llvm-6502/lib/Transforms/Utils/PromoteMemoryToRegister.cpp
Chris Lattner f12f8def39 rewrite the code used to construct pruned SSA form with the IDF method.
In the old way, we computed and inserted phi nodes for the whole IDF of 
the definitions of the alloca, then computed which ones were dead and
removed them.

In the new method, we first compute the region where the value is live,
and use that information to only insert phi nodes that are live.  This
eliminates the need to compute liveness later, and stops the algorithm
from inserting a bunch of phis which it then later removes.

This speeds up the testcase in PR1432 from 2.00s to 0.15s (14x) in a
release build and 6.84s->0.50s (14x) in a debug build.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@40825 91177308-0d34-0410-b5e6-96231b3b80d8
2007-08-04 22:50:14 +00:00

1000 lines
38 KiB
C++

//===- PromoteMemoryToRegister.cpp - Convert allocas to registers ---------===//
//
// 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 promote memory references to be register references. It promotes
// alloca instructions which only have loads and stores as uses. An alloca is
// transformed by using dominator frontiers to place PHI nodes, then traversing
// the function in depth-first order to rewrite loads and stores as appropriate.
// This is just the standard SSA construction algorithm to construct "pruned"
// SSA form.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "mem2reg"
#include "llvm/Transforms/Utils/PromoteMemToReg.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/AliasSetTracker.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/Compiler.h"
#include <algorithm>
using namespace llvm;
STATISTIC(NumLocalPromoted, "Number of alloca's promoted within one block");
STATISTIC(NumSingleStore, "Number of alloca's promoted with a single store");
STATISTIC(NumDeadAlloca, "Number of dead alloca's removed");
STATISTIC(NumPHIInsert, "Number of PHI nodes inserted");
// Provide DenseMapKeyInfo for all pointers.
namespace llvm {
template<>
struct DenseMapKeyInfo<std::pair<BasicBlock*, unsigned> > {
static inline std::pair<BasicBlock*, unsigned> getEmptyKey() {
return std::make_pair((BasicBlock*)-1, ~0U);
}
static inline std::pair<BasicBlock*, unsigned> getTombstoneKey() {
return std::make_pair((BasicBlock*)-2, 0U);
}
static unsigned getHashValue(const std::pair<BasicBlock*, unsigned> &Val) {
return DenseMapKeyInfo<void*>::getHashValue(Val.first) + Val.second*2;
}
static bool isPod() { return true; }
};
}
/// isAllocaPromotable - Return true if this alloca is legal for promotion.
/// This is true if there are only loads and stores to the alloca.
///
bool llvm::isAllocaPromotable(const AllocaInst *AI) {
// FIXME: If the memory unit is of pointer or integer type, we can permit
// assignments to subsections of the memory unit.
// Only allow direct loads and stores...
for (Value::use_const_iterator UI = AI->use_begin(), UE = AI->use_end();
UI != UE; ++UI) // Loop over all of the uses of the alloca
if (isa<LoadInst>(*UI)) {
// noop
} else if (const StoreInst *SI = dyn_cast<StoreInst>(*UI)) {
if (SI->getOperand(0) == AI)
return false; // Don't allow a store OF the AI, only INTO the AI.
} else {
return false; // Not a load or store.
}
return true;
}
namespace {
struct AllocaInfo;
// Data package used by RenamePass()
class VISIBILITY_HIDDEN RenamePassData {
public:
typedef std::vector<Value *> ValVector;
RenamePassData() {}
RenamePassData(BasicBlock *B, BasicBlock *P,
const ValVector &V) : BB(B), Pred(P), Values(V) {}
BasicBlock *BB;
BasicBlock *Pred;
ValVector Values;
void swap(RenamePassData &RHS) {
std::swap(BB, RHS.BB);
std::swap(Pred, RHS.Pred);
Values.swap(RHS.Values);
}
};
struct VISIBILITY_HIDDEN PromoteMem2Reg {
/// Allocas - The alloca instructions being promoted.
///
std::vector<AllocaInst*> Allocas;
SmallVector<AllocaInst*, 16> &RetryList;
DominatorTree &DT;
DominanceFrontier &DF;
/// AST - An AliasSetTracker object to update. If null, don't update it.
///
AliasSetTracker *AST;
/// AllocaLookup - Reverse mapping of Allocas.
///
std::map<AllocaInst*, unsigned> AllocaLookup;
/// NewPhiNodes - The PhiNodes we're adding.
///
DenseMap<std::pair<BasicBlock*, unsigned>, PHINode*> NewPhiNodes;
/// PhiToAllocaMap - For each PHI node, keep track of which entry in Allocas
/// it corresponds to.
