llvm-6502/lib/Transforms/Utils/PromoteMemoryToRegister.cpp
Chris Lattner cb2610ea03 - Rename AnalysisUsage::preservesAll to getPreservesAll & preservesCFG to
setPreservesCFG to be less confusing.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@4255 91177308-0d34-0410-b5e6-96231b3b80d8
2002-10-21 20:00:28 +00:00

281 lines
9.9 KiB
C++

//===- PromoteMemoryToRegister.cpp - Convert memory refs to regs ----------===//
//
// This pass is used to promote memory references to be register references. A
// simple example of the transformation performed by this pass is:
//
// FROM CODE TO CODE
// %X = alloca int, uint 1 ret int 42
// store int 42, int *%X
// %Y = load int* %X
// ret int %Y
//
// To do this transformation, a simple analysis is done to ensure it is safe.
// Currently this just loops over all alloca instructions, looking for
// instructions that are only used in simple load and stores.
//
// After this, the code is transformed by...something magical :)
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/iMemory.h"
#include "llvm/iPHINode.h"
#include "llvm/iTerminators.h"
#include "llvm/Function.h"
#include "llvm/Constant.h"
#include "llvm/Type.h"
#include "Support/Statistic.h"
using std::vector;
using std::map;
using std::set;
namespace {
Statistic<> NumPromoted("mem2reg", "Number of alloca's promoted");
struct PromotePass : public FunctionPass {
vector<AllocaInst*> Allocas; // the alloca instruction..
map<Instruction*, unsigned> AllocaLookup; // reverse mapping of above
vector<vector<BasicBlock*> > PhiNodes; // index corresponds to Allocas
// List of instructions to remove at end of pass
vector<Instruction *> KillList;
map<BasicBlock*,vector<PHINode*> > NewPhiNodes; // the PhiNodes we're adding
public:
// runOnFunction - To run this pass, first we calculate the alloca
// instructions that are safe for promotion, then we promote each one.
//
virtual bool runOnFunction(Function &F);
// getAnalysisUsage - We need dominance frontiers
//
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<DominanceFrontier>();
AU.setPreservesCFG();
}
private:
void Traverse(BasicBlock *BB, BasicBlock *Pred, vector<Value*> &IncVals,
set<BasicBlock*> &Visited);
bool QueuePhiNode(BasicBlock *BB, unsigned AllocaIdx);
void FindSafeAllocas(Function &F);
};
RegisterOpt<PromotePass> X("mem2reg", "Promote Memory to Register");
} // end of anonymous namespace
// isSafeAlloca - This predicate controls what types of alloca instructions are
// allowed to be promoted...
//
static inline bool isSafeAlloca(const AllocaInst *AI) {
if (AI->isArrayAllocation()) return false;
// 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))
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;
}
// FindSafeAllocas - Find allocas that are safe to promote
//
void PromotePass::FindSafeAllocas(Function &F) {
BasicBlock &BB = F.getEntryNode(); // Get the entry node for the function
// Look at all instructions in the entry node
for (BasicBlock::iterator I = BB.begin(), E = BB.end(); I != E; ++I)
if (AllocaInst *AI = dyn_cast<AllocaInst>(&*I)) // Is it an alloca?
if (isSafeAlloca(AI)) { // If safe alloca, add alloca to safe list
AllocaLookup[AI] = Allocas.size(); // Keep reverse mapping
Allocas.push_back(AI);
}
}
bool PromotePass::runOnFunction(Function &F) {
// Calculate the set of safe allocas
FindSafeAllocas(F);
// If there is nothing to do, bail out...
if (Allocas.empty()) return false;
// Add each alloca to the KillList. Note: KillList is destroyed MOST recently
// added to least recently.
KillList.assign(Allocas.begin(), Allocas.end());
// Calculate the set of write-locations for each alloca. This is analogous to
// counting the number of 'redefinitions' of each variable.
vector<vector<BasicBlock*> > WriteSets; // index corresponds to Allocas
WriteSets.resize(Allocas.size());
for (unsigned i = 0; i != Allocas.size(); ++i) {
AllocaInst *AI = Allocas[i];
for (Value::use_iterator U =AI->use_begin(), E = AI->use_end(); U != E; ++U)
if (StoreInst *SI = dyn_cast<StoreInst>(*U))
// jot down the basic-block it came from
WriteSets[i].push_back(SI->getParent());
}
// Get dominance frontier information...
