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9c31585ee4
llvm-svn: 3540
234 lines
9.4 KiB
C++
234 lines
9.4 KiB
C++
//===- LoadValueNumbering.cpp - Load Value #'ing Implementation -*- C++ -*-===//
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//
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// This file implements a value numbering pass that value #'s load instructions.
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// To do this, it finds lexically identical load instructions, and uses alias
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// analysis to determine which loads are guaranteed to produce the same value.
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//
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// This pass builds off of another value numbering pass to implement value
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// numbering for non-load instructions. It uses Alias Analysis so that it can
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// disambiguate the load instructions. The more powerful these base analyses
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// are, the more powerful the resultant analysis will be.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Analysis/LoadValueNumbering.h"
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#include "llvm/Analysis/ValueNumbering.h"
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/Pass.h"
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#include "llvm/iMemory.h"
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#include "llvm/BasicBlock.h"
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#include "llvm/Support/CFG.h"
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#include <algorithm>
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#include <set>
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namespace {
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// FIXME: This should not be a functionpass.
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struct LoadVN : public FunctionPass, public ValueNumbering {
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/// Pass Implementation stuff. This doesn't do any analysis.
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///
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bool runOnFunction(Function &) { return false; }
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/// getAnalysisUsage - Does not modify anything. It uses Value Numbering
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/// and Alias Analysis.
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///
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virtual void getAnalysisUsage(AnalysisUsage &AU) const;
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/// getEqualNumberNodes - Return nodes with the same value number as the
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/// specified Value. This fills in the argument vector with any equal
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/// values.
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///
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virtual void getEqualNumberNodes(Value *V1,
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std::vector<Value*> &RetVals) const;
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private:
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/// haveEqualValueNumber - Given two load instructions, determine if they
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/// both produce the same value on every execution of the program, assuming
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/// that their source operands always give the same value. This uses the
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/// AliasAnalysis implementation to invalidate loads when stores or function
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/// calls occur that could modify the value produced by the load.
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///
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bool haveEqualValueNumber(LoadInst *LI, LoadInst *LI2, AliasAnalysis &AA,
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DominatorSet &DomSetInfo) const;
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};
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// Register this pass...
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RegisterOpt<LoadVN> X("load-vn", "Load Value Numbering");
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// Declare that we implement the ValueNumbering interface
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RegisterAnalysisGroup<ValueNumbering, LoadVN> Y;
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}
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Pass *createLoadValueNumberingPass() { return new LoadVN(); }
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/// getAnalysisUsage - Does not modify anything. It uses Value Numbering and
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/// Alias Analysis.
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///
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void LoadVN::getAnalysisUsage(AnalysisUsage &AU) const {
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AU.setPreservesAll();
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AU.addRequired<AliasAnalysis>();
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AU.addRequired<ValueNumbering>();
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AU.addRequired<DominatorSet>();
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}
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// getEqualNumberNodes - Return nodes with the same value number as the
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// specified Value. This fills in the argument vector with any equal values.
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//
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void LoadVN::getEqualNumberNodes(Value *V,
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std::vector<Value*> &RetVals) const {
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if (LoadInst *LI = dyn_cast<LoadInst>(V)) {
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// If we have a load instruction find all of the load instructions that use
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// the same source operand. We implement this recursively, because there
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// could be a load of a load of a load that are all identical. We are
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// guaranteed that this cannot be an infinite recursion because load
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// instructions would have to pass through a PHI node in order for there to
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// be a cycle. The PHI node would be handled by the else case here,
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// breaking the infinite recursion.
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//
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std::vector<Value*> PointerSources;
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getEqualNumberNodes(LI->getOperand(0), PointerSources);
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PointerSources.push_back(LI->getOperand(0));
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Function *F = LI->getParent()->getParent();
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// Now that we know the set of equivalent source pointers for the load
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// instruction, look to see if there are any load candiates that are
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// identical.
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//
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std::vector<LoadInst*> CandidateLoads;
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while (!PointerSources.empty()) {
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Value *Source = PointerSources.back();
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PointerSources.pop_back(); // Get a source pointer...
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for (Value::use_iterator UI = Source->use_begin(), UE = Source->use_end();
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UI != UE; ++UI)
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if (LoadInst *Cand = dyn_cast<LoadInst>(*UI)) // Is a load of source?
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if (Cand->getParent()->getParent() == F && // In the same function?
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Cand != LI) // Not LI itself?
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CandidateLoads.push_back(Cand); // Got one...
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}
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// Remove duplicates from the CandidateLoads list because alias analysis
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// processing may be somewhat expensive and we don't want to do more work
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// than neccesary.