DenseMap<PHINode*, unsigned> PhiToAllocaMap;
/// PointerAllocaValues - If we are updating an AliasSetTracker, then for
/// each alloca that is of pointer type, we keep track of what to copyValue
/// to the inserted PHI nodes here.
///
std::vector<Value*> PointerAllocaValues;
/// Visited - The set of basic blocks the renamer has already visited.
///
SmallPtrSet<BasicBlock*, 16> Visited;
/// BBNumbers - Contains a stable numbering of basic blocks to avoid
/// non-determinstic behavior.
DenseMap<BasicBlock*, unsigned> BBNumbers;
/// BBNumPreds - Lazily compute the number of predecessors a block has.
DenseMap<const BasicBlock*, unsigned> BBNumPreds;
public:
PromoteMem2Reg(const std::vector<AllocaInst*> &A,
SmallVector<AllocaInst*, 16> &Retry, DominatorTree &dt,
DominanceFrontier &df, AliasSetTracker *ast)
: Allocas(A), RetryList(Retry), DT(dt), DF(df), AST(ast) {}
void run();
/// properlyDominates - Return true if I1 properly dominates I2.
///
bool properlyDominates(Instruction *I1, Instruction *I2) const {
if (InvokeInst *II = dyn_cast<InvokeInst>(I1))
I1 = II->getNormalDest()->begin();
return DT.properlyDominates(I1->getParent(), I2->getParent());
}
/// dominates - Return true if BB1 dominates BB2 using the DominatorTree.
///
bool dominates(BasicBlock *BB1, BasicBlock *BB2) const {
return DT.dominates(BB1, BB2);
}
private:
void RemoveFromAllocasList(unsigned &AllocaIdx) {
Allocas[AllocaIdx] = Allocas.back();
Allocas.pop_back();
--AllocaIdx;
}
unsigned getNumPreds(const BasicBlock *BB) {
unsigned &NP = BBNumPreds[BB];
if (NP == 0)
NP = std::distance(pred_begin(BB), pred_end(BB))+1;
return NP-1;
}
void DetermineInsertionPoint(AllocaInst *AI, unsigned AllocaNum,
AllocaInfo &Info);
void ComputeLiveInBlocks(AllocaInst *AI, AllocaInfo &Info,
const SmallPtrSet<BasicBlock*, 32> &DefBlocks,
SmallPtrSet<BasicBlock*, 32> &LiveInBlocks);
void RewriteSingleStoreAlloca(AllocaInst *AI, AllocaInfo &Info);
bool PromoteLocallyUsedAlloca(BasicBlock *BB, AllocaInst *AI);
void PromoteLocallyUsedAllocas(BasicBlock *BB,
const std::vector<AllocaInst*> &AIs);
void RenamePass(BasicBlock *BB, BasicBlock *Pred,
RenamePassData::ValVector &IncVals,
std::vector<RenamePassData> &Worklist);
bool QueuePhiNode(BasicBlock *BB, unsigned AllocaIdx, unsigned &Version,
SmallPtrSet<PHINode*, 16> &InsertedPHINodes);
};
struct AllocaInfo {
std::vector<BasicBlock*> DefiningBlocks;
std::vector<BasicBlock*> UsingBlocks;
StoreInst *OnlyStore;
BasicBlock *OnlyBlock;
bool OnlyUsedInOneBlock;
Value *AllocaPointerVal;
void clear() {
DefiningBlocks.clear();
UsingBlocks.clear();
OnlyStore = 0;
OnlyBlock = 0;
OnlyUsedInOneBlock = true;
AllocaPointerVal = 0;
}
/// AnalyzeAlloca - Scan the uses of the specified alloca, filling in our
/// ivars.
void AnalyzeAlloca(AllocaInst *AI) {
clear();
// As we scan the uses of the alloca instruction, keep track of stores,
// and decide whether all of the loads and stores to the alloca are within
// the same basic block.
for (Value::use_iterator U = AI->use_begin(), E = AI->use_end();
U != E; ++U) {
Instruction *User = cast<Instruction>(*U);
if (StoreInst *SI = dyn_cast<StoreInst>(User)) {
// Remember the basic blocks which define new values for the alloca
DefiningBlocks.push_back(SI->getParent());
AllocaPointerVal = SI->getOperand(0);
OnlyStore = SI;
} else {
LoadInst *LI = cast<LoadInst>(User);
// Otherwise it must be a load instruction, keep track of variable
// reads.
