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Initial implementation of value numbering for load instructions 

llvm-svn: 3540
This commit is contained in:
Chris Lattner 2002-08-30 20:28:10 +00:00
parent 3e4e6974f2
commit 2dd49e525c
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llvm/lib/Analysis/LoadValueNumbering.cpp Normal file
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//===- LoadValueNumbering.cpp - Load Value #'ing Implementation -*- C++ -*-===//
//
// This file implements a value numbering pass that value #'s load instructions.
// To do this, it finds lexically identical load instructions, and uses alias
// analysis to determine which loads are guaranteed to produce the same value.
//
// This pass builds off of another value numbering pass to implement value
// numbering for non-load instructions. It uses Alias Analysis so that it can
// disambiguate the load instructions. The more powerful these base analyses
// are, the more powerful the resultant analysis will be.
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/LoadValueNumbering.h"
#include "llvm/Analysis/ValueNumbering.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Pass.h"
#include "llvm/iMemory.h"
#include "llvm/BasicBlock.h"
#include "llvm/Support/CFG.h"
#include <algorithm>
#include <set>
namespace {
// FIXME: This should not be a functionpass.
struct LoadVN : public FunctionPass, public ValueNumbering {
/// Pass Implementation stuff. This doesn't do any analysis.
///
bool runOnFunction(Function &) { return false; }
/// getAnalysisUsage - Does not modify anything. It uses Value Numbering
/// and Alias Analysis.
///
virtual void getAnalysisUsage(AnalysisUsage &AU) const;
/// getEqualNumberNodes - Return nodes with the same value number as the
/// specified Value. This fills in the argument vector with any equal
/// values.
///
virtual void getEqualNumberNodes(Value *V1,
std::vector<Value*> &RetVals) const;
private:
/// haveEqualValueNumber - Given two load instructions, determine if they
/// both produce the same value on every execution of the program, assuming
/// that their source operands always give the same value. This uses the
/// AliasAnalysis implementation to invalidate loads when stores or function
/// calls occur that could modify the value produced by the load.
///
bool haveEqualValueNumber(LoadInst *LI, LoadInst *LI2, AliasAnalysis &AA,
DominatorSet &DomSetInfo) const;
};
// Register this pass...
RegisterOpt<LoadVN> X("load-vn", "Load Value Numbering");
// Declare that we implement the ValueNumbering interface
RegisterAnalysisGroup<ValueNumbering, LoadVN> Y;
}
Pass *createLoadValueNumberingPass() { return new LoadVN(); }
/// getAnalysisUsage - Does not modify anything. It uses Value Numbering and
/// Alias Analysis.
///
void LoadVN::getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesAll();
AU.addRequired<AliasAnalysis>();
AU.addRequired<ValueNumbering>();
AU.addRequired<DominatorSet>();
}
// getEqualNumberNodes - Return nodes with the same value number as the
// specified Value. This fills in the argument vector with any equal values.
//
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.
//
std::vector<Value*> PointerSources;
getEqualNumberNodes(LI->getOperand(0), PointerSources);
PointerSources.push_back(LI->getOperand(0));
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.
//
std::vector<LoadInst*> CandidateLoads;
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 (Cand->getParent()->getParent() == F && // In the same function?
Cand != LI) // Not LI itself?
CandidateLoads.push_back(Cand); // Got one...
}
// 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.
//
std::sort(CandidateLoads.begin(), CandidateLoads.end());
CandidateLoads.erase(std::unique(CandidateLoads.begin(),
CandidateLoads.end()),
CandidateLoads.end());
// Get Alias Analysis...
AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
DominatorSet &DomSetInfo = getAnalysis<DominatorSet>();
// Loop over all of the candindate loads. If they are not invalidated by
// stores or calls between execution of them and LI, then add them to
// RetVals.
for (unsigned i = 0, e = CandidateLoads.size(); i != e; ++i)
if (haveEqualValueNumber(LI, CandidateLoads[i], AA, DomSetInfo))
RetVals.push_back(CandidateLoads[i]);
} else {
// Make sure passmanager doesn't try to fulfill our request with ourself!
assert(&getAnalysis<ValueNumbering>() != (ValueNumbering*)this &&
"getAnalysis() returned this!");
// Not a load instruction? Just chain to the base value numbering
// implementation to satisfy the request...
return getAnalysis<ValueNumbering>().getEqualNumberNodes(V, RetVals);
}
}
// CheckForInvalidatingInst - Return true if BB or any of the predecessors of BB
// (until DestBB) contain an instruction that might invalidate Ptr.
//
static bool CheckForInvalidatingInst(BasicBlock *BB, BasicBlock *DestBB,
Value *Ptr, AliasAnalysis &AA,
std::set<BasicBlock*> &VisitedSet) {
// Found the termination point!
if (BB == DestBB || VisitedSet.count(BB)) return false;
// Avoid infinite recursion!
VisitedSet.insert(BB);
// Can this basic block modify Ptr?
if (AA.canBasicBlockModify(*BB, Ptr))
return true;
// Check all of our predecessor blocks...
for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE; ++PI)
if (CheckForInvalidatingInst(*PI, DestBB, Ptr, AA, VisitedSet))
return true;
// None of our predecessor blocks contain an invalidating instruction, and we
// don't either!
return false;
}
/// haveEqualValueNumber - Given two load instructions, determine if they both
/// produce the same value on every execution of the program, assuming that
/// their source operands always give the same value. This uses the
/// AliasAnalysis implementation to invalidate loads when stores or function
/// calls occur that could modify the value produced by the load.
///
bool LoadVN::haveEqualValueNumber(LoadInst *L1, LoadInst *L2,
AliasAnalysis &AA,
DominatorSet &DomSetInfo) const {
// Figure out which load dominates the other one. If neither dominates the
// other we cannot eliminate them.
//
// FIXME: This could be enhanced to some cases with a shared dominator!
//
if (DomSetInfo.dominates(L2, L1))
std::swap(L1, L2); // Make L1 dominate L2
else if (!DomSetInfo.dominates(L1, L2))
return false; // Neither instruction dominates the other one...
BasicBlock *BB1 = L1->getParent(), *BB2 = L2->getParent();
Value *LoadAddress = L1->getOperand(0);
// L1 now dominates L2. Check to see if the intervening instructions between
// the two loads 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 L1 & L2...
//
if (AA.canInstructionRangeModify(*L1, *L2, LoadAddress))
return false; // Cannot eliminate load
// No instructions invalidate the loads, they produce the same value!
return true;
} else {
// Make sure that there are no store instructions between L1 and the end of
// it's basic block...
//
if (AA.canInstructionRangeModify(*L1, *BB1->getTerminator(), LoadAddress))
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(), *L2, LoadAddress))
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, 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;
}
}