forked from OSchip/llvm-project
380 lines
15 KiB
C++
380 lines
15 KiB
C++
//===- LoadValueNumbering.cpp - Load Value #'ing Implementation -*- C++ -*-===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file was developed by the LLVM research group and is distributed under
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// the University of Illinois Open Source License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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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/Target/TargetData.h"
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#include "llvm/Pass.h"
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#include "llvm/Type.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 <set>
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using namespace llvm;
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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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};
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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 *llvm::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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AU.addRequired<TargetData>();
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}
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static bool isPathTransparentTo(BasicBlock *CurBlock, BasicBlock *Dom,
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Value *Ptr, unsigned Size, AliasAnalysis &AA,
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std::set<BasicBlock*> &Visited,
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std::map<BasicBlock*, bool> &TransparentBlocks){
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// If we have already checked out this path, or if we reached our destination,
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// stop searching, returning success.
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if (CurBlock == Dom || !Visited.insert(CurBlock).second)
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return true;
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// Check whether this block is known transparent or not.
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std::map<BasicBlock*, bool>::iterator TBI =
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TransparentBlocks.lower_bound(CurBlock);
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if (TBI == TransparentBlocks.end() || TBI->first != CurBlock) {
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// If this basic block can modify the memory location, then the path is not
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// transparent!
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if (AA.canBasicBlockModify(*CurBlock, Ptr, Size)) {
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TransparentBlocks.insert(TBI, std::make_pair(CurBlock, false));
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return false;
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}
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TransparentBlocks.insert(TBI, std::make_pair(CurBlock, true));
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} else if (!TBI->second)
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// This block is known non-transparent, so that path can't be either.
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return false;
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// The current block is known to be transparent. The entire path is
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// transparent if all of the predecessors paths to the parent is also
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// transparent to the memory location.
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for (pred_iterator PI = pred_begin(CurBlock), E = pred_end(CurBlock);
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PI != E; ++PI)
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if (!isPathTransparentTo(*PI, Dom, Ptr, Size, AA, Visited,
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TransparentBlocks))
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return false;
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return true;
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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 the alias analysis has any must alias information to share with us, we
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// can definitely use it.
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if (isa<PointerType>(V->getType()))
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getAnalysis<AliasAnalysis>().getMustAliases(V, RetVals);
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if (!isa<LoadInst>(V)) {
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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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assert(&getAnalysis<ValueNumbering>() != (ValueNumbering*)this &&
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"getAnalysis() returned this!");
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return getAnalysis<ValueNumbering>().getEqualNumberNodes(V, RetVals);
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}
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// Volatile loads cannot be replaced with the value of other loads.
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LoadInst *LI = cast<LoadInst>(V);
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if (LI->isVolatile())
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return getAnalysis<ValueNumbering>().getEqualNumberNodes(V, RetVals);
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// If we have a load instruction, find all of the load and store instructions
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// that use the same source operand. We implement this recursively, because
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// there 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 be
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// a cycle. The PHI node would be handled by the else case here, breaking the
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// 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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BasicBlock *LoadBB = LI->getParent();
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Function *F = LoadBB->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 or store candidates that are
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// identical.
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//
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std::map<BasicBlock*, std::vector<LoadInst*> > CandidateLoads;
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std::map<BasicBlock*, std::vector<StoreInst*> > CandidateStores;
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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 && !Cand->isVolatile()) // Not LI itself?
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CandidateLoads[Cand->getParent()].push_back(Cand); // Got one...
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} else if (StoreInst *Cand = dyn_cast<StoreInst>(*UI)) {
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if (Cand->getParent()->getParent() == F && !Cand->isVolatile() &&
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Cand->getOperand(1) == Source) // It's a store THROUGH the ptr...
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CandidateStores[Cand->getParent()].push_back(Cand);
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}
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}
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// Get alias analysis & dominators.
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AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
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DominatorSet &DomSetInfo = getAnalysis<DominatorSet>();
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Value *LoadPtr = LI->getOperand(0);
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// Find out how many bytes of memory are loaded by the load instruction...