UsingBlocks.push_back(LI->getParent());
AllocaPointerVal = LI;
}
if (OnlyUsedInOneBlock) {
if (OnlyBlock == 0)
OnlyBlock = User->getParent();
else if (OnlyBlock != User->getParent())
OnlyUsedInOneBlock = false;
}
}
}
};
} // end of anonymous namespace
void PromoteMem2Reg::run() {
Function &F = *DF.getRoot()->getParent();
// LocallyUsedAllocas - Keep track of all of the alloca instructions which are
// only used in a single basic block. These instructions can be efficiently
// promoted by performing a single linear scan over that one block. Since
// individual basic blocks are sometimes large, we group together all allocas
// that are live in a single basic block by the basic block they are live in.
std::map<BasicBlock*, std::vector<AllocaInst*> > LocallyUsedAllocas;
if (AST) PointerAllocaValues.resize(Allocas.size());
AllocaInfo Info;
for (unsigned AllocaNum = 0; AllocaNum != Allocas.size(); ++AllocaNum) {
AllocaInst *AI = Allocas[AllocaNum];
assert(isAllocaPromotable(AI) &&
"Cannot promote non-promotable alloca!");
assert(AI->getParent()->getParent() == &F &&
"All allocas should be in the same function, which is same as DF!");
if (AI->use_empty()) {
// If there are no uses of the alloca, just delete it now.
if (AST) AST->deleteValue(AI);
AI->eraseFromParent();
// Remove the alloca from the Allocas list, since it has been processed
RemoveFromAllocasList(AllocaNum);
++NumDeadAlloca;
continue;
}
// Calculate the set of read and write-locations for each alloca. This is
// analogous to finding the 'uses' and 'definitions' of each variable.
Info.AnalyzeAlloca(AI);
// If there is only a single store to this value, replace any loads of
// it that are directly dominated by the definition with the value stored.
if (Info.DefiningBlocks.size() == 1) {
RewriteSingleStoreAlloca(AI, Info);
// Finally, after the scan, check to see if the store is all that is left.
if (Info.UsingBlocks.empty()) {
// Remove the (now dead) store and alloca.
Info.OnlyStore->eraseFromParent();
if (AST) AST->deleteValue(AI);
AI->eraseFromParent();
// The alloca has been processed, move on.
RemoveFromAllocasList(AllocaNum);
++NumSingleStore;
continue;
}
}
// If the alloca is only read and written in one basic block, just perform a
// linear sweep over the block to eliminate it.
if (Info.OnlyUsedInOneBlock) {
LocallyUsedAllocas[Info.OnlyBlock].push_back(AI);
// Remove the alloca from the Allocas list, since it will be processed.
RemoveFromAllocasList(AllocaNum);
continue;
}
// If we haven't computed a numbering for the BB's in the function, do so
// now.
if (BBNumbers.empty()) {
unsigned ID = 0;
for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
BBNumbers[I] = ID++;
}
// If we have an AST to keep updated, remember some pointer value that is
// stored into the alloca.
if (AST)
PointerAllocaValues[AllocaNum] = Info.AllocaPointerVal;
// Keep the reverse mapping of the 'Allocas' array for the rename pass.
AllocaLookup[Allocas[AllocaNum]] = AllocaNum;
// At this point, we're committed to promoting the alloca using IDF's, and
// the standard SSA construction algorithm. Determine which blocks need phi
// nodes and see if we can optimize out some work by avoiding insertion of
// dead phi nodes.
DetermineInsertionPoint(AI, AllocaNum, Info);
}
// Process all allocas which are only used in a single basic block.
for (std::map<BasicBlock*, std::vector<AllocaInst*> >::iterator I =
LocallyUsedAllocas.begin(), E = LocallyUsedAllocas.end(); I != E; ++I){
const std::vector<AllocaInst*> &LocAllocas = I->second;
assert(!LocAllocas.empty() && "empty alloca list??");
// It's common for there to only be one alloca in the list. Handle it
// efficiently.
if (LocAllocas.size() == 1) {
// If we can do the quick promotion pass, do so now.
if (PromoteLocallyUsedAlloca(I->first, LocAllocas[0]))
RetryList.push_back(LocAllocas[0]); // Failed, retry later.
} else {
// Locally promote anything possible. Note that if this is unable to
// promote a particular alloca, it puts the alloca onto the Allocas vector
// for global processing.
PromoteLocallyUsedAllocas(I->first, LocAllocas);
}
}
if (Allocas.empty())
return; // All of the allocas must have been trivial!
// Set the incoming values for the basic block to be null values for all of
// the alloca's. We do this in case there is a load of a value that has not
// been stored yet. In this case, it will get this null value.
//
RenamePassData::ValVector Values(Allocas.size());
for (unsigned i = 0, e = Allocas.size(); i != e; ++i)
Values[i] = UndefValue::get(Allocas[i]->getAllocatedType());
// Walks all basic blocks in the function performing the SSA rename algorithm
// and inserting the phi nodes we marked as necessary
//
std::vector<RenamePassData> RenamePassWorkList;
RenamePassWorkList.push_back(RenamePassData(F.begin(), 0, Values));
while (!RenamePassWorkList.empty()) {
RenamePassData RPD;
RPD.swap(RenamePassWorkList.back());
RenamePassWorkList.pop_back();
// RenamePass may add new worklist entries.