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//
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std::sort(CandidateLoads.begin(), CandidateLoads.end());
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CandidateLoads.erase(std::unique(CandidateLoads.begin(),
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CandidateLoads.end()),
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CandidateLoads.end());
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// Get Alias Analysis...
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AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
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DominatorSet &DomSetInfo = getAnalysis<DominatorSet>();
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// Loop over all of the candindate loads. If they are not invalidated by
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// stores or calls between execution of them and LI, then add them to
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// RetVals.
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for (unsigned i = 0, e = CandidateLoads.size(); i != e; ++i)
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if (haveEqualValueNumber(LI, CandidateLoads[i], AA, DomSetInfo))
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RetVals.push_back(CandidateLoads[i]);
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} else {
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// Make sure passmanager doesn't try to fulfill our request with ourself!
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assert(&getAnalysis<ValueNumbering>() != (ValueNumbering*)this &&
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"getAnalysis() returned this!");
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// Not a load instruction? Just chain to the base value numbering
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// implementation to satisfy the request...
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return getAnalysis<ValueNumbering>().getEqualNumberNodes(V, RetVals);
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}
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}
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// CheckForInvalidatingInst - Return true if BB or any of the predecessors of BB
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// (until DestBB) contain an instruction that might invalidate Ptr.
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//
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static bool CheckForInvalidatingInst(BasicBlock *BB, BasicBlock *DestBB,
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Value *Ptr, AliasAnalysis &AA,
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std::set<BasicBlock*> &VisitedSet) {
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// Found the termination point!
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if (BB == DestBB || VisitedSet.count(BB)) return false;
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// Avoid infinite recursion!
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VisitedSet.insert(BB);
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// Can this basic block modify Ptr?
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if (AA.canBasicBlockModify(*BB, Ptr))
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return true;
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// Check all of our predecessor blocks...
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for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE; ++PI)
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if (CheckForInvalidatingInst(*PI, DestBB, Ptr, AA, VisitedSet))
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return true;
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// None of our predecessor blocks contain an invalidating instruction, and we
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// don't either!
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return false;
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}
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/// haveEqualValueNumber - Given two load instructions, determine if they both
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/// produce the same value on every execution of the program, assuming that
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/// their source operands always give the same value. This uses the
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/// AliasAnalysis implementation to invalidate loads when stores or function
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/// calls occur that could modify the value produced by the load.
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///
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bool LoadVN::haveEqualValueNumber(LoadInst *L1, LoadInst *L2,
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AliasAnalysis &AA,
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DominatorSet &DomSetInfo) const {
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// Figure out which load dominates the other one. If neither dominates the
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// other we cannot eliminate them.
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//
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// FIXME: This could be enhanced to some cases with a shared dominator!
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//
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if (DomSetInfo.dominates(L2, L1))
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std::swap(L1, L2); // Make L1 dominate L2
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else if (!DomSetInfo.dominates(L1, L2))
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return false; // Neither instruction dominates the other one...
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BasicBlock *BB1 = L1->getParent(), *BB2 = L2->getParent();
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Value *LoadAddress = L1->getOperand(0);
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// L1 now dominates L2. Check to see if the intervening instructions between
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// the two loads include a store or call...
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//
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if (BB1 == BB2) { // In same basic block?
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// In this degenerate case, no checking of global basic blocks has to occur
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// just check the instructions BETWEEN L1 & L2...
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//
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if (AA.canInstructionRangeModify(*L1, *L2, LoadAddress))
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return false; // Cannot eliminate load
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// No instructions invalidate the loads, they produce the same value!
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return true;
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} else {
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// Make sure that there are no store instructions between L1 and the end of
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// it's basic block...
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//
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if (AA.canInstructionRangeModify(*L1, *BB1->getTerminator(), LoadAddress))
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return false; // Cannot eliminate load
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// Make sure that there are no store instructions between the start of BB2
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// and the second load instruction...
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//
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if (AA.canInstructionRangeModify(BB2->front(), *L2, LoadAddress))
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return false; // Cannot eliminate load
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// Do a depth first traversal of the inverse CFG starting at L2's block,
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// looking for L1's block. The inverse CFG is made up of the predecessor
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// nodes of a block... so all of the edges in the graph are "backward".
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//
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std::set<BasicBlock*> VisitedSet;
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for (pred_iterator PI = pred_begin(BB2), PE = pred_end(BB2); PI != PE; ++PI)
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if (CheckForInvalidatingInst(*PI, BB1, LoadAddress, AA, VisitedSet))
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return false;
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// If we passed all of these checks then we are sure that the two loads
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// produce the same value.
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return true;
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}
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}
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