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unsigned LoadSize = getAnalysis<TargetData>().getTypeSize(LI->getType());
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// Find all of the candidate loads and stores that are in the same block as
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// the defining instruction.
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std::set<Instruction*> Instrs;
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Instrs.insert(CandidateLoads[LoadBB].begin(), CandidateLoads[LoadBB].end());
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CandidateLoads.erase(LoadBB);
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Instrs.insert(CandidateStores[LoadBB].begin(), CandidateStores[LoadBB].end());
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CandidateStores.erase(LoadBB);
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// Figure out if the load is invalidated from the entry of the block it is in
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// until the actual instruction. This scans the block backwards from LI. If
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// we see any candidate load or store instructions, then we know that the
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// candidates have the same value # as LI.
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bool LoadInvalidatedInBBBefore = false;
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for (BasicBlock::iterator I = LI; I != LoadBB->begin(); ) {
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--I;
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// If this instruction is a candidate load before LI, we know there are no
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// invalidating instructions between it and LI, so they have the same value
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// number.
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if (isa<LoadInst>(I) && Instrs.count(I)) {
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RetVals.push_back(I);
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Instrs.erase(I);
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}
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if (AA.getModRefInfo(I, LoadPtr, LoadSize) & AliasAnalysis::Mod) {
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// If the invalidating instruction is a store, and its in our candidate
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// set, then we can do store-load forwarding: the load has the same value
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// # as the stored value.
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if (isa<StoreInst>(I) && Instrs.count(I)) {
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Instrs.erase(I);
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RetVals.push_back(I->getOperand(0));
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}
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LoadInvalidatedInBBBefore = true;
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break;
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}
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}
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// Figure out if the load is invalidated between the load and the exit of the
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// block it is defined in. While we are scanning the current basic block, if
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// we see any candidate loads, then we know they have the same value # as LI.
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//
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bool LoadInvalidatedInBBAfter = false;
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for (BasicBlock::iterator I = LI->getNext(); I != LoadBB->end(); ++I) {
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// If this instruction is a load, then this instruction returns the same
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// value as LI.
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if (isa<LoadInst>(I) && Instrs.count(I)) {
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RetVals.push_back(I);
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Instrs.erase(I);
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}
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if (AA.getModRefInfo(I, LoadPtr, LoadSize) & AliasAnalysis::Mod) {
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LoadInvalidatedInBBAfter = true;
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break;
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}
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}
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// If there is anything left in the Instrs set, it could not possibly equal
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// LI.
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Instrs.clear();
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// TransparentBlocks - For each basic block the load/store is alive across,
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// figure out if the pointer is invalidated or not. If it is invalidated, the
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// boolean is set to false, if it's not it is set to true. If we don't know
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// yet, the entry is not in the map.
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std::map<BasicBlock*, bool> TransparentBlocks;
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// Loop over all of the basic blocks that also load the value. If the value
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// is live across the CFG from the source to destination blocks, and if the
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// value is not invalidated in either the source or destination blocks, add it
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// to the equivalence sets.
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for (std::map<BasicBlock*, std::vector<LoadInst*> >::iterator
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I = CandidateLoads.begin(), E = CandidateLoads.end(); I != E; ++I) {
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bool CantEqual = false;
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// Right now we only can handle cases where one load dominates the other.
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// FIXME: generalize this!
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BasicBlock *BB1 = I->first, *BB2 = LoadBB;
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if (DomSetInfo.dominates(BB1, BB2)) {
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// The other load dominates LI. If the loaded value is killed entering
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// the LoadBB block, we know the load is not live.
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if (LoadInvalidatedInBBBefore)
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CantEqual = true;
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} else if (DomSetInfo.dominates(BB2, BB1)) {
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std::swap(BB1, BB2); // Canonicalize
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// LI dominates the other load. If the loaded value is killed exiting
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// the LoadBB block, we know the load is not live.