RenamePass(RPD.BB, RPD.Pred, RPD.Values, RenamePassWorkList);
}
// The renamer uses the Visited set to avoid infinite loops. Clear it now.
Visited.clear();
// Remove the allocas themselves from the function.
for (unsigned i = 0, e = Allocas.size(); i != e; ++i) {
Instruction *A = Allocas[i];
// If there are any uses of the alloca instructions left, they must be in
// sections of dead code that were not processed on the dominance frontier.
// Just delete the users now.
//
if (!A->use_empty())
A->replaceAllUsesWith(UndefValue::get(A->getType()));
if (AST) AST->deleteValue(A);
A->eraseFromParent();
}
// Loop over all of the PHI nodes and see if there are any that we can get
// rid of because they merge all of the same incoming values. This can
// happen due to undef values coming into the PHI nodes. This process is
// iterative, because eliminating one PHI node can cause others to be removed.
bool EliminatedAPHI = true;
while (EliminatedAPHI) {
EliminatedAPHI = false;
for (DenseMap<std::pair<BasicBlock*, unsigned>, PHINode*>::iterator I =
NewPhiNodes.begin(), E = NewPhiNodes.end(); I != E;) {
PHINode *PN = I->second;
// If this PHI node merges one value and/or undefs, get the value.
if (Value *V = PN->hasConstantValue(true)) {
if (!isa<Instruction>(V) ||
properlyDominates(cast<Instruction>(V), PN)) {
if (AST && isa<PointerType>(PN->getType()))
AST->deleteValue(PN);
PN->replaceAllUsesWith(V);
PN->eraseFromParent();
NewPhiNodes.erase(I++);
EliminatedAPHI = true;
continue;
}
}
++I;
}
}
// At this point, the renamer has added entries to PHI nodes for all reachable
// code. Unfortunately, there may be unreachable blocks which the renamer
// hasn't traversed. If this is the case, the PHI nodes may not
// have incoming values for all predecessors. Loop over all PHI nodes we have
// created, inserting undef values if they are missing any incoming values.
//
for (DenseMap<std::pair<BasicBlock*, unsigned>, PHINode*>::iterator I =
NewPhiNodes.begin(), E = NewPhiNodes.end(); I != E; ++I) {
// We want to do this once per basic block. As such, only process a block
// when we find the PHI that is the first entry in the block.
PHINode *SomePHI = I->second;
BasicBlock *BB = SomePHI->getParent();
if (&BB->front() != SomePHI)
continue;
// Only do work here if there the PHI nodes are missing incoming values. We
// know that all PHI nodes that were inserted in a block will have the same
// number of incoming values, so we can just check any of them.
if (SomePHI->getNumIncomingValues() == getNumPreds(BB))
continue;
// Get the preds for BB.
SmallVector<BasicBlock*, 16> Preds(pred_begin(BB), pred_end(BB));
// Ok, now we know that all of the PHI nodes are missing entries for some
// basic blocks. Start by sorting the incoming predecessors for efficient
// access.
std::sort(Preds.begin(), Preds.end());
// Now we loop through all BB's which have entries in SomePHI and remove
// them from the Preds list.
for (unsigned i = 0, e = SomePHI->getNumIncomingValues(); i != e; ++i) {
// Do a log(n) search of the Preds list for the entry we want.
SmallVector<BasicBlock*, 16>::iterator EntIt =
std::lower_bound(Preds.begin(), Preds.end(),
SomePHI->getIncomingBlock(i));
assert(EntIt != Preds.end() && *EntIt == SomePHI->getIncomingBlock(i)&&
"PHI node has entry for a block which is not a predecessor!");
// Remove the entry
Preds.erase(EntIt);
}
// At this point, the blocks left in the preds list must have dummy
// entries inserted into every PHI nodes for the block. Update all the phi
// nodes in this block that we are inserting (there could be phis before
// mem2reg runs).
unsigned NumBadPreds = SomePHI->getNumIncomingValues();
BasicBlock::iterator BBI = BB->begin();
while ((SomePHI = dyn_cast<PHINode>(BBI++)) &&
SomePHI->getNumIncomingValues() == NumBadPreds) {
Value *UndefVal = UndefValue::get(SomePHI->getType());
for (unsigned pred = 0, e = Preds.size(); pred != e; ++pred)
SomePHI->addIncoming(UndefVal, Preds[pred]);
}
}
NewPhiNodes.clear();
}
/// ComputeLiveInBlocks - Determine which blocks the value is live in. These
/// are blocks which lead to uses. Knowing this allows us to avoid inserting
/// PHI nodes into blocks which don't lead to uses (thus, the inserted phi nodes
/// would be dead).
void PromoteMem2Reg::
ComputeLiveInBlocks(AllocaInst *AI, AllocaInfo &Info,
const SmallPtrSet<BasicBlock*, 32> &DefBlocks,
SmallPtrSet<BasicBlock*, 32> &LiveInBlocks) {
// To determine liveness, we must iterate through the predecessors of blocks
// where the def is live. Blocks are added to the worklist if we need to
// check their predecessors. Start with all the using blocks.