DominanceFrontier &DF = getAnalysis<DominanceFrontier>();
// Compute the locations where PhiNodes need to be inserted. Look at the
// dominance frontier of EACH basic-block we have a write in
//
PhiNodes.resize(Allocas.size());
for (unsigned i = 0; i != Allocas.size(); ++i) {
for (unsigned j = 0; j != WriteSets[i].size(); j++) {
// Look up the DF for this write, add it to PhiNodes
DominanceFrontier::const_iterator it = DF.find(WriteSets[i][j]);
DominanceFrontier::DomSetType S = it->second;
for (DominanceFrontier::DomSetType::iterator P = S.begin(), PE = S.end();
P != PE; ++P)
QueuePhiNode(*P, i);
}
// Perform iterative step
for (unsigned k = 0; k != PhiNodes[i].size(); k++) {
DominanceFrontier::const_iterator it = DF.find(PhiNodes[i][k]);
DominanceFrontier::DomSetType S = it->second;
for (DominanceFrontier::DomSetType::iterator P = S.begin(), PE = S.end();
P != PE; ++P)
QueuePhiNode(*P, i);
}
}
// 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.
//
vector<Value *> Values(Allocas.size());
for (unsigned i = 0, e = Allocas.size(); i != e; ++i)
Values[i] = Constant::getNullValue(Allocas[i]->getAllocatedType());
// Walks all basic blocks in the function performing the SSA rename algorithm
// and inserting the phi nodes we marked as necessary
//
set<BasicBlock*> Visited; // The basic blocks we've already visited
Traverse(F.begin(), 0, Values, Visited);
// Remove all instructions marked by being placed in the KillList...
//
while (!KillList.empty()) {
Instruction *I = KillList.back();
KillList.pop_back();
I->getParent()->getInstList().erase(I);
}
NumPromoted += Allocas.size();
// Purge data structurse so they are available the next iteration...
Allocas.clear();
AllocaLookup.clear();
PhiNodes.clear();
NewPhiNodes.clear();
return true;
}
// 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 PromotePass::QueuePhiNode(BasicBlock *BB, unsigned AllocaNo) {
// Look up the basic-block in question
vector<PHINode*> &BBPNs = NewPhiNodes[BB];
if (BBPNs.empty()) BBPNs.resize(Allocas.size());
// If the BB already has a phi node added for the i'th alloca then we're done!
if (BBPNs[AllocaNo]) return false;
// Create a PhiNode using the dereferenced type... and add the phi-node to the
// BasicBlock
PHINode *PN = new PHINode(Allocas[AllocaNo]->getAllocatedType(),
Allocas[AllocaNo]->getName()+".mem2reg",
BB->begin());
BBPNs[AllocaNo] = PN;
PhiNodes[AllocaNo].push_back(BB);
return true;
}
void PromotePass::Traverse(BasicBlock *BB, BasicBlock *Pred,
vector<Value*> &IncomingVals,
set<BasicBlock*> &Visited) {
// If this is a BB needing a phi node, lookup/create the phinode for each
// variable we need phinodes for.
vector<PHINode *> &BBPNs = NewPhiNodes[BB];
for (unsigned k = 0; k != BBPNs.size(); ++k)
if (PHINode *PN = BBPNs[k]) {
// at this point we can assume that the array has phi nodes.. let's add
// the incoming data
PN->addIncoming(IncomingVals[k], Pred);
// also note that the active variable IS designated by the phi node
IncomingVals[k] = PN;
}
// don't revisit nodes
if (Visited.count(BB)) return;
// mark as visited
Visited.insert(BB);
// keep track of the value of each variable we're watching.. how?
for (BasicBlock::iterator II = BB->begin(); II != BB->end(); ++II) {
Instruction *I = II; // get the instruction
if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
Value *Ptr = LI->getPointerOperand();
if (AllocaInst *Src = dyn_cast<AllocaInst>(Ptr)) {
map<Instruction*, unsigned>::iterator AI = AllocaLookup.find(Src);
if (AI != AllocaLookup.end()) {
Value *V = IncomingVals[AI->second];
// walk the use list of this load and replace all uses with r
LI->replaceAllUsesWith(V);
KillList.push_back(LI); // Mark the load to be deleted
}
}
} else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
// delete this instruction and mark the name as the current holder of the
// value
Value *Ptr = SI->getPointerOperand();
if (AllocaInst *Dest = dyn_cast<AllocaInst>(Ptr)) {
map<Instruction *, unsigned>::iterator ai = AllocaLookup.find(Dest);
if (ai != AllocaLookup.end()) {
// what value were we writing?
IncomingVals[ai->second] = SI->getOperand(0);
KillList.push_back(SI); // Mark the store to be deleted
}
}
} else if (TerminatorInst *TI = dyn_cast<TerminatorInst>(I)) {
// Recurse across our successors
for (unsigned i = 0; i != TI->getNumSuccessors(); i++) {
vector<Value*> OutgoingVals(IncomingVals);
Traverse(TI->getSuccessor(i), BB, OutgoingVals, Visited);
}
}
}
}
// createPromoteMemoryToRegister - Provide an entry point to create this pass.
//
Pass *createPromoteMemoryToRegister() {
return new PromotePass();
}