); ++DI) {
BasicBlock *DFBlock = *DI;
unsigned DFBlockID = BlockNumbering[DFBlock];
if (AnticipatibleBlocks[DFBlockID]) {
// Check to see if any of the predecessors of this block on the
// frontier are not available...
bool AnyNotAvailable = false;
for (pred_iterator PI = pred_begin(DFBlock),
PE = pred_end(DFBlock); PI != PE; ++PI)
if (!AvailableBlocks.count(*PI)) {
AnyNotAvailable = true;
break;
}
// If any predecessor blocks are not available, add the node to
// the current expression dominance frontier.
if (AnyNotAvailable) {
AvailabilityFrontier.insert(DFBlockID);
} else {
// This block is no longer in the availability frontier, it IS
// available.
AvailabilityFrontier.erase(DFBlockID);
// If all of the predecessor blocks are available (and the block
// anticipates a definition along the path to the exit), we need
// to insert a new PHI node in this block. This block serves as
// a new definition for the expression, extending the available
// region.
//
PHINode *PN = new PHINode(ExprType, Expr->getName()+".pre",
DFBlock->begin());
ProcessedExpressions.insert(PN);
DEBUG(std::cerr << " INSERTING PHI on frontier: " << *PN);
// Add the incoming blocks for the PHI node
for (pred_iterator PI = pred_begin(DFBlock),
PE = pred_end(DFBlock); PI != PE; ++PI)
if (*PI != DFBlock)
PN->addIncoming(AvailableBlocks[*PI], *PI);
else // edge from the current block
PN->addIncoming(PN, DFBlock);
Instruction *&BlockOcc = Definitions[DFBlockID];
if (BlockOcc) {
DEBUG(std::cerr <<" PHI superceeds occurrence: "<<
*BlockOcc);
BlockOcc->replaceAllUsesWith(PN);
BlockOcc->getParent()->getInstList().erase(BlockOcc);
++NumRedundant;
}
BlockOcc = PN;
}
}
}
}
}
} else {
// Otherwise we must be looking at a node in the availability frontier!
unsigned AFBlockID = *AvailabilityFrontier.begin();
AvailabilityFrontier.erase(AvailabilityFrontier.begin());
BasicBlock *AFBlock = BlockMapping[AFBlockID];
// We eliminate the partial redundancy on this frontier by inserting a PHI
// node into this block, merging any incoming available versions into the
// PHI and inserting a new computation into predecessors without an
// incoming value. Note that we would have to insert the expression on
// the edge if the predecessor didn't anticipate the expression and we
// didn't break critical edges.
//
PHINode *PN = new PHINode(ExprType, Expr->getName()+".PRE",
AFBlock->begin());
DEBUG(std::cerr << "INSERTING PHI for PR: " << *PN);
// If there is a pending occurrence in this block, make sure to replace it
// with the PHI node...
std::map<unsigned, Instruction*>::iterator EDFI =
Definitions.find(AFBlockID);
if (EDFI != Definitions.end()) {
// There is already an occurrence in this block. Replace it with PN and
// remove it.
Instruction *OldOcc = EDFI->second;
DEBUG(std::cerr << " Replaces occurrence: " << *OldOcc);
OldOcc->replaceAllUsesWith(PN);
AFBlock->getInstList().erase(OldOcc);
Definitions.erase(EDFI);
++NumRedundant;
}
for (pred_iterator PI = pred_begin(AFBlock), PE = pred_end(AFBlock);
PI != PE; ++PI) {
BasicBlock *Pred = *PI;
AvailableBlocksTy::iterator LBI = AvailableBlocks.find(Pred);
if (LBI != AvailableBlocks.end()) { // If there is a available value
PN->addIncoming(LBI->second, Pred); // for this pred, use it.
} else { // No available value yet...
unsigned PredID = BlockNumbering[Pred];
// Is the predecessor the same block that we inserted the PHI into?
// (self loop)
if (Pred == AFBlock) {
// Yes, reuse the incoming value here...
PN->addIncoming(PN, Pred);
} else {
// No, we must insert a new computation into this block and add it
// to the definitions list...
assert(NonPHIOccurrence && "No non-phi occurrences seen so far???");
Instruction *New = NonPHIOccurrence->clone();
New->setName(NonPHIOccurrence->getName() + ".PRE-inserted");
ProcessedExpressions.insert(New);
DEBUG(std::cerr << " INSERTING OCCURRRENCE: " << *New);
// Insert it into the bottom of the predecessor, right before the
// terminator instruction...
Pred->getInstList().insert(Pred->getTerminator(), New);
// Make this block be the available definition for any blocks it
// dominates. The ONLY case that this can affect more than just the
// block itself is when we are moving a computation to a loop
// header. In all other cases, because we don't have critical
// edges, the node is guaranteed to only dominate itself.
//
ReplaceDominatedAvailableOccurrencesWith(New, DT->getNode(Pred));
// Add it as an incoming value on this edge to the PHI node
PN->addIncoming(New, Pred);
NonPHIOccurrence = New;
NumInserted++;
}
}
}
// Find out if there is already an available value in this block. If so,
// we need to replace the available value with the PHI node. This can
// only happen when we just inserted a PHI node on a backedge.
//
AvailableBlocksTy::iterator LBBlockAvailableValIt =
AvailableBlocks.find(AFBlock);
if (LBBlockAvailableValIt != AvailableBlocks.end()) {
if (LBBlockAvailableValIt->second->getParent() == AFBlock) {
Instruction *OldVal = LBBlockAvailableValIt->second;
OldVal->replaceAllUsesWith(PN); // Use the new PHI node now
++NumRedundant;
DEBUG(std::cerr << " PHI replaces available value: %"
<< OldVal->getName() << "\n");
// Loop over all of the blocks dominated by this PHI node, and change
// the AvailableBlocks entries to be the PHI node instead of the old
// instruction.
MarkOccurrenceAvailableInAllDominatedBlocks(PN, AFBlock);
AFBlock->getInstList().erase(OldVal); // Delete old instruction!
// The resultant PHI node is a new definition of the value!
Definitions.insert(std::make_pair(AFBlockID, PN));
} else {
// If the value is not defined in this block, that means that an
// inserted occurrence in a predecessor is now the live value for the
// region (occurs when hoisting loop invariants, f.e.). In this case,
// the PHI node should actually just be removed.
assert(PN->use_empty() && "No uses should exist for dead PHI node!");
PN->getParent()->getInstList().erase(PN);
}
} else {
// The resultant PHI node is a new definition of the value!
Definitions.insert(std::make_pair(AFBlockID, PN));
}
}
}
AvailableBlocks.clear();
return Changed;
}
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