SmallVector<BasicBlock*, 64> LiveInBlockWorklist;
LiveInBlockWorklist.insert(LiveInBlockWorklist.end(),
Info.UsingBlocks.begin(), Info.UsingBlocks.end());
// If any of the using blocks is also a definition block, check to see if the
// definition occurs before or after the use. If it happens before the use,
// the value isn't really live-in.
for (unsigned i = 0, e = LiveInBlockWorklist.size(); i != e; ++i) {
BasicBlock *BB = LiveInBlockWorklist[i];
if (!DefBlocks.count(BB)) continue;
// Okay, this is a block that both uses and defines the value. If the first
// reference to the alloca is a def (store), then we know it isn't live-in.
for (BasicBlock::iterator I = BB->begin(); ; ++I) {
if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
if (SI->getOperand(1) != AI) continue;
// We found a store to the alloca before a load. The alloca is not
// actually live-in here.
LiveInBlockWorklist[i] = LiveInBlockWorklist.back();
LiveInBlockWorklist.pop_back();
--i, --e;
break;
} else if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
if (LI->getOperand(0) != AI) continue;
// Okay, we found a load before a store to the alloca. It is actually
// live into this block.
break;
}
}
}
// Now that we have a set of blocks where the phi is live-in, recursively add
// their predecessors until we find the full region the value is live.
while (!LiveInBlockWorklist.empty()) {
BasicBlock *BB = LiveInBlockWorklist.back();
LiveInBlockWorklist.pop_back();
// The block really is live in here, insert it into the set. If already in
// the set, then it has already been processed.
if (!LiveInBlocks.insert(BB))
continue;
// Since the value is live into BB, it is either defined in a predecessor or
// live into it to. Add the preds to the worklist unless they are a
// defining block.
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
BasicBlock *P = *PI;
// The value is not live into a predecessor if it defines the value.
if (DefBlocks.count(P))
continue;
// Otherwise it is, add to the worklist.
LiveInBlockWorklist.push_back(P);
}
}
}
/// DetermineInsertionPoint - At this point, we're committed to promoting the
/// alloca using IDF's, and the standard SSA construction algorithm. Determine
/// which blocks need phi nodes and see if we can optimize out some work by
/// avoiding insertion of dead phi nodes.
void PromoteMem2Reg::DetermineInsertionPoint(AllocaInst *AI, unsigned AllocaNum,
AllocaInfo &Info) {
// Unique the set of defining blocks for efficient lookup.
SmallPtrSet<BasicBlock*, 32> DefBlocks;
DefBlocks.insert(Info.DefiningBlocks.begin(), Info.DefiningBlocks.end());
// Determine which blocks the value is live in. These are blocks which lead
// to uses.
SmallPtrSet<BasicBlock*, 32> LiveInBlocks;
ComputeLiveInBlocks(AI, Info, DefBlocks, LiveInBlocks);
// Compute the locations where PhiNodes need to be inserted. Look at the
// dominance frontier of EACH basic-block we have a write in.
unsigned CurrentVersion = 0;
SmallPtrSet<PHINode*, 16> InsertedPHINodes;
std::vector<std::pair<unsigned, BasicBlock*> > DFBlocks;
while (!Info.DefiningBlocks.empty()) {
BasicBlock *BB = Info.DefiningBlocks.back();
Info.DefiningBlocks.pop_back();
// Look up the DF for this write, add it to defining blocks.
DominanceFrontier::const_iterator it = DF.find(BB);
if (it == DF.end()) continue;
const DominanceFrontier::DomSetType &S = it->second;
// In theory we don't need the indirection through the DFBlocks vector.