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if (LoadInvalidatedInBBAfter)
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CantEqual = true;
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} else {
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// None of these loads can VN the same.
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CantEqual = true;
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}
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if (!CantEqual) {
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// Ok, at this point, we know that BB1 dominates BB2, and that there is
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// nothing in the LI block that kills the loaded value. Check to see if
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// the value is live across the CFG.
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std::set<BasicBlock*> Visited;
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for (pred_iterator PI = pred_begin(BB2), E = pred_end(BB2); PI!=E; ++PI)
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if (!isPathTransparentTo(*PI, BB1, LoadPtr, LoadSize, AA,
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Visited, TransparentBlocks)) {
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// None of these loads can VN the same.
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CantEqual = true;
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break;
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}
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}
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// If the loads can equal so far, scan the basic block that contains the
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// loads under consideration to see if they are invalidated in the block.
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// For any loads that are not invalidated, add them to the equivalence
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// set!
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if (!CantEqual) {
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Instrs.insert(I->second.begin(), I->second.end());
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if (BB1 == LoadBB) {
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// If LI dominates the block in question, check to see if any of the
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// loads in this block are invalidated before they are reached.
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for (BasicBlock::iterator BBI = I->first->begin(); ; ++BBI) {
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if (isa<LoadInst>(BBI) && Instrs.count(BBI)) {
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// The load is in the set!
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RetVals.push_back(BBI);
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Instrs.erase(BBI);
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if (Instrs.empty()) break;
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} else if (AA.getModRefInfo(BBI, LoadPtr, LoadSize)
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& AliasAnalysis::Mod) {
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// If there is a modifying instruction, nothing below it will value
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// # the same.
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break;
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}
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}
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} else {
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// If the block dominates LI, make sure that the loads in the block are
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// not invalidated before the block ends.
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BasicBlock::iterator BBI = I->first->end();
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while (1) {
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--BBI;
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if (isa<LoadInst>(BBI) && Instrs.count(BBI)) {
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// The load is in the set!
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RetVals.push_back(BBI);
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Instrs.erase(BBI);
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if (Instrs.empty()) break;
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} else if (AA.getModRefInfo(BBI, LoadPtr, LoadSize)
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& AliasAnalysis::Mod) {
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// If there is a modifying instruction, nothing above it will value
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// # the same.
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break;
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}
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}
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}
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Instrs.clear();
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}
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}
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// Handle candidate stores. If the loaded location is clobbered on entrance
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// to the LoadBB, no store outside of the LoadBB can value number equal, so
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// quick exit.
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if (LoadInvalidatedInBBBefore)
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return;
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for (std::map<BasicBlock*, std::vector<StoreInst*> >::iterator
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I = CandidateStores.begin(), E = CandidateStores.end(); I != E; ++I)
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if (DomSetInfo.dominates(I->first, LoadBB)) {
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// Check to see if the path from the store to the load is transparent
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// w.r.t. the memory location.
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bool CantEqual = false;
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std::set<BasicBlock*> Visited;
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for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB);
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PI != E; ++PI)
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if (!isPathTransparentTo(*PI, I->first, LoadPtr, LoadSize, AA,
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Visited, TransparentBlocks)) {
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// None of these stores can VN the same.
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CantEqual = true;
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break;
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}
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Visited.clear();
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if (!CantEqual) {
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// Okay, the path from the store block to the load block is clear, and
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// we know that there are no invalidating instructions from the start
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// of the load block to the load itself. Now we just scan the store
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// block.
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BasicBlock::iterator BBI = I->first->end();
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while (1) {
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--BBI;
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if (AA.getModRefInfo(BBI, LoadPtr, LoadSize)& AliasAnalysis::Mod){
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// If the invalidating instruction is one of the candidates,
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// then it provides the value the load loads.
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if (StoreInst *SI = dyn_cast<StoreInst>(BBI))
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if (std::find(I->second.begin(), I->second.end(), SI) !=
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I->second.end())
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RetVals.push_back(SI->getOperand(0));
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break;
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}
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}
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}
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}
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}
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