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Implement forwarding from stores to loads of must-aliased pointers.

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This implements: GCSE/2003-06-13-LoadStoreEliminate.ll

llvm-svn: 6694
This commit is contained in:
Chris Lattner 2003-06-16 12:06:41 +00:00
parent 25e6231cd8
commit 24441a15c5
1 changed files with 96 additions and 14 deletions
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110
lib/Analysis/LoadValueNumbering.cpp
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@ -24,7 +24,7 @@
#include <set>
namespace {
// FIXME: This should not be a functionpass.
// FIXME: This should not be a FunctionPass.
struct LoadVN : public FunctionPass, public ValueNumbering {
/// Pass Implementation stuff. This doesn't do any analysis.
@ -51,6 +51,8 @@ namespace {
///
bool haveEqualValueNumber(LoadInst *LI, LoadInst *LI2, AliasAnalysis &AA,
DominatorSet &DomSetInfo) const;
bool haveEqualValueNumber(LoadInst *LI, StoreInst *SI, AliasAnalysis &AA,
DominatorSet &DomSetInfo) const;
};
// Register this pass...
@ -83,13 +85,13 @@ void LoadVN::getEqualNumberNodes(Value *V,
std::vector<Value*> &RetVals) const {
if (LoadInst *LI = dyn_cast<LoadInst>(V)) {
// If we have a load instruction, find all of the load instructions that use
// the same source operand. We implement this recursively, because there
// could be a load of a load of a load that are all identical. We are
// guaranteed that this cannot be an infinite recursion because load
// instructions would have to pass through a PHI node in order for there to
// be a cycle. The PHI node would be handled by the else case here,
// breaking the infinite recursion.
// If we have a load instruction, find all of the load and store
// instructions that use the same source operand. We implement this
// recursively, because there could be a load of a load of a load that are
// all identical. We are guaranteed that this cannot be an infinite
// recursion because load instructions would have to pass through a PHI node
// in order for there to be a cycle. The PHI node would be handled by the
// else case here, breaking the infinite recursion.
//
std::vector<Value*> PointerSources;
getEqualNumberNodes(LI->getOperand(0), PointerSources);
@ -98,30 +100,40 @@ void LoadVN::getEqualNumberNodes(Value *V,
Function *F = LI->getParent()->getParent();
// Now that we know the set of equivalent source pointers for the load
// instruction, look to see if there are any load candiates that are
// identical.
// instruction, look to see if there are any load or store candiates that
// are identical.
//
std::vector<LoadInst*> CandidateLoads;
std::vector<StoreInst*> CandidateStores;
while (!PointerSources.empty()) {
Value *Source = PointerSources.back();
PointerSources.pop_back(); // Get a source pointer...
for (Value::use_iterator UI = Source->use_begin(), UE = Source->use_end();
UI != UE; ++UI)
if (LoadInst *Cand = dyn_cast<LoadInst>(*UI)) // Is a load of source?
if (LoadInst *Cand = dyn_cast<LoadInst>(*UI)) {// Is a load of source?
if (Cand->getParent()->getParent() == F && // In the same function?
Cand != LI) // Not LI itself?
CandidateLoads.push_back(Cand); // Got one...
} else if (StoreInst *Cand = dyn_cast<StoreInst>(*UI)) {
if (Cand->getParent()->getParent() == F &&
Cand->getOperand(1) == Source) // It's a store THROUGH the ptr...
CandidateStores.push_back(Cand);
}
}
// Remove duplicates from the CandidateLoads list because alias analysis
// processing may be somewhat expensive and we don't want to do more work
// than neccesary.
//
unsigned OldSize = CandidateLoads.size();
std::sort(CandidateLoads.begin(), CandidateLoads.end());
CandidateLoads.erase(std::unique(CandidateLoads.begin(),
CandidateLoads.end()),
CandidateLoads.end());
// FIXME: REMOVE THIS SORTING AND UNIQUING IF IT CAN'T HAPPEN
assert(CandidateLoads.size() == OldSize && "Shrunk the candloads list?");
// Get Alias Analysis...
AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
@ -133,9 +145,11 @@ void LoadVN::getEqualNumberNodes(Value *V,
for (unsigned i = 0, e = CandidateLoads.size(); i != e; ++i)
if (haveEqualValueNumber(LI, CandidateLoads[i], AA, DomSetInfo))
RetVals.push_back(CandidateLoads[i]);
for (unsigned i = 0, e = CandidateStores.size(); i != e; ++i)
if (haveEqualValueNumber(LI, CandidateStores[i], AA, DomSetInfo))
RetVals.push_back(CandidateStores[i]->getOperand(0));
} else {
// Make sure passmanager doesn't try to fulfill our request with ourself!
assert(&getAnalysis<ValueNumbering>() != (ValueNumbering*)this &&
"getAnalysis() returned this!");
@ -215,7 +229,7 @@ bool LoadVN::haveEqualValueNumber(LoadInst *L1, LoadInst *L2,
return true;
} else {
// Make sure that there are no store instructions between L1 and the end of
// it's basic block...
// its basic block...
//
if (AA.canInstructionRangeModify(*L1, *BB1->getTerminator(), LoadAddress,
LoadSize))
@ -242,3 +256,71 @@ bool LoadVN::haveEqualValueNumber(LoadInst *L1, LoadInst *L2,
return true;
}
}
/// haveEqualValueNumber - Given a load instruction and a store instruction,
/// determine if the stored value reaches the loaded value unambiguously on
/// every execution of the program. This uses the AliasAnalysis implementation
/// to invalidate the stored value when stores or function calls occur that
/// could modify the value produced by the load.
///
bool LoadVN::haveEqualValueNumber(LoadInst *Load, StoreInst *Store,
AliasAnalysis &AA,
DominatorSet &DomSetInfo) const {
// If the store does not dominate the load, we cannot do anything...
if (!DomSetInfo.dominates(Store, Load))
return false;
BasicBlock *BB1 = Store->getParent(), *BB2 = Load->getParent();
Value *LoadAddress = Load->getOperand(0);
assert(LoadAddress->getType() == Store->getOperand(1)->getType() &&
"How could the same source pointer return different types?");
// Find out how many bytes of memory are loaded by the load instruction...
unsigned LoadSize = getAnalysis<TargetData>().getTypeSize(Load->getType());
// Compute a basic block iterator pointing to the instruction after the store.
BasicBlock::iterator StoreIt = Store; ++StoreIt;
// Check to see if the intervening instructions between the two store and load
// include a store or call...
//
if (BB1 == BB2) { // In same basic block?
// In this degenerate case, no checking of global basic blocks has to occur
// just check the instructions BETWEEN Store & Load...
//
if (AA.canInstructionRangeModify(*StoreIt, *Load, LoadAddress, LoadSize))
return false; // Cannot eliminate load
// No instructions invalidate the stored value, they produce the same value!
return true;
} else {
// Make sure that there are no store instructions between the Store and the
// end of its basic block...
//
if (AA.canInstructionRangeModify(*StoreIt, *BB1->getTerminator(),
LoadAddress, LoadSize))
return false; // Cannot eliminate load
// Make sure that there are no store instructions between the start of BB2
// and the second load instruction...
//
if (AA.canInstructionRangeModify(BB2->front(), *Load, LoadAddress,LoadSize))
return false; // Cannot eliminate load
// Do a depth first traversal of the inverse CFG starting at L2's block,
// looking for L1's block. The inverse CFG is made up of the predecessor
// nodes of a block... so all of the edges in the graph are "backward".
//
std::set<BasicBlock*> VisitedSet;
for (pred_iterator PI = pred_begin(BB2), PE = pred_end(BB2); PI != PE; ++PI)
if (CheckForInvalidatingInst(*PI, BB1, LoadAddress, LoadSize, AA,
VisitedSet))
return false;
// If we passed all of these checks then we are sure that the two loads
// produce the same value.
return true;
}
}