// In practice, the order of calling QueuePhiNode would depend on the
// (unspecified) ordering of basic blocks in the dominance frontier,
// which would give PHI nodes non-determinstic subscripts. Fix this by
// processing blocks in order of the occurance in the function.
for (DominanceFrontier::DomSetType::const_iterator P = S.begin(),
PE = S.end(); P != PE; ++P) {
// If the frontier block is not in the live-in set for the alloca, don't
// bother processing it.
if (!LiveInBlocks.count(*P))
continue;
DFBlocks.push_back(std::make_pair(BBNumbers[*P], *P));
}
// Sort by which the block ordering in the function.
if (DFBlocks.size() > 1)
std::sort(DFBlocks.begin(), DFBlocks.end());
for (unsigned i = 0, e = DFBlocks.size(); i != e; ++i) {
BasicBlock *BB = DFBlocks[i].second;
if (QueuePhiNode(BB, AllocaNum, CurrentVersion, InsertedPHINodes))
Info.DefiningBlocks.push_back(BB);
}
DFBlocks.clear();
}
}
/// RewriteSingleStoreAlloca - If there is only a single store to this value,
/// replace any loads of it that are directly dominated by the definition with
/// the value stored.
void PromoteMem2Reg::RewriteSingleStoreAlloca(AllocaInst *AI,
AllocaInfo &Info) {
StoreInst *OnlyStore = Info.OnlyStore;
bool StoringGlobalVal = !isa<Instruction>(OnlyStore->getOperand(0));
// Be aware of loads before the store.
SmallPtrSet<BasicBlock*, 32> ProcessedBlocks;
for (unsigned i = 0, e = Info.UsingBlocks.size(); i != e; ++i) {
BasicBlock *UseBlock = Info.UsingBlocks[i];
// If we already processed this block, don't reprocess it.
if (!ProcessedBlocks.insert(UseBlock)) {
Info.UsingBlocks[i] = Info.UsingBlocks.back();
Info.UsingBlocks.pop_back();
--i; --e;
continue;
}
// If the store dominates the block and if we haven't processed it yet,
// do so now. We can't handle the case where the store doesn't dominate a
// block because there may be a path between the store and the use, but we
// may need to insert phi nodes to handle dominance properly.
if (!StoringGlobalVal && !dominates(OnlyStore->getParent(), UseBlock))
continue;
// If the use and store are in the same block, do a quick scan to
// verify that there are no uses before the store.
if (UseBlock == OnlyStore->getParent()) {
BasicBlock::iterator I = UseBlock->begin();
for (; &*I != OnlyStore; ++I) { // scan block for store.
if (isa<LoadInst>(I) && I->getOperand(0) == AI)
break;
}
if (&*I != OnlyStore)
continue; // Do not promote the uses of this in this block.
}
// Otherwise, if this is a different block or if all uses happen
// after the store, do a simple linear scan to replace loads with
// the stored value.
for (BasicBlock::iterator I = UseBlock->begin(), E = UseBlock->end();
I != E; ) {
if (LoadInst *LI = dyn_cast<LoadInst>(I++)) {
if (LI->getOperand(0) == AI) {
LI->replaceAllUsesWith(OnlyStore->getOperand(0));
if (AST && isa<PointerType>(LI->getType()))
AST->deleteValue(LI);
LI->eraseFromParent();
}
}
}
// Finally, remove this block from the UsingBlock set.
Info.UsingBlocks[i] = Info.UsingBlocks.back();
Info.UsingBlocks.pop_back();
--i; --e;
}
}
/// PromoteLocallyUsedAlloca - Many allocas are only used within a single basic
/// block. If this is the case, avoid traversing the CFG and inserting a lot of
/// potentially useless PHI nodes by just performing a single linear pass over
/// the basic block using the Alloca.
///
/// If we cannot promote this alloca (because it is read before it is written),
/// return true. This is necessary in cases where, due to control flow, the
/// alloca is potentially undefined on some control flow paths. e.g. code like
/// this is potentially correct:
///
/// for (...) { if (c) { A = undef; undef = B; } }
///
/// ... so long as A is not used before undef is set.
///
bool PromoteMem2Reg::PromoteLocallyUsedAlloca(BasicBlock *BB, AllocaInst *AI) {
assert(!AI->use_empty() && "There are no uses of the alloca!");
// Handle degenerate cases quickly.
if (AI->hasOneUse()) {
Instruction *U = cast<Instruction>(AI->use_back());
if (LoadInst *LI = dyn_cast<LoadInst>(U)) {
// Must be a load of uninitialized value.
LI->replaceAllUsesWith(UndefValue::get(AI->getAllocatedType()));
if (AST && isa<PointerType>(LI->getType()))
AST->deleteValue(LI);
} else {
// Otherwise it must be a store which is never read.
assert(isa<StoreInst>(U));
}
BB->getInstList().erase(U);
} else {
// Uses of the uninitialized memory location shall get undef.
Value *CurVal = 0;
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
Instruction *Inst = I++;
if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) {
if (LI->getOperand(0) == AI) {
if (!CurVal) return true; // Could not locally promote!
// Loads just returns the "current value"...
LI->replaceAllUsesWith(CurVal);
if (AST && isa<PointerType>(LI->getType()))
AST->deleteValue(LI);
BB->getInstList().erase(LI);
}
} else if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
if (SI->getOperand(1) == AI) {
// Store updates the "current value"...
CurVal = SI->getOperand(0);
BB->getInstList().erase(SI);
}
}
}
}
// After traversing the basic block, there should be no more uses of the
// alloca: remove it now.
assert(AI->use_empty() && "Uses of alloca from more than one BB??");
if (AST) AST->deleteValue(AI);
AI->eraseFromParent();
++NumLocalPromoted;
return false;
}
/// PromoteLocallyUsedAllocas - This method is just like
/// PromoteLocallyUsedAlloca, except that it processes multiple alloca
/// instructions in parallel. This is important in cases where we have large
/// basic blocks, as we don't want to rescan the entire basic block for each
/// alloca which is locally used in it (which might be a lot).
void PromoteMem2Reg::
PromoteLocallyUsedAllocas(BasicBlock *BB, const std::vector<AllocaInst*> &AIs) {
DenseMap<AllocaInst*, Value*> CurValues;
for (unsigned i = 0, e = AIs.size(); i != e; ++i)
CurValues[AIs[i]] = 0; // Insert with null value
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
Instruction *Inst = I++;
if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) {
// Is this a load of an alloca we are tracking?
if (AllocaInst *AI = dyn_cast<AllocaInst>(LI->getOperand(0))) {
DenseMap<AllocaInst*, Value*>::iterator AIt = CurValues.find(AI);
if (AIt != CurValues.end()) {
// If loading an uninitialized value, allow the inter-block case to
// handle it. Due to control flow, this might actually be ok.
if (AIt->second == 0) { // Use of locally uninitialized value??
RetryList.push_back(AI); // Retry elsewhere.
CurValues.erase(AIt); // Stop tracking this here.
if (CurValues.empty()) return;
} else {
// Loads just returns the "current value"...
LI->replaceAllUsesWith(AIt->second);
if (AST && isa<PointerType>(LI->getType()))
AST->deleteValue(LI);
BB->getInstList().erase(LI);
}
}
}
} else if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
if (AllocaInst *AI = dyn_cast<AllocaInst>(SI->getOperand(1))) {
DenseMap<AllocaInst*, Value*>::iterator AIt = CurValues.find(AI);
if (AIt != CurValues.end()) {
// Store updates the "current value"...
AIt->second = SI->getOperand(0);
SI->eraseFromParent();
}
}
}
}
// At the end of the block scan, all allocas in CurValues are dead.
for (DenseMap<AllocaInst*, Value*>::iterator I = CurValues.begin(),
E = CurValues.end(); I != E; ++I) {
AllocaInst *AI = I->first;
assert(AI->use_empty() && "Uses of alloca from more than one BB??");
if (AST) AST->deleteValue(AI);
AI->eraseFromParent();
}
NumLocalPromoted += CurValues.size();
}
// QueuePhiNode - queues a phi-node to be added to a basic-block for a specific
// Alloca returns true if there wasn't already a phi-node for that variable
//
bool PromoteMem2Reg::QueuePhiNode(BasicBlock *BB, unsigned AllocaNo,
unsigned &Version,
SmallPtrSet<PHINode*, 16> &InsertedPHINodes) {
// Look up the basic-block in question.
PHINode *&PN = NewPhiNodes[std::make_pair(BB, AllocaNo)];
// If the BB already has a phi node added for the i'th alloca then we're done!
if (PN) return false;
// Create a PhiNode using the dereferenced type... and add the phi-node to the
// BasicBlock.
PN = new PHINode(Allocas[AllocaNo]->getAllocatedType(),
Allocas[AllocaNo]->getName() + "." +
utostr(Version++), BB->begin());
++NumPHIInsert;
PhiToAllocaMap[PN] = AllocaNo;
PN->reserveOperandSpace(getNumPreds(BB));
InsertedPHINodes.insert(PN);
if (AST && isa<PointerType>(PN->getType()))
AST->copyValue(PointerAllocaValues[AllocaNo], PN);
return true;
}
// RenamePass - Recursively traverse the CFG of the function, renaming loads and
// stores to the allocas which we are promoting. IncomingVals indicates what
// value each Alloca contains on exit from the predecessor block Pred.
//
void PromoteMem2Reg::RenamePass(BasicBlock *BB, BasicBlock *Pred,
RenamePassData::ValVector &IncomingVals,
std::vector<RenamePassData> &Worklist) {
NextIteration:
// If we are inserting any phi nodes into this BB, they will already be in the
// block.
if (PHINode *APN = dyn_cast<PHINode>(BB->begin())) {
// Pred may have multiple edges to BB. If so, we want to add N incoming
// values to each PHI we are inserting on the first time we see the edge.
// Check to see if APN already has incoming values from Pred. This also
// prevents us from modifying PHI nodes that are not currently being
// inserted.
bool HasPredEntries = false;
for (unsigned i = 0, e = APN->getNumIncomingValues(); i != e; ++i) {
if (APN->getIncomingBlock(i) == Pred) {
HasPredEntries = true;
break;
}
}
// If we have PHI nodes to update, compute the number of edges from Pred to
// BB.
if (!HasPredEntries) {
TerminatorInst *PredTerm = Pred->getTerminator();
unsigned NumEdges = 0;
for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i) {
if (PredTerm->getSuccessor(i) == BB)
++NumEdges;
}
assert(NumEdges && "Must be at least one edge from Pred to BB!");
// Add entries for all the phis.
BasicBlock::iterator PNI = BB->begin();
do {
unsigned AllocaNo = PhiToAllocaMap[APN];
// Add N incoming values to the PHI node.
for (unsigned i = 0; i != NumEdges; ++i)
APN->addIncoming(IncomingVals[AllocaNo], Pred);
// The currently active variable for this block is now the PHI.
IncomingVals[AllocaNo] = APN;
// Get the next phi node.
++PNI;
APN = dyn_cast<PHINode>(PNI);
if (APN == 0) break;
// Verify it doesn't already have entries for Pred. If it does, it is
// not being inserted by this mem2reg invocation.
HasPredEntries = false;
for (unsigned i = 0, e = APN->getNumIncomingValues(); i != e; ++i) {
if (APN->getIncomingBlock(i) == Pred) {
HasPredEntries = true;
break;
}
}
} while (!HasPredEntries);
}
}
// Don't revisit blocks.
if (!Visited.insert(BB)) return;
for (BasicBlock::iterator II = BB->begin(); !isa<TerminatorInst>(II); ) {
Instruction *I = II++; // get the instruction, increment iterator
if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
AllocaInst *Src = dyn_cast<AllocaInst>(LI->getPointerOperand());
if (!Src) continue;
std::map<AllocaInst*, unsigned>::iterator AI = AllocaLookup.find(Src);
if (AI == AllocaLookup.end()) continue;
Value *V = IncomingVals[AI->second];
// Anything using the load now uses the current value.
LI->replaceAllUsesWith(V);
if (AST && isa<PointerType>(LI->getType()))
AST->deleteValue(LI);
BB->getInstList().erase(LI);
} else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
// Delete this instruction and mark the name as the current holder of the
// value
AllocaInst *Dest = dyn_cast<AllocaInst>(SI->getPointerOperand());
if (!Dest) continue;
std::map<AllocaInst *, unsigned>::iterator ai = AllocaLookup.find(Dest);
if (ai == AllocaLookup.end())
continue;
// what value were we writing?
IncomingVals[ai->second] = SI->getOperand(0);
BB->getInstList().erase(SI);
}
}
// 'Recurse' to our successors.
TerminatorInst *TI = BB->getTerminator();
unsigned NumSuccs = TI->getNumSuccessors();
if (NumSuccs == 0) return;
// Add all-but-one successor to the worklist.
for (unsigned i = 0; i != NumSuccs-1; i++)
Worklist.push_back(RenamePassData(TI->getSuccessor(i), BB, IncomingVals));
// Handle the last successor without using the worklist. This allows us to
// handle unconditional branches directly, for example.
Pred = BB;
BB = TI->getSuccessor(NumSuccs-1);
goto NextIteration;
}
/// PromoteMemToReg - Promote the specified list of alloca instructions into
/// scalar registers, inserting PHI nodes as appropriate. This function makes
/// use of DominanceFrontier information. This function does not modify the CFG
/// of the function at all. All allocas must be from the same function.
///
/// If AST is specified, the specified tracker is updated to reflect changes
/// made to the IR.
///
void llvm::PromoteMemToReg(const std::vector<AllocaInst*> &Allocas,
DominatorTree &DT, DominanceFrontier &DF,
AliasSetTracker *AST) {
// If there is nothing to do, bail out...
if (Allocas.empty()) return;
SmallVector<AllocaInst*, 16> RetryList;
PromoteMem2Reg(Allocas, RetryList, DT, DF, AST).run();
// PromoteMem2Reg may not have been able to promote all of the allocas in one
// pass, run it again if needed.
std::vector<AllocaInst*> NewAllocas;
while (!RetryList.empty()) {
// If we need to retry some allocas, this is due to there being no store
// before a read in a local block. To counteract this, insert a store of
// undef into the alloca right after the alloca itself.
for (unsigned i = 0, e = RetryList.size(); i != e; ++i) {
BasicBlock::iterator BBI = RetryList[i];
new StoreInst(UndefValue::get(RetryList[i]->getAllocatedType()),
RetryList[i], ++BBI);
}
NewAllocas.assign(RetryList.begin(), RetryList.end());
RetryList.clear();
PromoteMem2Reg(NewAllocas, RetryList, DT, DF, AST).run();
NewAllocas.clear();
}
}